HomeMy WebLinkAboutCOUNCIL - AGENDA ITEM - 03/02/1999 - FIRST READING OF ORDINANCE NO. 42, 1999, AMENDING AGENDA ITEM SUMMARY ITEM NUMBER: 31
DATE: March 2, 1999
• STAFF
B
FORT COLLINS CITY COUNCIL • Mike Smith/
ob Smith/Susan Hayes j
SUBJECT : j
First Reading of Ordinance No. 42, 1999, Amending the "Storm Drainage Design Criteria and
Construction Standards" by Revising the Rainfall Intensity Duration Curve Graph.
I
RECOMMENDATION:
Staff recommends adoption of the Ordinance on First Reading.
INANCIAL IMPACT:
Assessing the specific economic impacts on basin wide master planned facilities is not possible
j without conducting full master plan updates. However, we can make several generalizations can
be made about the effects of increasing rainfall.
Increasing the rainfall criterion to 3.67 inches affects basins with detention ponds more than basins f
without detention ponds. Peak flows typically do not increase significantly in basins without
detention,therefore the cost for improvements shown in existing master plans should not increase
significantly. In basins with detention, the increased volume of runoff causes ponds to overtop,
thereby increasing the peak flows in channels downstream. The construction of larger ponds to j
contain the additional volume will cost more. Cost increases are very site specific based on the
constraints of the site. Two case studies of detention ponds show cost increases varying from 14% j
to 35%.
tNewly developing land is required to provide protection for the 100-year design storm, regardless
of the benefit-cost ratio. In general, impacts due to increased rainfall would be similar to the basin
i impacts. The main impact of using the 3.67 inch rainfall criterion is the need for additional pond j
volume. The cost for providing this can be minimal if the site is laid out initially to handle the
p higher rainfall. Two case studies showed estimated increase to the cost of a typical lot to be 0.5%.
BACKGROUND:
E "
This Ordinance will modify the current family of design storms used by the City of Fort Collins,
which are currently the 100-year, 50-year, 25-year, 10-year, 5-year and 2-year storm frequencies,
based on a revised analysis of rainfall data. The following discussion focuses on the 100-year design
storm. i
Council discussed this issue at its January 12 and February 23, 1999 study sessions.
J
DATE: March 2, 1999 2 ITEM NUMBER: 31
I
In January of 1998,the Utilities embarked on a study to reevaluate the amount of rainfall associated
with the 100-year frequency storm. A Precipitation Study Task Force(the "Task Force")made up
of technical experts,regulatory agencies, and citizens guided this study.
The recommendation of a majority of the members on the Task Force is to increase the 100-year
design rainfall criterion from 2.89 inches to 3.67 inches,and make a similar adjustment to the other
storm frequencies,to be applied uniformly over the entire city. Letters of support from the Colorado
Water Conservation Board (CWCB), Larimer County, and the Federal Emergency Management
Agency (FEMA) are included in the attached packet.
The Water Board has also recommended Council adopt an increase in the 100-year design rainfall
criterion from 2.89 inches to 3.67 inches, applied uniformly over the entire city.
A minority of the members on the Task Force recommend Council adopt 4.37 inches east of Taft Hill
Road, and 5.5 inches west of Taft Hill Road. The 5.5 inch value was determined using the Fort
Collins gauge data with a higher value for the 1997 storm than was recorded at the gauge station.
In the late 1970's City Council adopted the policy to use the 100-year storm for the planning and
design of storm drainage facilities. It is used as a standard by the FEMA for floodplain mapping,
and is the standard of the CWCB for the analysis of drainage basins. Both of these agencies have
regulations that govern the criteria of the City.
The current 100-year value used by the City was determined from the National Oceanic and
I Atmospheric Administration (NOAA) Atlas 2, published in 1973. NOAA typically conducts the
precipitation frequency studies for the United States guided by standard engineering and statistical
practices. Based on data up to 1969, the City adopted a 100-year design stone criterion of 2.89
inches over a two-hour period.
The statistical analysis from the City's recent study resulted in two values for the 100-year stone:
• 3.67 inches over two hours:based on a regional analysis of Front Range gauge data from Fort
Collins, Longmont, and Boulder.
• 4.37 inches over two hours: based on data from CSU's gauge in Fort Collins only.
To put the numbers in some perspective:
• The cities of Denver,Loveland, and Colorado Springs use a 100-year design storm of slightly
over three inches over a two-hour period. The proposed 3.67 inches would result in the Fort
Collins value being higher than these other Front Range communities. All of these communities
use the 100-year stone as their adopted level of protection for the major drainage system.
• The rainfall criterion used by the Denver area Urban Drainage and Flood Control District ranges
from 3.0 inches to 3.01 inches over two-hours. The region under the District's jurisdiction
ranges from Boulder to six miles east of Denver International Airport.
• Prior to the July 1997 storm there had never been a recorded two-hour rainfall amount that
exceeded 3.10 inches.
. ,
DATE: March2, 1999 3 ITEM NUMBER: 31
• Prior to 1940 there were only two instances where the daily total at the CSU gauge exceeded 4
inches.
Given the comparison to other Front Range communities and the feedback received from regulatory t
agencies, staff strongly believes the regional analysis (3.67 inches) is the best estimate of the 100-
I
year value. Adoption of a higher value would be, in essence, adoption of a higher level of
protection than 100-year. Using the regional analysis curve,the 4.37 inch value lies between the 150
and 175 year events, and the 5.5 inch value corresponds to the 325 year event.
The 100-year design rainfall criterion is used to calculate runoff flows,map floodplains,and design
drainage facilities in new subdivisions and already developed areas. Increasing the design rainfall
criterion to 3.67 inches, or even the higher 4.37 inches, will not prevent flooding. Storms of even
greater magnitude still have a chance to occur.The City's stormwater management program includes
components to address the effects of a larger rainfall.
The rainfall used to design drainage facilities is just a start. The remaining components of the City's
stormwater management program include:
• Storm Drainage Basin Master Planning: the City prepares basin master plans which identify
a collection of drainage projects to solve existing and potential drainage problems. The plans
are based on 100-year runoff flows, assuming the basin is fully developed. The master plans
must show the overall benefits to the basin are outweighed by the cost of the improvements. In
some basins, 100-year protection can not be provided since the benefits of the improvements
needed to contain 100-year flows do not outweigh the costs. However,adopting the 3.67" design
storm criterion does not preclude the City from providing greater than 100-year protection in
master planned facilities when the need is shown and the economics warrant it. Currently,the
City is in the process of re-studying two west-side basin master plans, Canal Importation and
West Vine. Through this process, greater protection on the west side can be evaluated.
• Factors of safety in design: one foot of freeboard on channels and ponds,as well as controlled
spillways for ponds, allow these facilities to safely pass more than the design storm. The City
is currently revising the Storm Drainage Design Criteria to incorporate additional items such as
analyzing the impacts of larger storm events to help guide the design of new developments and
provide information to future homeowners about potential problems,requiring controlled spill
locations on irrigation ditches that run through new developments,and assuming partial blockage
of culverts under roads when designing the size of road crossings.
• Sensible floodplain regulations to avoid putting people and property at risk: the City
already regulates to higher standards than FEMA, as well as regulating many local floodplain.
Certain critical facilities are not allowed in 100-year and 500-year floodplains. A Task Force
has been formed to look at additional changes to the current floodplain regulations.
• Emergency response: the City is in the process of updating its early warning system. As part
of a FEMA grant,streamflow and precipitation gauges,enhanced weather information,an auto-
telephone dialing system, an AM radio station for emergency management, and an enhanced
cable TV override system will be installed. The goal is to be able to provide real time flood
DATE: March2, 1999 4 ITEM NUMBER: 31
inundation mapping to enhance response to flooding and notify and evacuate residents if
necessary.
• Education and outreach:the City currently provides floodplain determinations and continues
to send fliers to all floodplain residents yearly. Awareness is also raised through articles in City
News and the newspaper. A Flood Education Day was held at the mall in 1998 and will be held
again in 1999. The City's Office of Emergency Management has created a web page that has
information about past flooding history and emergency preparedness. Many of the proposed
projects for FEMA's Project Impact will be focusing on education about emergency management
and hazard mitigation. Staff will be creating education programs for the schools,movie theatre
based outreach, videos, web site information, and more newspaper articles. This education
outreach will include common sense components such as how to grade lots to prevent local
flooding,how to protect valuables in case of a flood,cautions about what to do and not do(such
as driving in water) during a flood.
• Flood proofing and Flood insurance:the City continues to provide floodproofing information
to interested residents. The program was greatly expanded in 1998 utilizing FEMA funds. Staff
is currently working with 32 homeowners to construct floodproofing measures for window wells,
outside stairways, and doors. This program is being considered for continuation in the future.
The City also encourages all residents to obtain flood insurance, even if they are not in a
designated floodplain. The City is currently rated a Class 6 in FEMA's Community Rating
System. This entitles all flood insurance policy holders to a 20% discount on their insurance
premium.
• Maintenance of the stormwater system:the City continues to inspect the drainage system and
perform debris removal as needed. This ensures the system will perform as designed during a
storm event.
There is no way to know if the recommended value is, or ever will be,the absolutely correct value
for the 100-year event. As more information is gathered over the next few decades the value will
be refined, and it will change again. The recommended value of 3.67 inches over two hours
represents a reasonable value based on the information currently available. This will give the City
a drainage system which will not necessarily be "flood proof' but will, in conjunction with all the
other components of the stormwater program,minimize property damage and loss of life during all
storm events.
• ORDINANCE NO. 42, 1999
OF THE COUNCIL OF THE CITY OF FORT COLLINS
AMENDING THE "STORM DRAINAGE DESIGN CRITERIA
AND CONSTRUCTION STANDARDS" BY REVISING THE
RAINFALL INTENSITY DURATION CURVE GRAPH
WHEREAS, in 1984 the Council of the City of Fort Collins (the "Council') approved
Resolution 84-41 pursuant to which it adopted the"Storm Drainage Design Criteria and Construction
Standards"for the purpose of setting forth the technical criteria to be used in the analysis,design and
construction of storm drainage systems within the City and its urban growth area; and
WHEREAS, in 1991 the Council adopted Resolution 91-44 pursuant to which it amended
the Storm Drainage Design Criteria and Construction Standards by the addition of the"Construction
Erosion Control Criteria" (the "Erosion Control Criteria"); and
WHEREAS,the Storm Drainage Design Criteria and Construction Standards, as amended
by the addition of the Erosion Control Criteria, constitute the Storm Drainage Design Criteria and
Construction Standards that all new developments in the City are currently required to comply with,
pursuant to City Code Section 26-544 and Section 3.3.3 of the City's Land Use Code; and
WHEREAS, in January of 1998,the City embarked on a study to reevaluate the amount of
rainfall associated with the 100-year frequency storm, as well as with other storm frequencies(i.e.,
2-year,5-year, 1 0-year,25-year and 50-year),and in doing so established a Precipitation Study Task
Force(the"Task Force")made up of technical experts,representatives from regulatory agencies and
citizens; and
WHEREAS, a majority of the members on the Task Force has recommended an increase in
the 100-year design rainfall criterion from 2.89 inches to 3.67 inches over a two-hour period,as well
as a similar increase for other stone frequencies, to be applied uniformly over the entire city; and
WHEREAS, the Colorado Water Conservation Board, Latimer County and the Federal
Emergency Management Agency have all expressed support for this proposed change; and
WHEREAS,the Fort Collins Water Board has also recommended this proposed change; and
WHEREAS,a minority of the members on the Task Force has recommended an increase in
the 100-year design rainfall criterion from 2.89 inches to 4.3 7 inches over a two-hour period,as well
as similar increases for other storm frequencies, to be applied uniformly over the entire city; and
WHEREAS,the Council has determined that it is in the best interests of the citizens of the
City and necessary for the public's health, safety and welfare, that the Storm Drainage Design
Criteria and Construction Standards be amended as recommended by a majority of the members on
the Task Force; and
WHEREAS, in order to so amend the Storm Drainage Design Criteria and Construction
Standards it is necessary to replace in it the existing Figure 3-1 which constitutes the "Rainfall
Intensity Duration Curve" graph dated November, 1975, (the "Existing Figure 3-1") with a new
"Rainfall Intensity Duration Curve" graph dated February, 1999, a copy of which new Figure 3-1
is on file in the Office of the Fort Collins City Clerk(the "New Figure 3-1").
NOW, THEREFORE,BE IT ORDAINED BY THE COUNCIL OF THE CITY OF FORT
COLLINS as follows:
Section 1. That the Storm Drainage Design Criteria and Construction Standards be amended
by substituting and replacing the Existing Figure 3-1 with the New Figure 3-1.
Section 2. That the Storm Drainage Design Criteria and Construction Standards,as amended
by Resolution 91-44 and by this Ordinance, are hereby reaffirmed,readopted and reenacted by the
Council.
Introduced and considered favorably on first reading and ordered published this 2nd day of
March,A.D. 1999, and to be presented for final passage on the 16th day of March, A.D. 1999.
Mayor
ATTEST:
City Clerk
Passed and adopted on final reading this 16th day of March,A.D. 1999.
Mayor
ATTEST:
City Clerk
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ADOPTION OF
100-YEAR RAINFALL CRITERIA
MARCHCOUNCIL MEETING
• •
COUNCIL STUDY SESSIONS
Utilities
light S power • stormwater • wastewater • water
47tv;of Fort Collins
MEMORANDUM
DATE: February 23, 1999
TO: Mayor Azari and Council Members
THRU: John F. Fischbach, City Manager
FROM: Michael B. Smith,Utilities General Manager
RE: February 23, 1999 Study Session Summary— 100-Year Rainfall Study
At the January 12, 1999 study session, a recommendation from staff, the Water Board and
a majority of the Precipitation Study Task Force was presented to City Council to change
the 100-year design storm to 3.67 inches over two hours. A minority opinion of the
Precipitation Study Task Force was to change the rainfall amount to 4.37 inches with 5.5
inches for those areas on the west side of the City. During the discussion at the January
12th Study Session, City Council requested information pertaining to the overall
stormwater management program that would mitigate the effects of larger storm events,
and asked the Task Force and the Water Board to reaffirm their recommendations. At the
February 23, 1999 Study Session, Utilities Staff, members of the Precipitation Study Task
Force, and the Water Board presented to Council the additional information requested
regarding the changing of the design rainfall.
Key Discussion Points
1.Council members are comfortable saying the existing criteria is inappropriate.
2. The values presented are estimates based on the data available. There is no one
true value.
3. Regardless of the new value chosen, there will be an impact on the cost of new
development and master planned drainage facilities.
4. Other elements of the stormwater management program that help protect
against extreme events and which can mitigate the effects of an event higher than
3.67 inches were discussed. Those included:
- Emergency response which includes rain gages, notification, evacuation.
700 Wood St. • P.O. Box 580 • Fort Collins,CO 80522-0580 • (970)221-6700 • FAX(970)221-6619 • FAX(970)221-6593 • TDD(970)224-6003
e-mail: utilities(ici.fort-collins.co.us • www.ci.fort•collins.co.us/UTILITIES
- Education programs about flooding and its impacts.
-Floodproofing and flood insurance.
-Design factors of safety in the design of stormwater facilities.
-Basin master planning which plans for the drainage system holistically
and facilities are justified on a benefit to cost analysis.
-Floodplain regulations that are more restrictive than federal
requirements.
-Maintenance of stormwater facilities.
5. Council would like the minority opinion to be presented at Council meeting.
6. Adoption of the 3.67-inch 100-year design storm does not preclude City
Council from adopting a policy for higher levels of protection.
7. It does rain more on the west side of Fort Collins,but there is no strong data
that shows it rains harder.
8. There is uncertainty and differences of opinion about global warming and its
impacts on rainfall rates and intensities.
Next Steps
1. Staff will bring a recommendation in the form of an ordinance to Council for
final decision.
Cc: Water Board
Precipitation Study Task Force
DATE: February 23, 1999 STUDY SESSION ITEM
STAFF: Mike Smith/ FORT COLLINS CITY COUNCIL
ift Bob Smith/Susan Hayes
JECT FOR DISCUSSION:
The 100-year Design Rainfall.
GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED:
Staff would like to provide additional information requested at the January study session and get
additional guidance from Council.
Council Feedback Desired:
• Does Council have additional questions about historic rainfall or rainfall patterns in Fort Collins?
• From the previous study session it appears the Council supports the 100-year protection level
of 3.67 inches over 2-hours. Is there Council interest to provide a greater level of protection on
the west side of the City? For example: 175-year event, 325-year event, or some other degree
of protection?
In January of 1998,the Utilities embarked on a study to reevaluate the amount of rainfall associated
with the 100-year frequency storm. A Precipitation Study Task Force (the "Task Force") made up
of technical experts, regulatory agencies, and citizens guided this study.
Summary of the recommendations:
The recommendation of a majority of the members on the Task Force is to increase the 100-year
design rainfall from 2.89 inches to 3.67 inches, and make a similar adjustment to the other storm
frequencies, to be applied uniformly over the entire City. Letters of support from the Colorado
Water Conservation Board (CWCB), Larimer County, and the Federal Emergency Management
Agency (FEMA) are included in the attached packet.
The Water Board has also recommended Council adopt an increase in the 100-year design rainfall
from 2.89 inches to 3.67 inches, applied uniformly over the entire City. This recommendation was
reconfirmed at the Board's January 1999 meeting. A memo from the Water Board Chair is included
in the attached packet.
A minority of the members on the Task Force recommend Council adopt 4.37 inches east of Taft Hill
Road, and 5.5 inches west of Taft Hill Road. The 5.5 inch value was determined using the Fort
Collins gage data using a higher value for the 1997 storm than was recorded at the gage station.
i
February 23, 19991 2
DATE:
In late January the Task Force reconvened. All members reconfirmed their opinions stated in the
Task Force memo.
The statistical analysis from the City's recent study resulted in two values for the 100-year storm:
• 3.67 inches over two hours: based on a regional analysis of Front Range gage data from Fort
Collins, Longmont, and Boulder.
• 4.37 inches over two hours: based on data from CSU's gage in Fort Collins only.
To put the numbers in some perspective:
• The cities of Denver, Loveland, and Colorado Springs use a 100-year design storm of slightly-
over three inches over a two-hour period. The proposed 3.67 inches would result in Fort Collins
being higher than these other Front Range communities. All of these communities use the 100-
year storm as their adopted level of protection for the major drainage system.
• The rainfall used by the Denver area Urban Drainage and Flood Control District (UDFCD)
ranges from 3.0 inches to 3.01 inches over 2-hours. The region under the District's jurisdiction
ranges from Boulder to six miles east of Denver International Airport. The UDFCD has also
adopted the 100-year level of protection for the design of major drainage systems.
• Prior to the July 1997 storm there had never been a recorded two-hour rainfall amount that
exceeded 3.10 inches.
• In the first 50 years of record keeping, there were no recorded daily totals that reached 3.67
inches.
I
Given the comparison to other Front Range communities and the feedback received from regulatory
agencies, staff strongly believes the regional analysis (3.67 inches) is the best estimate of the 100-
year value. Adoption of a higher value would be, in essence, adoption of a higher level of
protection than 100-year. Using the regional analysis curve,the 4.3 7 inch value lies between the 150
and 175 year events, and the 5.5 inch value corresponds to the 325 year event.
At the January 12, 1999 meeting, Council requested additional information about the City's
stormwater program and how it can mitigate the impacts of a storm greater than 3.67 inches.
Components of the City's stormwater management program include:
• Storm Drainage Basin Master Planning: the City prepares basin master plans which identify
a collection of drainage projects to solve existing and potential drainage problems. The plans
are based on 100-year runoff flows, assuming the basin is fully developed. The master plans
must show the overall benefits to the basin are outweighed by the cost of the improvements. In
some basins, 100-year protection can not be provided since the benefits of the improvements
needed to contain 100-year flows do not outweigh the costs. However,adopting the 3.67"design
storm does not preclude the City from providing greater then 100-year protection in master
planned facilities when the need is shown and the economics warrant it. Currently,the City is
DATE: February 23, 1999 3
in the process of re-studying two west-side basin master plans, Canal Importation and West
. Vine. Through this process, greater protection on the west side can be evaluated.
• Factors of safety in design:one foot of freeboard on channels and ponds,as well as controlled
spillways for ponds,allow these facilities to safely pass more than the design storm. The City
is currently revising the Storm Drainage Design Criteria to incorporate additional items such as
analyzing the impacts of larger storm events to help guide the design of new developments and
provide information to future homeowners about potential problems,requiring controlled spill
locations on irrigation ditches that run through new developments,and assuming partial blockage
of culverts under roads when designing the size of road crossings.
• Sensible floodplain regulations to avoid putting people and property at risk: the City
already regulates to higher standards than FEMA,as well as regulating many local floodplains.
Certain critical facilities are not allowed in 100-year and 500-year floodplains. A Task Force
has been formed to look at additional changes to the current floodplain regulations.
• Emergency response: the City is in the process of updating its early warning system. As part
of a FEMA grant we will be installing streamflow and precipitation gages, enhanced weather
information,an auto-telephone dialing system,an AM radio station for emergency management,
and an enhanced cable TV override system. The goal is to be able to provide real time flood
inundation mapping to enhance our response to flooding and notify and evacuate residents if
necessary.
• Education and outreach:the City currently provides floodplain determinations and continues
to send fliers to all floodplain residents yearly. Awareness is also raised through articles in City
News and the newspaper. A Flood Education Day was held at the mall in 1998 and will be held
again in 1999. The City's Office of Emergency Management has created a web page that has
information about past flooding history and emergency preparedness. Many of the proposed
projects for FEMA's Project Impact will be focusing on education about emergency management
and hazard mitigation. We will be creating education programs for the schools, movie theatre
based outreach, videos, web site information, and more newspaper articles. This education
outreach will include common sense components such as how to grade your lot to prevent local
flooding,how to protect your valuables in case of a flood, cautions about what to do and not do
(such as driving in water) during a flood.
• Flood proofing and Flood insurance:the City continues to provide floodproofrng information
to interested residents. The program was greatly expanded in 1998 utilizing FEMA funds. We
are currently working with 32 homeowners to construct floodproofrng measures for window
wells, outside stairways, and doors. This program is being considered for continuation in the
future. The City also encourages all residents to obtain flood insurance,even if they are not in
a designated floodplain. The City is currently rated a Class 6 in FEMA's Community Rating
System. This entitles all flood insurance policy holders to a 20% discount on their insurance
premium.
DATE: February 23, 1999 4
• Maintenance of the stormwater system:the City continues to inspect the drainage system and
perform debris removal as needed. This ensures the system will perform as designed during a
storm event.
There is no way to know if the recommended value is,or ever will be,the absolutely correct value
for the 100-year event. As more information is gathered over the next few decades the value will
be refined, and it will change again. The recommended value of 3.67 inches over two hours
represents a reasonable value based on the information currently available. This will give the City
a drainage system which,while not"flood proof',will,in conjunction with all the other components
of the stormwater program, minimize property damage and loss of life during all storm events.
An issue has also surfaced, since the previous study session, regarding the process the Task Force
used to make its recommendation. There's a concern the Task Force initially decided on the 4.37"
inches value as the recommended 100-year storm,but after the introduction of information regarding
the economic impact of adopting this value,the majority of the Task Force members changed their
support to the 3.67 inches. At the May 1998 Task Force meeting the majority of the members
supported the 4.37 inches value based on the information they had at the time. However, after the
meeting,concerns were raised by members ofthe Task Force regarding the adequacy of the decision.
At the August meeting this was further discussed,and it was decided to continue to look at the data
trends because the effect of raising the rainfall criteria could be costly, and it was worthwhile to
spend the time and money on further analysis. Based on the discussion at the August meeting,many
members chose to freely change their opinions and support the 3.67 inches. The further analysis
confirmed their decisions.
No specific economic information was provided to the Task Force. The detailed case studies
presented to the Water Board and City Council were not even started until after the final
recommendation was made.However,the experience of many members on the Task Force who work
in this field made it intuitively obvious to them that adopting higher rainfall would translate into
higher costs for drainage facilities. The analysis of the data was not influenced by economics, but
individual Task Force members may have chosen to support a particular value based on their
experience and the ramifications of adopting a higher value not as well supported by the available
data.
i
r—,DATE: February 23 1999 5
SUMMARY OF IMPACTS
3.67 INCHES 4.37 INCHES
Provides more protection for new - Provides more protection for new
developments than current criteria. developments than current criteria.
May provide greater protection for - May provide greater protection for
existing developments. Design of existing developments. Design of
improvements that solve existing improvements that solve existing
problems is based on benefits problems is based on benefits
outweighing costs. Final projects may outweighing costs. Final projects may
provide less than 100-year protection. provide less than 100-year protection.
The residual 100-year floodplain will be The residual 100-year floodplain will be "
larger and subject to floodplain larger and subject to floodplain
regulations. regulations.
- Peak rainfall intensities increase slightly. - Peak rainfall intensities increase over
30%.
Pond volumes can increase 0%to 60%.
- Pond volumes can increase 60% to more
- Peak flows increase slightly, +/- 5% to than 100%.
10% in undetained basins.
Peak flows increase 40% to 60% in
Will cost more to build new development undetained basins.
facilities due to increased pond volume.
- Will cost more to build new development
- May cost more for projects which solve facilities due to both increased pond
existing problems. volume and larger storm sewers and
inlets.
- The master planned facilities already
constructed are more likely to be able to - May cost more for projects which solve
handle increased rainfall within freeboard existing problems.
limits. Additional volume required, but
less retro-fit of channels needed. - The master planned facilities already
constructed will not be able to handle the
increased rainfall; additional retrofit may
be required,or a larger residual floodplain
may be mapped.
i
LIST OF ATTACHMENTS (if appropriate):
January 12, 1999 Study Session memorandum.
Letters of Support from Federal Emergency Management Agency, Colorado Water Conservation
Board, Larimer County and Water Board.
Excerpts from Water Board Minutes,January 28, 1999
Utilities
light & power • stormwater • wastewater • water
MEMORANDUM
ty of Fort Collins
DATE: January 14, 1999
TO: Mayor Azari and Council Members
THRU: John F. Fischbach, City Manager
FROM: James C. Hibbard, Water Operations & Planning Manager
RE: January 12, 1999 Study Session Summary— I00-Year Rainfall Study
At the January 12, 1999 Study Session, Utilities Staff, Members of the Precipitation
Study Task Force, and the Chairman of the Water Board presented to Council
information regarding the changing of the design rainfall for the 100-year design storm.
The recommendation from staff,the Water Board and a majority opinion of the Task
Force was to change the design storm to 3.67 inches, while a minority opinion of the
Task Force was to change the rainfall amount to 4.37 inches with 5.5 inches for those
areas on the west side of the City.
Key Discussion Points
1.Council members are comfortable saying the existing criteria is inappropriate.
2. The values presented are estimates based on the data available. There is no one
true value.
3. Regardless of the new value chosen, there will be an impact on the cost of new
development and master planned drainage facilities.
4. Other elements of stormwater management program help protect against more
extreme events. Council would like additional information about the other
activities,which can mitigate the effects of an event higher than 3.67 inches.
- Rain gages, warning, evacuation
-Education programs
-Floodproofing and flood insurance
- Design factors of safety
5. Acknowledged a group of experts worked on this. Experts can disagree.
700 Wood St. • P.O.Box 580 • Fort Collins,CO 80522-0580 • (970)221-6700 • FAX(970)221-6619 • FAX(970)221-6593 • TDD(970)224-6003
e-mail: utilitiest:+.ci.fort-collins.co.us • www.ci.fort-collins.co.us/JTILITIES
6. Council would like minority opinion to be presented at Council meeting.
Next Steps
1. Reconvene the Task Force for a meeting with the Utilities General Manager to
review the Task Force recommendation and the Council Study Session discussion.
2. Staff will bring a recommendation in the form of an ordinance to Council for
final decision. Additional information will be provided about other activities that
can mitigate the effects of an extreme event.
Cc: Water Board
Precipitation Study Task Force
DATEJanuary 12, 1999 STUDY SESSION ITEM
STAFFMike Smith FORT COLLINS CITY COUNCIL
ak Bob Smith/Susan Hayes
CT FOR DISCUSSION:
The 100-Year Design Rainfall.
GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED:
In the late 1970's the Fort Collins City Council adopted the policy to use the 100-year storm for the
planning and design of storm drainage facilities. The 100-year rainfall is used to calculate runoff
flows, map floodplains, and design drainage facilities in new subdivisions and already developed
areas. It's used as a standard by the Federal Emergency Management Agency and the Colorado
Water Conservation Board,two agencies which govern Fort Collins criteria.
In January of 1998,the Utilities embarked on a study to reevaluate the amount of rainfall associated
with the 100-year frequency storm. This study was guided by a Precipitation Study Task Force
made up of technical experts,regulatory agencies, and citizens.
The statistical 'analysis from the study resulted in two values for the 100-year storm:
3.67 inches over two hours: based on a regional analysis of Front Range gage data
from Fort Collins,Longmont, and Boulder.
I 4.37 inches over two hours: based on data from the Fort Collins (CSU) gage only.
The majority recommendation of the Task Force is to increase the 100-year design rainfall from 2.89
inches to 3.67 inches, applied uniformly over the entire City.
The Water Board has recommended Council adopt an increase in the 100-year design rainfall from
2.89 inches to 3.67 inches, applied uniformly over the entire City.
A minority opinion from the Task Force recommends the adoption of 4.37 inches for areas east of
Taft Hill Road and 5.5 inches for areas west of Taft Hill Road. The 5.5 inch value was determined
using the Fort Collins gage data with a higher value for the 1997 storm than was recorded at the gage
station.
DATE: January 12, 1999 I 2 -
i
I SUMMARY OF IMPACTS
3.67 INCHES 4.37 INCHES
- Provides more protection for new Provides more protection for new
developments than current criteria. developments than current criteria.
- May provide greater protection for - May provide greater protection for
existing developments. Design of existing developments. Design of
improvements which solve existing improvements which solve existing
problems is based on benefits problems is based on benefits
outweighing costs. Final projects may outweighing costs. Final projects
provide less than 100-year protection. may provide less than 100-year
The residual I00-year floodplain will be protection. The residual 100-year
larger and subject to floodplain floodplain will be larger and
regulations. subject to floodplain regulations.
Peak rainfall intensities increase slightly. Peak rainfall intensities increase
over 30%.
Pond volumes can increase 0%to 60%.
Peak flows increase slightly,+/- 5 %to - Pond volumes can increase 60%to
10%in undetained basins. more than 100%.
Will cost more to build new - Peak flows increase 40%to 60%in
development facilities due to increased undetained basins.
pond volume.
May cost more for projects that solve - Will cost more to build new
existing problems. development facilities due to both
increased pond volume and larger
storm sewers and inlets.
The master planned facilities already May cost more for projects which
constructed are more likely to be able to solve existing problems.
handle increased rainfall within
freeboard limits. Additional volume
required, but less retro-fit of channels
needed.
The master planned facilities
already constructed will not be able
to handle the increased rainfall;
additional retrofit may be required,
or a larger residual floodplain may
be mapped.
January 12, 1999 3
DATE:
Council Feedback Desired
• Does Council have enough information to make a decision regarding the 100-year design
rainfall?
• Does Council support increasing the 100-year design rainfall from 2.89 inches to 3.67
inches?
LIST OF ATTACHMENTS (if appropriate):
Background memorandum to Council
Excerpts from Water Board Minutes
Memorandum from Precipitation Study Task Force and list of Task Force members
Summary information on basin wide and land development impacts
Public Outreach information
General weather related articles
MEMORANDUM
DT: January 6, 1999
TO: City Council Members
TH: John Fischbach, City Manager
Michael Smith,Utilities General Manage''{{rr
FM: Bob Smith,Water Planning Manager y lL
Susan Duba Hayes, Senior Stormwater Engineer,Master Planning° p
RE: 100-Year Design Rainfall
In January of 1998 the Utilities embarked on a study of the rainfall amount used to design
drainage facilities,both for new development and the construction of facilities in already
developed areas.
At their regular monthly meeting on October 22, 1998,the Water Board adopted a
recommendation for the 100-year design rainfall. The minutes from Water Board meetings are
included in Exhibit A. The remainder of this memorandum, and attachments, summarize the
background and alternatives considered, along with the recommendation.
Background
The design of drainage facilities is based on a design rainfall. Typically, as with Fort Collins, the
100-year storm is used to design major facilities which can safely convey the runoff through the
City with a minimum of damage. In the late 1970's the Fort Collins City Council adopted the
policy to use the 100-year storm for planning and design of storm drainage facilities.
The 100-year storm is an often-used term,and sometimes misunderstood. The 100-year storm
has a precise meaning based on the statistical study of many years of data gathered from official
weather stations. The more data you have,the more confident you are with the value calculated,
however, it is still an estimate. The technical definition is "the amount of rainfall during a
specified length of time that has a 1%chance of being equaled or exceeded in any given year."
An important part of the definition of a storm is the length of time, (duration), associated with the
heavy rainfall. The storm duration we use for design purposes is 2-hours,typical of the type of
intense rainstorms which occur here and cause flash flooding.
Another important aspect of the calculated rainfall is that the value represents total rainfall,over
a certain duration, for one point. In any given year, many heavy storms occur over a large area.
It is totally normal, over a state the size of Colorado, to experience many,many storms each year
that locally equal or exceed the rainfall associated with the "100-year" storm. There are probably
between one hundred and three hundred 100-year storms somewhere in the state of Colorado in a
typical year. For more detailed information,please refer to the weather related articles included
in Exhibit E.
Does it mean if we use the 100-year storm for design that we'll be safe from a flood for 100
years? No. For example,over a period of 30-years, a typical mortgage, a drainage facility
designed for a 100-year storm still has 25%chance of being exceeded because of the possible
occurrence of an even larger event. Adoption of a higher rainfall does not preclude flooding or
flood damage. We can not guarantee flooding will never happen again.
The current 100-year value used by the City was determined from the National Oceanic and
Atmospheric Administration (NOAA)Atlas 2,published in 1973. This Federal agency typically
conducts the precipitation frequency studies for the United States guided by standard engineering
and statistical practices. Based on data up to 1969,the City of Fort Collins previously adopted a
100-year design storm of 2.89 inches over a two-hour period.
What do we use the 100-year design storm(and its related storms of 2-year, 50-year,etc.) for?
Rainfall is just one part of the whole stormwater management picture. It is used to calculate
runoff flows,map floodplains,and design drainage facilities in new subdivisions and already
developed areas. It is used as a standard by the Federal Emergency Management Agency for
floodplain mapping,and is the standard of the Colorado Water Conservation Board for the
analysis of drainage basins. Both of these agencies have regulations which govern the criteria of
the City.
The design rainfall is a start. The remaining components of a strong stormwater management
program include:
• factors of safety in design, such as freeboard and spillways for ponds;
• sensible floodplain regulations to avoid putting people and property at risk-,
• emergency response: weather forecasting,precipitation and stream gages,real time flood
inundation mapping,warnings,evacuation, and rescue;
• flood proofing and flood insurance;
• common sense: lot grading,protection of valuables,not driving in water, etc.;
• maintenance of the stormwater system.
Study Process and Recommendation
This study was performed with the guidance of a technical advisory group,the Precipitation
Study Task Force. Members represent a cross section of regulatory agencies (Federal, State,
County and City),technical expertise(CSU,private consultants), and private citizens.
Composition of the Task Force is included in Exhibit B.
There were diverse opinions amongst the members which resulted in much discussion about
difficult issues. All members were listened to, analyses asked for were done, and additional
members were added at the request of Task Force members.
It's important to note the Task Force members agreed on many aspects of the study,including the
statistical analysis method to use and the data to be included. Basic disagreements centered on:
"at-site"vs.regional analysis of the data,using a higher value for the 1997 storm than actually
recorded at the gage, and the application of a non-uniform rainfall rate over the City(using a
higher rainfall on the west side of the City.)
The final recommendation of the Task Force is:
• Adopt a I00-year design storm of 3.67 inches over two hours(regional analysis). This
amount is a reasonable value, which takes into account the storm of 1997 but balances the
lack of data from the first half of the century with other regional data. This value may be
higher than the future NOAA study will recommend,however,this study is more accurate for
Fort Collins.
• Adopt a uniform rainfall amount across Fort Collins. The data we have does not show a
significant difference between rainfall values for the shorter duration storms to warrant a
varied rainfall rate across the City.
Consensus on the recommendation was not reached among the Task Force members. It was
agreed the minority opinion would be presented to the Water Board for its consideration.
The minority recommendation of the Task Force is:
• Adopt a 100-year design storm of 4.37 inches over two hours(at-site analysis). A minority
believes analysis shows Fort Collins is an extreme within the region and experiences
preferred storm tracks.
• Adopt a higher rainfall, 5.5 inches over two hours, for use on the west side of the City, in
particular west of Taft Hill Road.
The full Task Force memo is included in Exhibit B, and provides the detailed arguments for all
recommendations.
Impact
The rainfall values calculated are based on the standard practice of rainfall analysis,not on the
economic impacts. We do recognize decisionmakers'desires to know what the economic impacts
may be. A detailed discussion is included in Exhibit C. A summary is provided here.
In general, adoption of a higher rainfall will result in the construction of larger drainage facilities
in newly developed areas,providing a greater level of protection than we currently have.
Problems in already developed areas may not experience a higher level of protection. The
facilities built in developed areas,to solve existing problems,are based on the benefits of the
project outweighing the cost of the improvements. Therefore, full protection to the 100-year
storm may not be provided. The Old Town Basin is an example of this.
Basin Impacts
Assessing the specific economic impacts on basin wide master planned facilities is not possible
without conducting a full master plan update. However,we can make several generalizations
about the effects of increasing rainfall.
Incease to 3.67 inches: this rainfall affects basins with detention ponds more than basins without
detention ponds. Since the peak intensities do not increase,peak flows typically do not increase
in basins without detention. In basins with detention, the increased volume of rain causes ponds
to overtop,thereby increasing the peak flows in channels downsteam. Moreland needed and
additional construction to build larger ponds increases the cost of these facilities. This is very
site specific based on the constraints of the site. Two case studies are provided in Exhibit C,
which show cost increases varying from 14%to 35%.
Increase to 4.37 inches:peak intensities as well as total volume of water increase. Therefore,we
see higher peak flows in basins without detention,as well as large increases in volume required
in basins with detention ponds. Increased costs would be associated with larger detention ponds,
as well as larger channels and storm sewers. Due to the large impact on the basin runoff it is
difficult to assess the cost impact without doing a full master plan update. The recommended
solutions in our current master plans would not work for the higher rainfall and would need to be
reevaluated.
Land Development Impacts
Newly developing land is required to provide protection for the 100-year design storm,regardless
of the benefit-cost ratio. In general,impacts due to increased rainfall would be similar to the
basin impacts. Using the 3.67"rainfall,the main impact will be the need for additional pond
volume. The cost for providing this can be minimal if the site is laid out initially to handle the
higher rainfall. Using the 4.37"rainfall,not only will pond volume increase, the on-site peak
flows will also increase. This will most likely require larger on-site infrastructure such as storm
sewers and inlets. Two case studies were prepared by Sear-Brown Engineering, detailing the
design changes and increased costs associated with the two rainfalls. A summary can be found in
Exhibit C.
Public Outreach
The rainfall study-and its results have been public knowledge for several months. The Water
Board has considered the issue and made a recommendation. There has been an open house,
newspaper articles, and information given to applicants during Conceptual Review meetings.
We have also been providing local consultants with the technical information needed to design
with the recommended 3.67"rainfall. Copies of the open house material,comments received
from the public,and the newspaper articles are included in Exhibit D.
File:precipcouncilmemo.doc
LETTERS OF SUPPORT
STATE OF COLORADO
Colorado Water Conservation Board
Department of Natural Resources
721 Centennial Building
1313 Sherman Sheet (D
Denver,Colorado 80203
Phone: (303)866.3441
FAX: (303)666.4474 Bill Owens
Governor
February 9, 1999 Greg E.Walcher
Executive Director,DNR
Hayes Peter H.Evans
Ms. Susan Ha
y Acting Director,CWCB
City of Fort Collins Utilities
P.O. Box 580
Fort Collins, Colorado 80522
RE: Fort Collins Precipitation Task Force
Dear Susan:
This letter is written in response to your request for a follow-up review of the findings of the City's
Precipitation Task Force. The Colorado Water Conservation Board would like to acknowledge that
the Precipitation Frequency Analysis, submitted to you by WRC Engineering under contract with
the City, was performed using sound engineering and statistical approaches. The original
recommendation of using a revised 100-year, 2-hour rainfall amount of 3.67" within the city was
supported by CWCB staff along with the majority vote of the Task Force. Upon further review and
• discussions, the CWCB still supports the use of 3.67" for the 2-hour storm.
Should the City choose to adopt a higher rainfall value, such as 4.37" or even 5.5" (for the west side
of Fort Collins), the CWCB may not be able to support or officially designate future floodplain
studies that make use of those rainfall values. The CWCB designates and approves floodplain
studies based on the concept of 100-year flood frequency. The 100-year flood concept is accepted in
Colorado and nationwide. Should the City choose to utilize a rainfall value that is higher than the
WRC study results (i.e. higher than 3.67" for the 2-hour storm) then that would be considered as
greater than a 100-year storm. However, the CWCB does encourage communities to provide flood
protection to the highest level that is economically feasible. Therefore, the City may wish to design
and build projects that make use of the 100-year rainfall/runoff values, and then add flood protection
by using freeboard or increased conveyance capacity.
In addition, there does not seem to be a sound quantitative approach to accurately define a
numerical difference between rainfall on the west side of the City vs. rainfall on the east side. The
CWCB would support the use of one rainfall value across the entire city. Please call me at (303)
866-3443 if you have any questions or need additional information.
Sind
Thomas W. Browning, P.E.
Flood Control and Floodplain Management Section
02/17/99 WED 09:16 FAX 303 235 4849 FEMA R8 MITIGATION U 001
AN
Federal Emergency Management Agency
{ m Region VIII Denver Federal Center P.O. Box 25267
Denver, CO 80225-0267
February 16, 1999
Ms. Susan Hayes, Stormwater Engineer
Stormwater Utility Dept.
235 Mathews Street
P.O. Box 580
Fort Collins, CO 80522-0580
First, I would like to thank you for inviting me to be one of the member of the
Fort Collins Precipitation Frequency Study Task Force to review the study results by
WRC Engineering under the contract with the City. The accurate precipitation frequency
study is so important to produce Fort Collins flood mapping under the National Flood
Insurance Program (NFIP) which is managed by the Federal Emergency Management
Agency (FEMA).
The final 100-year, 2-hour duration rainfall value of 3.67 inches within the City
of Fort Collins is supported by the majority members of the Task Force and me, Regional
Project Engineer of NFIP Mapping for Region VIII, FEMA. This final precipitation
frequency results was performed using sound engineering and statistical methods which
were accepted by the meteorologist and flood hydrologist societies in USA.
Again, I am pleased City's efforts and the final study results. If you need more
help, please let me know.
Sincerely,
John Liou, P.E.
Senior Hydrologist
RVIII, FEMA
aCO
MERENGINEERING DEPARTMENT
NTY Post Once Box 1190
Fort Collins,Colorado 80522-1190
(970)498-5700
i FAX(970)498-7986
January 20, 1999
City of Fort Collins
City Council
P.O. Box 580 -
Fort Collins, Colorado 80522
Re: Design Rainfall Intensities
Dear Council Members:
This letter is written with regard to the recent review of design rainfall amounts conducted by stormwater staff
members of the City of Fort Collins Utility Department. I have sat on the committee established for review of the
design rainfall.
. As you are probably aware, a two hour"model"rainfall serves as the basis for design of facilities meant to
handle stormwater.After months of attending the task force meetings and reading through the provided
materials,the majority of the task force members agreed to revise the 100-year,2-hour rainfall to 3.67 inches.
Larimer County staff members endorsed the majority opinion. We felt that the procedure used in determining
the 3.67"value was the most valid. As you know,the minority opinion voted for a two hour rainfall of 4.37
inches. We do not argue that the 4.37 inches number could be not correct,only that at this time we feel there is
not enough data to justify this number.
As stormwater engineers,we are interested in obtaining and utilizing the most scientifically accurate numbers
available as the basis for design of our facilities. After reviewing the data and listening to the discussions of
experts in the fields of hydrology and statistics,I feel that a model 2 hour storm of 3.67 inches is the most
reasonable estimate of a storm with a one percent chance of occurrence in any given year. I believe that with
this rainfall depth we can make the best estimations of the true frequency of occurrence of various storms.
Larimer County, through an Intergovernmental Agreement,has adopted the City's rainfall intensities for areas
within the Growth management Area of the City.We are in the process of designing a stormwater project in the
West Vine Basin.The project will benefit low/moderate income residents.The project is being partially funded
by the Community Development Block Grant Program. The project is projected to cost around $725,000 for a'/z
mile reach with residual 50 year flooding in some areas(based on the 3.67 inch rainfall). While the County
strives to provide 100-year protection,our funding level will not allow us to meet that standard.In this case,we
have chosen to use a more economical 50 year design for the improvements, and use floodproofing techniques to
reduce the frequency of flooding of residences in areas of residual flooding to 100 year.It is the County's desire
to continue to enforce the City's Stormwater Management practices throughout the Growth Management Area.
h:%westvinekityw=ci1.d=
I would like to express my thanks for the efforts the City of Fort Collins Utility Department has put forth in
examining the rainfall intensities used by the City. Susan Hayes has done an excellent job heading up the task
force. She has managed to bring many different agencies together to study this matter.
i;1y
URex A. Bums,P.E.
Larimer County Drainage Engineer
cc: Bob Smith, Water Utilities
Susan Hayes Water Utilities
h:\wesrvine\citycoonciLdm Page 2 of 2
Utility Services
Water Board
it
Wy of Fort Collins
MEMORANDUM
DATE: February 10, 1999
TO: Mayor Ann Azari and Council Members
John F. Fischbach, City Manager yM
FROM: Paul Clopper, Chair '
City of Fort Collins Wa oard
RE: Recommendation to Adopt Higher 100-year Rainfall
On behalf of the Water Board I would like to reiterate the Board's recommendation to
increase the 100-year design rainfall to 3.67 inches over a two-hour period.
The Board discussed this issue at their September and October, 1998 meetings.
Subsequent to the Council's Study Session, the Board also discussed and reconfirmed
its position at their January 1999 meeting.
The Board would like to make these points as Council considers the decision:
The decision was not unanimous. The Board voted 7-1 to recommend the
3.67 inches.
This is not an absolute science. The 3.67" value is based on sound rationale
using a regional (Front Range) approach.
Considering the magnitude of the 1997 storm, the current drainage system,
designed with a 2.89" design storm, performed remarkably well.
The one member who recommended the 4.37 inches believes the City should
"err on the conservative side" when adopting a higher rainfall.
In a separate but related action, the Board is also recommending that the City's Storm
Drainage Design Criteria be modified to include a requirement that the July 1997 event
(referred to as the Maximum Historical Event, or MHE) be analyzed and mapped as a
"what-if" scenario. The MHE analysis would be submitted, for informational purposes
only, as part of the drainage report for new development or for master plan updates.
Staff will be preparing the details of the MHE recommendation in the coming months.
The minutes of the three Board meetings are included in the packet for your
information. If you have any questions, please feel free to contact me at
223-5556. Thank you.
Cc: Water Board
Mike Smith, Utilities General Manager
File:precip board.doc
700 Wood Street • P.O. Boa 580 • Fort Collins, CO 80522-0580 • (303) 221-6681
EXHIBIT A
WATER BOARD
MINUTES
i
EXCERPT FROM WATER BOARD MINUTES
January 28, 1999
IJPT)A E: RARMAL.L STUDY
A memo summarizing the January 12, 1999 City Council study session was included in Board
packets. Utilities Staff, members of the Precipitation Study Task Force and the Chairman of the
Water Board presented information regarding the changing of the design rainfall for the 100-year
design storm. The recommendation from staff, the Water Board and a majority opinion of the
Task Force was to change the design storm to 3.67 inches, while a minority opinion of the Task
Force was to change the rainfall amount to 4.37 inches with 5.5 inches for those areas on the west
side of the City.
Key Discussion Points at Council Session
• Council members are comfortable saying the existing criteria are inappropriate.
The values presented are estimates based on the data available. There is no one true value.
• Regardless of the new value chosen, there will be an impact on the cost of new
development and master planned drainage facilities.
Other elements of stormwater management program help protect against more extreme
events. Council would like additional information about the other activities which can
mitigate the effects of an event higher than 3.67 inches.
• They acknowledged a.group of experts worked on this and experts can disagree.
Council would like the minority opinion presented at the Council meeting.
Susan Hayes felt the discussion with City Council went well. "We were able to talk about a lot of
the technical issues," she said. Don Heyse gave the presentation for the minority opinion. Dave
Frick spoke to the majority opinion. Paul Clopper summarized the Water Board discussion and
recommendations. Nols. Hayes said it was not absolutely clear to her which way the Council will be
heading. She reiterated that they are still very much interested in the minority opinion, and would
like to hear that at the Council presentation on February 16th. "The Council was extremely
interested in knowing what kinds of measures we have in place to address what happens if we get
more than 3.67 inches of rainfall, and that will be the focus of our presentation on the l6th. We
will also be talking about safety measures with respect to design criteria, freeboard, good
education programs, new emergency response measures,Project Impact,Reverse 911, having
precip. gages and being tied in with the National Weather Service for predictions." She added that
it is getting Council to a comfort level that, even though it happens to be the lowest number they
happen to be selecting from, it is not in any way, shape or form a low number. It is definitely an
increase over what we have now and it's higher than anything used along the Front Range.
Ms. Hayes went on to say that there will be an additional Task Force meeting Friday, January
29th at 3:30 p.m. "We are getting back together to let members reconfirm their opinions that were
submitted in the original Task Force memo, and that information will be passed along to Council.
If the Water Board would like to reconfirm its decision to the Council in the form of a memo or
letter, we would be happy to accept that and put it in the Council packet," she said. She asked
Paul Clopper if he had anything to add about the work session. He said the decision about a
reconfirmation would be at the pleasure of the Board. "If it would be helpful to staff, that would
be a consideration," he added. "From a staff perspective, it doesn't hurt to have the
reconfirmation of the Board,"Ms. Hayes responded, "particularly after hearing what the Council
had to say, and what their concerns were."
Mr. Clopper said his impressions of the work session were a recognition and awareness that the
Council members expressed on the economic ramifications of the larger numbers and what that
does to our infrastructure needs, etc.He explained to the Council that, although that information
was presented to the Board, it was presented as"FYI" and didn't really enter into the Board's
deliberations on making a recommendation.
Mr. Clopper asked the Board to consider what they thought about sending a formal letter from
the Board. He asked staff what the next step is procedurally. "Staff will bring a recommendation
in the form of an ordinance to Council on February 16th for a final decision," Bob Smith stated.
"The Council wants both options to be considered,"Mike Smith noted. "But the staff
recommendation will be 3.67 inches,"Ms. Hayes emphasized.
ACTION: Motion, Second and Discussion
Joe Bergquist moved that the Board support sending a letter to Council confirming the Board's
October decision. Tom Brown seconded the motion. David Lauer still feels the Board should
reconsider the minority opinion. The vote at the October meeting was 7-1 with Mr. Lauer
favoring the minority opinion of the Precipitation Task Force. "It would be smart for us to re-
consider that vote, and it would be smart for the Council reconsider it," he asserted. "I would
rather err on that side than on the side of not being as prepared as we possibly can."Mr. Clopper
pointed out that there is a connection to what the Board will be considering later on in the
meeting. That is Dave Frick's proposal of a check flood or maximum historical event which will
be the July 1997 event.
Dave Rau pointed out that we use flat frequencies as a way of discussing amounts of rainfall, as
hydrologists, but it really isn't all that meaningful. It's just a way of communicating, e.g. a 100-
year event doesn't mean that it's going to happen once every 100 years. Statistically, based on the
data we have, there is a 1% chance of it occurring in any one year. If we wanted to provide the
maximum protection to the citizens of the City, we could, but we wouldn't have any money for
anything else. "I think that needs to be made clear to the City Council," he stressed. "That came
out very strongly in the work session that they are looking for one true number,"Ms. Hayes
2
related, "and it is very difficult to describe to them that there is no right answer. It's the best
`guesstimate' that we have based on the data we have. It will surely be wrong in 20 years just as
you could look at what we adopted 20 years ago as being incorrect," she explained. She added,
"that's why we have all the other programs in place to increase the factors of safety in case we are
wrong." "But the factors of safety are essentially related to an arbitrary number that we picked a
long time ago for a 100-year event,"Mr. Rau asserted. "Essentially it's a policy decision," Tom
Brown stated. "But it's a policy decision that's not even quantitative," Mr. Rau argued. "But it's
a policy decision,"Mr. Brown insisted, "a Council decision essentially. To look for a magic
number is just a way of avoiding a decision."Mr. Clopper said that Susan Hayes and Bob Smith
have done a tremendous job in presenting that very issue, not only to the Board but in the exact
same way to Council members at the work session. "I think it came through, warts and all, what
that science really entails. It was very fairly presented." Joe Bergquist suggested we put in the
letter to the Council that with all the data we have now, the number we selected makes a lot of
sense. "To pick a higher number would only be satisfying a small contingent on the west side," he
added, "and we don't want to do that yet, even though sometime in the future we may." "When
you look at the severity of the 1997 event, and look at what we had designed for, I think our
system performed remarkably well,"Mr. Rau observed. "I think that's one of the things to look
at."He thinks if you really got down to how much money you want to spend, that would be the
level of protection you could get.
"Is there some way to present a graph that gives the Council a feel of the relationship between the
increased rainfall and increased cost?An increase of just one inch difference could be millions of
dollars," John Morris pointed out. "If it's fairly flat, you could add inches, and it would cost only
$100,000." "It's definitely not flat. It's not necessarily even a linear relationship," Ms. Hayes
responded. "It varies by basin is my standard answer," she said. "Could we estimate how much it
would cost us to go that extra inch with some kind of general analysis?"Mr. Morris wondered.
"We don't have those numbers,"Ms. Hayes responded. Bob Smith said, "when you look at what
was presented to the Council, we have a slide that shows what the numbers look like as far as the
discharges going up." "A 50% increase in rain didn't translate into that same cost difference.
When we jumped up that 50%, the flows took ofl;"Ms. Hayes related.
Ms. Hayes went on to say that Mike Smith said it best at the work session when he pointed out
that we would have to have a much stronger statistical base for the 4.37 inches to justify the
economic impact. "If we had 300 years of data and came up with 4.37 we would be in there
saying 4.37 regardless of the cost," she said. "With only 50 years of data, it is really hard to justify
recommending 4.37. The 3.67 at least has 150 years of data. We have a lot more confidence in
that. There is still an economic impact. If we didn't want an economic impact, we would say no
change by keeping the 2.89." "Also, keep in mind that the Precipitation Task Force, as part of the
recommendation, added 4 stream gages and 5 rain gages real time," Paul Clopper said. Ms. Hayes
said there wasn't a specific number from the Task Force; they just recommended doing that.
"Those were added through the Project Impact funding." "We will begin to collect data from
different points around town in the near future, and not have to rely on the CSU gage alone,"Mr.
Clopper noted.
3
ACTION: Vote
Mr. Clopper pointed out there was a motion and a second on the floor. He asked if there was
further discussion. Bob Smith and Susan Hayes will draft a letter. Paul Clopper will review it, sign
it and forward it to the Council. George Reed said his inclination would be to keep the letter
simple. Joe Bergquist suggested that the letter confirm and stress that the Board is supportive of
what staff has done. Mr. Clopper called for the question. The Board voted unanimously in favor
of drafting a letter, including suggestions from the Board, and forward it to the Council.
4
WATER BOARD MINUTES
• October 22, 1998
3:05 p.m. - 5:15 p.m.
Ligbt and Power Training Room
700 Wood Street
CITY COUNCEL LIAISON
Chuck Wanner(not present)
WATER BOAIM CHATRMA_N
Paul Clopper- 223-5556
STAFF LIAISON
Molly Nortier-221-6681
MEMBERS PRESENT
Paul Clopper, Chair, Tom Sanders, Vice Chair, Alison Adams, David Lauer, John Moms, Robert
Ward, Joe Bergquist, Dave Rau
STAFF
Mike Smith, Wendy Williams, Dave Agee, Dennis Bode,Bob Smith, Tim Hibbard, Bob Smith,
Ellen Alward,Marsha Hilmes,Bill Switzer, Susan Hayes, Glen Schlueter,Kevin McBride, Basil
Hamden, Sue Paquette, Matt Fater, Clayton Kimi, Donnie Dustin, Beth Molenaar
GI JESTS
Don Heyse, Precipitation Task Force Representative
Jim Allen-Morley, Sear-Brown
MEIsMERS ABSENT
Tom Brown, Dave Frick, George Reed
Chair Paul Clopper opened the meeting. The following items were discussed:
Water Board Minutes
October 22, 1998
Page 2
MINUTES
David Lauer moved to accept the minutes of September 24, 1998. John Morris seconded the motion.
Joe Bergquist said the last full sentence on p. 6 should read: "Mr.Bergquist pointed out that there
is a move in the Denver region to throw out a 1935 storm like this because it was way out of line."
On p. 4, second paragraph, 4th sentence Tom Sanders said"is" should be changed to"it" and "at all"
should be deleted. The motion to accept the minutes, with the corrections, passed unanimously.
UPDATE' NORTHERN COLORADO WATER CONSERVANCY DISTRICT
Gene Schleiger, from the District, was not able to attend, so there was no report.
PRESENTATION ON STORMWATER DEVELOPMENT FEES
Dave Agee began by saying that staff is back with a proposal for the Board to review on Stormwater
development fees. Last week Mr. Agee met with the Council Finance Committee to discuss about
$31 million worth of water bonds that we are in the process of issuing. The only question they asked
is if growth is paying its own way. "I pointed out that we had just adopted new Plant Investment Fees
(PIFs), so it is."The development fees on the Stormwater side are basically the equivalent of the PIFs
on the water and sewer side.
When staff began with this, they wanted to choose a methodology that was simple, understandable
and easy to administer. "I think this proposal does that," he said. "You will recall when we did water
and wastewater PIFs, we used the equity method, and what that assumes is that existing customers
have provided equity in the system, and buying into the system is going to have to compensate them
for that built up equity."He explained that what equity consists of is your assents less your liabilities.
He provided an example to show how that works. "Let's say that Mike Smith and I decide to form
an investment partnership where each of us puts in $1000. We decide to buy and sell some
investments for a period of time. After 5 years,we decide to ask Wendy Williams to be a new partner.
Are we going to let Wendy in at $1000? Probably not if the equity that has been built up is greater
than the $2000 we have in it. If that's $6000 she is going to have to pay $3000 to get in because our
equity is each equal to $3000 if$6000 is what the investments were worth."
In the case of Stormwater, we are looking at the equity that customers have in the system. First we
take the replacement value of the improvements. We looked at all the improvements that have been
constructed in the Canal Importation Basin and inflated those to 1997 values using Engineering News
Report(ENR)factors. Then we estimated improvements that we would be building in 1998. We add
those two figures and that's the total replacement value of the $7.218 million improvements. From
that we subtract the liabilities;we have outstanding bond principal that is applicable to this basin. We
also have a small amount of borrowing from other basins, so our equity in this basin by existing
customers is $3,499,000. Currently, in that basin we have 6,797 ERUs (Equivalent Residential Unit).
That is an 8600 sq. ft. lot with a light runoff coefficient. We like to think of it as a single family
Water Board Minutes
October 22, 1998
Page 7
differences I thought about is the calculation turns out to be the same, but on the water and
wastewater side you are looking at capacity units to develop your per tap charge," Mr. Agee
explained. "With Stormwater it is not capacity per se because there really is no capacity; it's
improvements to the system. The reason the basins are broken down by ERUs is because per square
foot the improvements are the same. With water and wastewater it's not the same because you have
different size taps." "To stretch that a bit farther, the different size taps mean it's bigger," Mr. Smith
pointed out.
"Is there any link to how the fees were established before?"Mr. Bergquist asked. "We have 172 now
in Spring Creek." "For one thing, these fees were last updated five years ago. There have been a-
number of changes since then," Mr. Agee responded. "The other thing is the fees in the past were
not developed on the cost side but the revenue side. We compared revenue from new development
to revenue from existing development to see if those percentages were in sync with the amount of
developed and undeveloped land in the basin," he explained. "That's right," Bob Smith said. "That's
how you track it as basin fees are established.You go back to the master plan to find what percentage
is developed and undeveloped and compare the percentage to improvements. That gives you a base
to start as far as the basin fee. You look at your percentages to see if they are still accurate. For
example, if Spring Creek is still 60% - $40% as it was 5 years ago; if it's not, you can make an
adjustment. You also look at what is yet to be built,"M. Smith said, "and divide it up according to
the equity in the system. Those are the same discussions we had with the water and wastewater PIFs."
"Do you see how things will play out in a developers' agreement like this? Will it make things more
difficult under a city-wide concept?"Mr. Clopper asked. "For the developers who develop in a basin
now, it is relatively inexpensive. How are they going to see the city-wide plan?" "It will all be the
same, but there will be adjustments,"Mr. Smith replied. "Once that is established it will be simpler."
He acknowledged that some will get a windfall and some will "get a shock in the pocket. However,
it really shouldn't change that much."
Evergreen/Greenbriar is the most expensive. That basin is becoming built out so we see less activity
there.I don't know the stormwater fees that development pays. I think street oversizing fees might
make a difference,"John Moms observed. "It's just one element among many that isn't proportionally
compared to street oversizing," David Lauer said. "We hoped that development fees would push
development away from areas where you don't have to spend as much money," Mr. Rau commented.
Tom Sanders continues to think the basin by basin concept should be retained because of the variation
of costs among the basins. "I will vote to keep it basin by basin," he said.
ACTION: Motion and Vote
Alison Adams moved that the Board recommend the city-wide concept for Stormwater development
fees. John Moms seconded the motion. Those in favor were David Lauer, John Morris, Alison
Adams, Paul Clopper and Dave Rau. Those voting no were Tom Sanders, Robert Ward and Joe
Bergquist. The motion passed 5-3. Mr. Clopper asked Mr. Bergquist and Mr. Ward to briefly state
Water Board Minutes
October 22, 1998
Page 8
why they oppose the city-wide concept. Mr. Ward said that Dr. Sanders' argument is pretty
persuasive. It seems to me we have a situation where fairness is difficult to determine in terms of
growth paying its own way. Ifyou go one way with one fee and another with the other fees, you are
allowing both arguments to prevail," he explained. Joe Bergquist said for people who don't have
major problems the basin by basin is more fair to them. "It was made quite clear in the other
discussion that city-wide has the same problem," he said.
Mr.Lauer asked if there was any middle ground between basin by basin and city-wide that would be
considered in terms of the regionalization of various basins. Mr. Agee said that what you are really -
looking at is each parcel's contribution to the system in terms of runoff. That's why we have the
runoff coefficient as part the formula in the City Code, so City residents have less fees for the same
sized lot as you would fora business that has a parking lot. I don't know if you can look at it
regionally,but you can look at it parcel by parcel. You are still differentiating your cost that you are
allocating to someone based on the actual property's contribution to the system. "The value of going
city-wide is much easier because it would be a lot cheaper,"Dr. Sanders said. "Just because I don't
live in the Mail Creek Basin doesn't mean that I don't benefit from stormwater improvements on
Spring Creek," Mr. Rau asserted. "I don't take issue with that" Dr. Sanders replied. "There is a
benefit that our neighbors don't flood down the way, but there is also a disincentive financially when
you build your house right on the creek bed and expect the City to provide flood protection. City-
wide doesn't give an incentive to that person not to build on the creek bed," he argued. "There are
future issues coming up about discouraging development in the floodplain, With city-wide you can
build anywhere at the same cost" he insisted. "It's a completely different issue," Ms. Adams contends.
Dave Agee asked for a second recommendation on whether using the "equity buy-in' methodology
fairly distributes the cost of new development. "Should we use that method to develop fees?" "How
long do we do this?" Tom Sanders asked. "Do we update this every so often, like every 5 years?"
"How often we do that is one issue. If we get to the point of doing it as it's done with water we'll
build in, not only existing equity, but our improvement plan to set it up every 5 years," Mr Agee
replied. "This is something the former Storm Drainage Board used to look at periodically," Mr.
Clopper pointed out,"but this is the first time since we've merged that we've had a chance to discuss
it." "We would like to set it for every 5 years as it is for water and wastewater," Mr. Smith stated.
ACTION: Motion and Vote
Tom Sanders moved that the Board recommend that staff use the equity buy-in method for
calculating stormwater development fees. Joe Bergquist seconded the motion. The Board voted
unanimously for the motion.
PRECIPITATION FREQUENCY Y STUDY
Susan Hayes began by saying that at the last meeting, the Board discussed a lot of the technical
background of the Precipitation Frequency Study, so she won't be going over that again, but she will
Water Board Minutes
. October 22, 1998
Page 9
review it if anyone has questions. The purpose today is to present the final Task Force
recommendations, to review the Basin Impact results and to review the New Land Development
Impacts. She introduced guests Don Heyse, citizen representative from the Task Force and Jim Allen-
Morley from Sear-Brown Group who did the Land Impact Study. From staff Sue Paquette and Matt
Fater helped out with most of the analyses on the basin impacts.
Ms. Hayes handed out the final memorandum from the Task Force. It's nearly the same as the one
included in the packets. It has been reformatted and there is one new recommendation from the Task
Force, which was to add the installation of rain gages. "We had already intended to do that but Dave
Frick suggested we include it in the memorandum so Council gets the message that we should make
sure that happens," she said, "and that we implement a data gathering program," she added. The
second item the Board received was a refined Exhibit C Land Development Impacts. When she had
originally prepared this for the Board, she did not have the complete analysis. This replaces the
Exhibit C included in the memorandum. The thud item Ms.Hayes handed out was copies of the slides
she used for her presentation.
Rainfall Study Results
• Current City of Fort Collins 2-hour design rainfall, 2.89".
• 2-hour regional rainfall 3.67" -27% increase.
• At-gage analysis resulted in 4.37" two-hour rainfall - 5 1% increase.
• 3.67" rainfall distribution peak intensity same as 2.89" - 9 in/hr at 35 minutes.
• 4.37" rainfall distribution peak intensity 12 in/hr at 35 minutes.
Important to the presentation on Basin Impacts and Land Development Impacts was, for the 3.67"
rainfall, peak intensities did not go up; for the 4.37 they do. "That does affect the results for the
runoff remodeling," she said.
Summary of Task Force Recommendations
• The majority opinion was to adopt the regional 100-year, 2-hour design storm of 3.67".
• Adopt uniform rainfall across Fort Collins - The reasoning is included in the Task Force
Memo.
• Build additional recording rain gage sites. Start gathering data so if, in the future, we want
to adopt a pattern we can.
"Are we making this so the telemetries are at real time from the rain gages or are we just doing it for
data?"Tom Sanders asked. "The former,"Ms. Hayes replied. "We will use that as our data base as
well?'he continued. "Right, it's a dual purpose. We are designing this so we can collect information
on streamflow as well as the storm events,"Marsha Hilmes," replied.
Water Board Minutes
October 22, 1998
Page 10
"When we are talking about additional rain gage sites, how many will you be installing and whereT'
David Lauer asked. "We will be installing 7 new precipitation gages this year, six new streamflow
gages, and then tying into one existing streamflow, also tying into 4 existing precip. gages. They will
all be exactly the same telemetry for collecting data. That's our base information. We have developed
a master plan that includes many more sites across the City that we would like to implement over
time,"Ms, Hilmes explained.
One challenge the Board gave staff was to look at what kinds of basin impacts would be seen, what
kinds of impacts might be seen on the improvements proposed in our master plan, as well as the
impacts on new land development by adopting a higher rainfall, Ms. Hayes continued. She reminded
the Board that this information is not to be used to say no we shouldn't go to a higher rainfall. It's
background information so the Board knows what those impacts are. "We feel comfortable with the
3.67 as a good estimate," she said.
Preliminary Results-Basin Impacts
We had to look at the City as a whole. It was time consuming to look at every single basin because
we would have to update all our models, so we picked some representative basins. We looked at
Canal Importation, Foothills, Lower Dry Creek, Old Town, Spring Creek and West Vine Basins.
They are a good representative cross section of detained basins, undetained basins, highly urbanized
versus not so highly urbanized.
• Preliminary testing of those basins.
• Difference between 2.89" and 3.67" runoff not as significant as 2.89" and 4.37" difference,
due to peak intensity change.
• Basins without detention are not as affected by the rainfall increase as those with detention. -
The basins that have a lot of on-site detention are more severely impacted by an increase in
rainfall because those ponds begin to overtop once it goes beyond 3 inches, and that increases
the peak downstream. In basins such as Old Town where you don't have a lot of on-site
detention, we don't see a very high increase in peak flows, because there aren't any ponds
detaining and overtopping.Because that peak intensity didn't go up, that is what drives peak
flow rates.
Intensity Duration Frequency(IDF) Comparison
Ms.Hayes then projected a chart that showed the Intensity Duration Frequency(IDF) Comparison.
It is an illustration that shows how the peak intensity has not gone up. The left side of the chart goes
from 0-10 inches per hour. The purple is the old IDF curve which was approximately 9 inches per
hour,the new one is also about 9 inches per hour, a 100-year storm with a frequency of 3.67. Where
we see the increases are in the latter part of the storm. We see more intense rainfall for a longer
period of time in this storm. There is a lot of volume falling in the last hour of this storm. What drives
the peak flows is the 9 inches per hour rate for 5 minutes, and not the intensities on the lower half of
the curve. That will affect volume very greatly, but it does not affect peak flows.
Water Board Minutes
October 22, 1998
Page 11
Basin Comparisons
All the information about the various basins was included in Board packets. Ms. Hayes briefly went
through each of the representative basins, contrasting and comparing them. According to Exhibit B,
assessing the exact impact of increasing the rainfall criteria is difficult to do. Each basin is unique and
will respond differently from another basin, depending on the existing drainage facilities and those
proposed in the master plan.
Impacts of Increase in Rainfall on Master Plans
Staff looked at the Rodeo Arena Pond in the Canal Importation Basin on the western edge of the
basin upstream of Overland Trail.Under our current criteria we would need about 34 acre feet which
is an 11 ac-ft increase up to 45 when we increase our rainfall. In this case it's a fairly minor increase
in cost because land is fairly inexpensive, and it's virtually undeveloped. There is considerable open
space and it is not restricted by structures.We have utility conflicts and land use conflicts, but it's not
houses built around the edge.In this case it's a fairly minor impact. When you go to the Taft Hill Rd.
Pond, however, there was a significant increase in volume from 69 ac-ft up to 112 ac-ft. This
increased the cost by 35%. The bulk of this cost was in land cost. We would be ponding onto the land
where there are houses, which means we would have to buy them out. Improved land is a lot more
expensive than less improved land.
. The summary of how this increase will impact our master plans is:
• We can control the peaks in our channels where we have already built facilities by adding
more detention.
• Detention costs will vary depending on the site.If it's an existing pond with structures around
it, it will be more expensive. If it's a pond that has not yet been built, we have the opportunity
to design it in and get land cheaper.
Until we have done master plan updates for all of our basins, we won't know the exact impacts, Ms.
Hayes concluded.
Land Development Impacts
The Water Board also asked staff to examine the impacts on land development. "We hired Sear
Brown Group, who are experts in this field," she began. They do modeling and a lot of land
development in town. They took a couple of sites in the southern portion of the City and asked,
"How would we have designed these sites based on 3.67 inches?" Staff also looked at some of the
generalizations you can make about impacts on land development:
• Off-site flows may increase. Right now new development has to pass the 100-year existing
flows through their site. If they have detention ponds upstream of them that will now be
overtopping,they will have to handle that flow through their site. They don't have to detain
it on their site but they need to have a spillway they can pass it through.
Water Board Minutes
October 22, 1998
Page 12
• Street capacity is not expected to change. That's because the peak intensity has not gone up.
What they calculate for the localized flows in the street, in storm sewers and inlets, won't
change drastically.
• Flood plain development - If under current conditions, before we can go back and retrofit,
we have ponds overtopping and more flow downstream, we won't have accurate BFEs for
Ms. Hilmes to regulate to. We will have to run models and get that updated.
• Implementation Issues - How do we retrofit existing ponds owned and maintained by
homeowners associations? Do we say they are allowed to overtop or do we have some
methodology to help them retrofit their ponds and does the City take that on? Again, as we
do master plan reviews,we can evaluate the impact of each pond to determine if it should be
retrofitted. There will also be Larimer County coordination issues. Within the Urban Growth
Area (UGA), Larimer County will be adopting this, but they will have problems with what
they tell somebody in Loveland or Laporte because we are going to have different rainfall
criteria than the rest of Larimer County. They have told us they don't have a problem with
the City implementing it within the UGA, and even when basins go outside their UGA they
would agree to doing the master plans with the increased rainfall.
Land Development Impacts Pertaining to Detention Ponds
For detention ponds, the biggest impact is going to be an increase in volumes.
• Release rates for ponds may not be impacted. - We found there is not a typical site that
absolutely releases at an historic rate as required. Other issues are involved which control the
release rate.
• Increased volume for ponds.
• Ponds may be deeper. -Need to evaluate criteria on how deep they are allowed to be.
• Additional earthwork costs either for berming or excavation.
• Potentially more land needed for ponds. - This would increase the cost for development.
• Possible increase in inlet pipe size due to higher ponding elevation.
It is all very dependent on the site. The two sites that Sear Brown looked at were Ridgewood Hills
and Stetson Creek.
Water Board Minutes
October 22, 1998
Page 13
Summary of Impact Costs -Ridgewood Hills First and Second Filings
• Ridgewood Hills is in the Fossil Creek Basin, on Trilby Rd. between College and Shields.
• Total Site acreage= 122.43 acres, 215 lots and 7 detention ponds.
• Total cost increase for Development= $25,200
• Total Cost Increase per Lot= $117
• Using Lot price of$4.00/sq. foot.
Ms.Hayes referred to a graph. Increases in pond values were shown in red.There were very minor
increases, mainly into the freeboard that was there. In talking to Sear Brown, the information we
received was, "if you know this up front and you can lay out your site to include this additional
volume, there shouldn't be much of an impact at all on land. It is difficult if you have to retrofit, but
usually a developer can shift the lot somewhat, etc.
Summary of Impact Costs - Stetson Creek First Filing
The second site that was looked at was actually an existing site. It has already been built out and the
ponds are already there, so you can almost look at it from a retrofit standpoint.
Stetson Creek is in the McClellands Basin, south of Harmony along Timberline Rd.
• Total Site Acreage= 18.29, 44 lots, 2 detention ponds - Only one pond was impacted by an
increase in volume. They would have to do some berming along the road and berming along
the channel to get the volume they needed. They would have needed to regrade their pond
to make sure the lots were up high enough. In this case the cost was higher because there are
far fewer lots.
• Total Cost Increase for Development= $7,032
• Total Cost per Lot = $160
• Using Lot price of$4.00/sq. Ft.
Ms. Hayes asked Mr. Morley if this had been laid out from the beginning would it have been even
less. "Yes, considering that the detention ponds are used as green space areas, and there is always that
requirement anyway. You generally aren't heavily impacting lots anyway. If you raised the water from
a foot to a foot and a half,that's not going to go out on the adjacent lot boundaries. "It also depends
on the topography and the type of site. If you had a flat site, it would be more problematic than a
rolling site," he added. Sear Brown also found that the release rate had an impact. For example,
Stetson Creek had a much larger outflow rate, so it had less volume increase, whereas Ridgewood
had a far more restricted rate per acre and had a larger volume increase for the detention pond. "You.
have to view this as just two examples and not as a general rule," he cautioned.
"I think because the peak intensity is not increasing, some of the highly structural measures that cost
a lot, such as storm sewers and inlets, aren't being impacted,"Ms. Hayes remarked. "There is no cost
increase, which is helpful." She also pointed out that Mr. Morley's comment about variable release
rates is typical now, even under the 2.89".
Water Board Minutes
October 22, 1998
Page 14
Ms.Hayes said there were no good commercial sites to look at because typically they aren't modeled
with SWMM and they use all rational methods to the ponds. "As I mentioned at our last meeting, we
are going to have to deal with a better method for sizing the ponds on commercial sites because right
now it won't take into account all the volume at the end of a storm."
Ms.Hayes ended her presentation. She asked for questions and comments. Paul Clopper asked if Ms.
Hayes was requesting a formal recommendation from the Board of 3.67" "Yes, I am," she replied.
Mr. Clopper asked the people who were on the Precipitation Frequency Study Task Force to identify
themselves in case there were specific questions directed to them. "Is there any kind of formal.
presentation from the minority opinion?"Mr. Clopper asked. "Only in the letter, but Don Heyse is
here if you would like him to explain that opinion,"Ms. Hayes said.
ACTION: MOTION,DISCUSSION AND VOTE
Robert Ward moved that the Board recommend approval of the Task Force recommendations.
Alison Adams seconded the motion.
Minority Opinion
Mr. Clopper asked Mr. Heyse to present the minority opinion of the Task Force before a vote was
taken.Mr.Heyse said the 1997 storm actually covered a large area. On the west side of town there
were actually three separate storms over a period of 31 hours. There were more than 12 inches in
three separate spots over an 8 mile stretch of the western part of town. "Unfortunately, we didn't
have very good data for most of that area,"he said. "When we looked at the historical data, there was
virtually no data from that area. It leaves you wondering what happened in past history in this area.
A minority of the group felt that this storm was not that unusual. We looked at the campus volume
in 1902, 1938 and 1951 when the campus experienced severe flooding. In 1938 and 1951 there were
reports of very heavy rainfall on the west side of town. Our feeling was there was a tendency for
much heavier rainfall on the west side of town that was not being picked up by the CSU rain gage.
That is why the minority opinion group advocated two points:
(1) A differentiation from west to east and on the west it would be higher than 4.37.
(2) Use a higher value of 4.37" for the entire City
Joe Bergquist asked if the voting included the entire package. "I think we separated into two groups:
the majority felt they wanted to use the town as a unit, and a minority felt that the west and east parts
of town should be separate,"Mr. Heyse replied. "If you didn't get your separation, you wanted a
higher rainfall amount?"Mr.Bergquist asked. "There's a good argument for that," Mr. Heyse said.
"We shouldn't neglect other parts of town. In fact Marsha Hilmes raised that point. We might want
to raise it again that statistically one of these intense storms happening on the west side, could hit the
east side as well. We would not want to leave them unprotected." "We should also take into account
what happened this last time with the runoff," David Lauer pointed out. "The impact of the runoff
Water Board Minutes
• October 22, 1998
Page 15
was just as great on the east side and perhaps greater, was it not?" "That's the way the system works
though,"David Rau said. "If you have a storm on the west side of town,you have to get it all the way
through town. You have to have your main facilities able to carry the flow."Ms. Hayes clarified the
minority opinion.It wasn't expressed that if they couldn't get a differentiation from west to east their
fall-back was the higher rainfall; it was both. "They wanted both the higher rainfall plus a
differentiation from west to east and on the west side it would be higher than 4.37 inches."
"Did you look at the cost for the 4.37"?"Mr.Bergquist asked. "No, we didn't look at the cost," Ms.
Hayes replied. "You didn't raise the cost after you made a decision, did you?" Mr. Rau asked. "The.
impacts of the 4.37" in terms of total runoff are included in the tables," Ms. Hayes pointed out. "You
can tell the peaks are going to be significantly higher and the volumes are going to be significantly
higher with 4.37 inches," she stressed, "as a result, the costs would be significantly higher."
Tom Sanders commented that he attended only about half of the Task Force meetings so he felt he
wasn't as informed as he might have been had he been at every meeting. "I think most of us agree that
there probably is a difference between the west and east parts of town. The data to justify that was
insufficient. The best records we have are the CSU data which did not pick up the heavy parts of the
storm. It's the age old problem of not enough data. For the discussions I was involved in, there were
some legitimate arguments, but we just didn't have enough justification and data to back us up," he
explained. He also pointed out that we don't want to be too far away from what the regional analysis
will be for the foothills. "I think we all agree that the foothills area is the toughest for estimating
rainfalls."
David Lauer asked about the dates and instances when the campus was impacted. "The campus was
flooded,"Mr. Heyse replied. John Moms pointed out that that could be independent of the amount
of rain because of the infrastructure that has been constructed and the growth patterns. "Instead of
doing it all together like the City does, we broke it all up," he said. "I had a flood plan for several
years and I never had any support until the 1997 storm." he stressed.
"We realize that we will never have all the rain gages we need because we will never know where rain
will fall," Alison Adams said. "At least with the first phase of rain gage installation, do you think there
will be enough on the west side so we can start collecting data, so in 5-6 years we might have some
information to show there is some disparity between west and east? Maybe then we would have a
better justification for dividing the City if that looks like what needs to happen." Marsha Hilmes
explained that the initial gages are primarily on the west side. The reasoning behind it is that more
often the rain goes from west to east. The main focus behind it was early warning. That's why the
focus is there. There are some on Collindale Golf Course; they are located in many different places."
ACTION: Vote
Paul Clopper called for the question."All those in favor of adopting the Task Force recommendation
to accept 3.67" per 2-hour 100-year rainfall amount, raise your hands," he asked. David Lauer was
Water Board Minutes
October 22, 1998
Page 16
opposed. The motion passed 7-1. Mr. Lauer explained that he tends to agree with the minority
opinion that there appears to be a history of more intense and more volume of rainfall on the west
side,which would make the rainfall amount more in the range of 4.37".
Robert Ward commented that what's been done here has raised a lot of questions. "Why did NOAA
throw away 40 years of data? That shocked me. It appears there is an opportunity to do a lot more
work in this area, and there is with NOAA working on it. I don't think it's the type of thing where
it ends with this vote," he stated. "I would like to think that we would continue to try to look at it,
even to the point of involving graduate students at the University to do research in this area.
However, with the information we have now, I feel comfortable with the recommendation, but I
would also like to be in a position to revisit this as new information comes in," he concluded.
"I would eventually like to divide the town into different regions," Joe Bergquist stated.
"I think we need a lot more real data, but I think we are moving in the right direction," Ms. Adams
concluded.
STAEF REPORTS
Treated Water Production Summary
Dennis Bode reported that for September the City used 3,321 ac-ft of treated water which is about
94 % of what was projected. This was the warmest September on record . "How do you arrive at
that;the wannest temperature for the 30 days?"Tom Sanders asked. "You take the high plus the low
and divide by two," Beth Molenaar explained.Nolan Doesken does the calculations, she said. Mr.
Bode referred to the graph that showed the projected and actual water demand. "It's always
interesting to see the different patterns we've had,"Mr. Bode said.
Robert Ward was curious about the 93%water projection versus actual demand. "The actual demand
is consistently tracking under the average projected," he noticed. "I know it's jumping around a lot,
but I'm just curious, if you were to look over time, are you seeing any impact from our water
conservation efforts?""I think it's changing primarily because of the meters that are being installed,"
Mr. Bode replied. "Do you try to build that into your projections?" Mr. Ward continued. "That's
right," Mr. Bode answered. "You notice at the beginning of the year it is tracking almost what the
average projected was," David Lauer pointed out. "Then we start filling that gap beginning in April
and May,"he added. "That's when the meters kick in,"Mr. Ward said. "It's not quite that precise,"
Mr.Bode stated. "If you look along the line about 2/3 of the way down where it says ratio, you get
a lot of variance because of the weather. "I know that, but this curve seems to smooth that out,"Mr.
Ward asserted.He reiterated that he was looking at how the actual is consistently slightly under the
average that staff projected. He was wondering if the conservation programs are beginning to show
up in data like that. "I think they are, but the difference you see there is probably not because of
WATER BOARD MINUTES
September 24, 1998
3:15 - 5:10 p.m.
Ligbt and Power Training Room
700 Wood Street
CITY COTTNCrLLIAISON
Chuck Wanner(not present)
WATER BOARD CHAIRMAN
Paul Clopper- 223-5556
STAFF LIAISON
Molly Nortier - 221-6681
MEMBERS PRESENT
Paul Clopper, Chair, Joe Bergquist, George Reed, David Lauer, Dave Frick, John Morris, Tom
Sanders, Dave Rau, Tom Brown, Robert Ward
STAFF
Mike Smith, Wendy Williams,Dennis Bode, Dave Agee, Gale McGaha Miller, Bob Smith, Susan
Hayes, Sue Paquette,Matt Fater, Molly Nortier
QUEST
Don Heyse, Precipitation Task Force Representative
MEMBERS ABSENT
Alison Adams, Vice Chair
Chair Paul Clopper opened the meeting. The following items were discussed:
MINI ITRS
George Reed moved that the minutes of July 23, 1998 be approved as distributed. David Lauer
seconded the motion.Mr.Reed pointed out on p. 5 under Wastewater Utility, second paragraph, first
sentence that gallons should be gallon. The Board unanimously approved the minutes with that
correction.
Water Board Minutes
September 24, 1998
Page 2
UPDATE• NORTHERN COLORADO WATER CONSERVANCY DISTRICT
Gene Schlciger was unable to attend the meeting, so there was no update.
Dennis Bode distributed the revised NCWCD Carryover Policy passed by the District Board. The
policy becomes effective on October 31, 1998. The major change is that the amount of water carried
over by a water user will be limited to 20%of the water that would have been available from a 100%
quota. If the user wishes to carry over water from one year to the next there will be a charge. The
user will be able to use the water just like any other CBT water during the year. The biggest effect
the policy should have is raising the quota each year. "It should be a little higher because the
carryover water will not be considered in setting the quota." "Overall does staff think the change is
good as far as the Fort Collins is concerned?"Paul Clopper asked. "We are not too happy about the
fee part of it,"Mr.Bode replied,"so the City probably will not carry over a lot unless we really need
it."Nevertheless he thinks the policy should work. "Is it going to adversely affect our operations?"
Dave Frick asked. "I don't think so,"Mr. Bode answered.
FLECTION OF OFFICERS
Chair Paul Clopper called for the annual election of the chair and vice chair of the Water Board. He
asked for nominations for chair. He was re-elected as chair by acclamation. Alison Adams, the
current vice chair and Tom Sanders were nominated for vice chair. Tom Sanders was elected as the
new vice chair.
PRECIPITATION FREQUENCY STUDY
Board members received a portion of the Precipitation Frequency Study in their packets. Susan Hayes
began by asking for comments and questions from the Board on the draft report. She said that staff
would not be asking for a recommendation today. "There is a lot of information to absorb and
understand.Before the Task Force's recommendation comes to you it would probably be helpful to
at least see what we have," she explained.
Ms. Hayes introduced some of the task force members, three of whom were on the Water Board:
Dave Frick, John Moms and Tom Sanders.Don Heyse, a citizen member, was a visitor to the Board.
Ms. Hayes continued by emphasizing the complexity of the project. She wanted the Board to
appreciate what the group has been doing and why it is taking so long. "It has not been an easy
process," she related.
She went over the summary of the highlights of the report which appeared in the memo introducing
the report.
Water Board Minutes
. September 24, 1998
Page 3
• The current analysis does show an increase in the rainfall amount as follows:
(1) Current rainfall amount: 2.89" over two hours
(2) Proposed regional analysis amount: 3.67" over two hours
(3) At-gage analysis (CSU site) rainfall amount: 4.37" over two hours
Ms. Hayes emphasized that the 1997 storm greatly impacted the analysis.
• The majority of the Task Force currently favors the regional analysis value of 3.67'; however,
additional analysis of data trends is being conducted to get a better comfort level with the
results.
• There was not a consensus on the Task Force to use the regional value. Some favored the
higher, at-gage, value of 4.37".When the final report is submitted, it will include a letter from
the Task Force with the final recommendation as well as any minority opinions.
• This analysis does not necessarily mimic the way NOAA will conduct their analysis when
NOAA Atlas II is updated in the next several years. Their numbers will most likely be lower;
however, the Task Force felt our use of the data for the regional analysis was most
appropriate for Fort Collins.
• Impact on the design of drainage facilities due to an increase in rainfall is being studied. Some
preliminary results will be discussed at the meeting today.
Ms.Hayes emphasized that this analysis was done on limited data. She used a graphic to explain what
the data problems have been. (See Figure 1) The first NOAA study used data from 1900 up to the
late 1960s. "When we embarked on this study,we thought we had another 30 years of data we could
incorporate, plus we had a huge storm in 1997. This would affect our results, plus we would have
nearly 100 years of data. That's a good thing to have when you are trying to determine what your 100
year storm is.Unfortunately, it didn't turn out that way. What we have is 100 years of daily data. This
can be converted to 24 hour data, but we don't design for a 24 hour storm, so this data isn't that
useful to us."
"What we do have, and what we need for our study, is hourly data," Ms. Hayes continued. "What's
available right now, that is digitized and useful for analysis, is 1948 to 1997. There is data available
between 1940 and 1948,but it isn't digitized, so wasn't used for this analysis. We don't know why,
but NOAA no longer has the information from 1900 to 1940, so we lost 40 years of data. Even
though we gained 30 years of data, we lost 40. The revised rainfall numbers represent 1948 to 1997
data only. The future NOAA study, they have already informed us, will be using their current
digitized available data which is 1948-1997. They will not be going back to try to reclaim the early
data for the hourly events." Mr.Rau asked if they intend to use the undigitized data. "They haven't
indicated they are," Ms. Hayes responded.
Water Board Minutes
September 24, 1998
Page 4
"As a result,we don't have any more confidence in the numbers we are getting, based on the last 50
years of data, than we had in the first 70 years of data.Due to the concern over the loss of the first
half century of data,we decided to look at the rainfall trends at 5 gages. How did the first half of the
century compare to the last half of the century?We did a very simplistic analysis."
To date, we have looked at Boulder, Waterdale which is Loveland, and Fort Collins. It's rather
inconclusive. Fort Collins went up significantly comparing the latter half of the century to the first
half. There is almost an inch of difference in total rainfall.For Boulder is has hardly changed at all.
For Loveland it has actually been a little drier the last half of the century.
When we started talking about using a regional analysis versus an at-gage analysis, the data that is
available really makes a crucial difference. That's why there is a difference between 4.37" and 3.67".
Boulder and Longmont are having an effect on the numbers because they have a drier record than
Fort Collins. Certainly there are reasons to use at-gage because it represents what's actually
happening here, and it did include the 1997 storm. We felt strongly that we should use the actual
event recorded at the gage. But, because we have lost the first half century of data, it doesn't
recognize that the first half was drier. The 3.67" value may be conservative;we don't know. It is still
limited by the data available.
As of the last Task Force meeting, the majority of the members present were leaning towards the
regional analysis because of the lack of data available. She acknowledged there is a split on the
committee; there is a minority point of view. That minority view will come to the Board in letter
format from the Task Force.
She went on to explain how the 1997 storm affects the information. When you only have 50 years
of data and you have this huge storm that far exceeds anything else that is plotted, it really brings the
curve up. As we add more data over the years and the decades, the value will most likely go down.
"We won't know in time to implement our master plans," she stressed. "We have a 15-year build-out
schedule now. We are in a position where we can't wait to collect additional data."
There is going to be a future NOAA study. It will include Colorado and surrounding states. "They
won't use the same region we have chosen. Our Task Force felt comfortable with the region we
chose, whereas NOAA will likely take too large a region that would not be representative of Fort
Collins. NOAA will only use the 1948-1997 data; and it will take several years to complete. Our
consultant estimates 3-6 years. Their final rainfall value maybe lower because they will use a different
region. We don't think that's appropriate for us because we don't agree with their region. We also
think our method, the General Extreme Value method is more representative of our data."
Ms. Hayes went on to explain the effects on our facilities. One thing that total rainfall does not give
us is the distribution of that rainfall over time. When you think about going from 2.89" to 3.67" in
2 hours, what you are going to see is a more uniform rate of rainfall over that two hours. "It's all
Water Board Minutes
September 24, 1998
Page 5
volume, basically. Our peak rainfall intensity did not go up; we are just seeing it rain harder for
longer."
Matt Fater put together some distributions looking at early peaks, mid-peaks and late peaks and ran
it through some of the models. "What we are finding is for basins that don't have on-site detention,
such as Old Town, our peaks aren't increasing much at all. For the basins that have on-site detention,
such as Spring Creek and Canal Importation, those ponds are over-topping due to the increased
volume, and that's what is causing our increase in peak. We will be able to solve that problem by
increasing volume in the basin. How that will translate into dollars will be dependent on the basin, the
land value,whether or not we have room to expand, or if it's surrounded by houses. It will be unique
to each basin how it will pan out financially,"she explained.
Mike Smith asked if the numbers Matt Fater ran are based on 3.67"or 4.37". On 3.67", Ms. Hayes
replied. "Is the minority report going to based on the 4.37?"Mr. Smith continued. "That's correct,"
she replied, "and for the reason, that many of us agree with, that Fort Collins seems to be unique
along the Front Range.We appear to be in a preferred storm track and we get some of the heavier
storms and the data shows that. I think the concern with the rest of the committee is we have lost
50 years of data," she said.
"As a result of all this, are we going to get new IDF curves for the City and do 100 down to two
years? Would that take the place ofNOAA IDF curves we have been using?" Tom Sanders asked.
"Yes, and actually they may not change much."Ms. Hayes answered.
Dave Rau asked if Stormwater is going to upgrade all of our facilities to meet the new criteria. "I
would say not necessarily,"Ms. Hayes replied. "One, we won't have to in some cases," she added
"What about the ponds that are undersized?" Mr. Rau continued. "I think if we are going to add
volume,the benefits need to outweigh costs.We would have to look at an individual site to see if it's
worthwhile doing that. If it's fairly simple to increase the volume of a pond, we could incorporate that
into our master plan update,"Ms. Hayes responded. She added that they would look at the basin as
a whole and determine the cost/benefit ratio. "One individual project may not be cost effective, but
combining it with all the projects in the basin, the benefits may outweigh the costs."
Dave Frick pointed out there are a lot of ponds that were done early on that are already undersized
even with the existing criteria. "It's going to add to it but it's not going to be the sole reason that the
pond could over top."
"Are you expecting to come back next month with a formal recommendation to the Board to
approve, modify or do whatever we need to do with it?"Mr. Rau asked. "Yes, the next step will be
to pull our Task Force together for one last time to draft a recommendation.We will prepare more
detailed information about potential cost impacts etc., and expect to be back next month with a final
staff recommendation."Ms. Hayes said. "At this point I don't expect any delays," she added.
Water Board Minutes
September 24, 1998
Page 6
"One thing we need to keep in mind as we are considering this," Mr. Frick began, "is the fact that we
are going to end up with higher numbers than Loveland and Larimer County. When we do basin
studies with Larimer County,we will have inconsistencies in our data. We need to consider how we
are going to interact regionally when we look at our basins. There are going to be some differences,
and we are going to have to deal with that," he concluded. "It's going to become more of an issue
as Larimer County gets more involved in stormwater issues," Mr. Rau noted. "West Vine is an
example; it's part in the City and part out. Do we use County numbers in the County and City
numbers in the City?"Mr. Frick wondered.
"Another point that has been brought up is how this is going to affect the Poudre River," Ms. Hayes
said."We are not anticipating a change at all in the Poudre River because it is based on 100 years of
river gage data, so we feel pretty comfortable with those numbers," she related. "Plus, it's driven by
the upstream basin which is not along the Front Range.We don't expect that to change. It will change
the local inflows to the River,but those flows don't drive the floodplain on the River," she explained.
"Thus rainfall study is a$'ecting the development in the floodplain issue because people want to know
how our floodplains will change before we make a decision on how we should restrict them. With the
numbers we're seeing, they may not change dramatically if we take the tack that we are going to
increase the detention volume if we need to. With the Poudre River we can at least begin that
discussion without worrying about adopting this."
"There was a sense of controversy that came through in reading what was included in the packets,"
Mr. Clopper said. "I would like to hear from you or any of the other Task Force members, if there
were pro and con issues that didn't relate to the differences in the data sets and statistical issues. Were
there other issues that added to the sense of controversy?" "Yes, there was a discussion about the
methodology we used, but I think we are all in agreement that the General Extreme Value method
is most appropriate," Ms. Hayes responded. "There was also a discussion about how to handle the
1997 storrn, and whether we should use a known value of 10 inches at the center of the storm versus
what was recorded at the gage. It was determined we use what was recorded at the gage. There
certainly was the opportunity to reduce that value and treat it as an outline, but we chose not to," she
concluded.
"There is no question that it is a valid piece of data," Tom Brown began, "but it may really have a
large effect on mean values; larger than it should if you are really going to characterize it for the long
term. We have all the rough estimates, but the general impression is it's a very unusual storm," Mr.
Brown asserted, "and when you put it in with 50 years of data, it carries a lot of weight. I think you
need to be able to justify putting in such an extreme value in 50 years of data," he stressed. "I think
that's why the recommendation is to go to the regional analysis rather than 'at gage.' That does
mitigate the effect of the '97 stotm," Ms. Hayes explained.
Mr.Bergquist pointed out that there is a move in the Denver region to throw out a storm like this in
1935 because it was way out of line."They have isolated the storm to be an extreme event and tossed
Water Board Minutes
September 24, 1999
Page 7
it out.""I think the compromise that's been done is good; not to completely ignore the storm but to
mitigate its effect by using other data. We would have a hard time throwing it out," Ms. Hayes
responded.
"Would you explain what you mean by bringing in other data; i.e. Loveland and Boulder?" Mr.
Brown asked."We did an at-site analysis and a regional analysis. The regional analysis incorporated
Boulder, Longmont and Fort Collins data. We chose Front Range communities that had similar
characteristics to Fort Collins.Loveland didn't actually have enough hourly data,"Ms. Hayes replied.
"How about Denver?"Mr. Brown asked. "We did not choose Denver because we didn't feel it was
represented as a community right up against the foothills, and that's why we didn't choose Greeley,"
Ms. Hayes responded. "The principle here is if you use more data, the value will go down," Mr.
Brown clarified. "The effect of the '97 storm would be less,"Mr. Frick added.
"The other aspect of the study that we haven't talked about is the qualitative rainfall pattern part,"
Ms. Hayes continued. "We discussed the fact that it may rain more and harder on the west side of
town than it does on the east side. Currently the result is, yes it does appear to, but it is not
statistically significant enough to adopt a different criteria. Our Task Force has never approached the
subject.We have been so engrossed in the statistical analysis. That will be the one final decision we
have to make in our next meeting and come back to you with a recommendation," she explained.
George Reed said what would help him is a way to summarize the consequences of one decision or
the other. "Understand that I'm coming from the side of financial reasons, because we had
Stormwater Committee members who were really strong on holding the line on our capital fees, etc.
For many years we tended to, it appears,underestimate. What are the consequences of overestimating
in this case?I would like just an executive summary stating the reasons for going to 'this' number or
the main reasons for going to 'that' number,"he suggested.
"You had a consultant looking at specific development to determine what infrastructure up scaling
needed to be done, if any,by virtue of going to these higher numbers," Mr. Clopper began. "Will that
also be included for next month?""Yes, it will,"Ms.Hayes said. "Do you have any feel for what that
is?" Mr. Rau asked. "What it will most likely do is increase the detention volume they need."Ms.
Hayes replied. "I mean bottom line for costs,"Mr. Rau added. "It's hard to know. It will depend on
each site,Ms.Hayes replied. 'Each site will be unique. What we were concerned about is if this will
increase the peak flows coming off the site into the ponds. Would we have to put in more storm
sewers, bigger inlets, etc.?If you believe the preliminary results, it doesn't look like it will; it's just
more volume they will have to retain on their site," she explained. "That's going to take more land
and that isn't cheap," she added.
Mr. Brown further clarified his observations on the minority opinion issue. "I would like to point out
that, in addition to the minority opinion that would have you use a larger estimate, there is a
legitimate minority opinion that would have you throw out the '97 storm all together, like Joe
Water Board M•tnutes
September 24, 1998
Page 8
Bergquist said has been considered in other places.I'm not suggesting that but I don't think it's more
extreme than it is to overemphasize that"
Ms.Hayes concluded that the Board will be making a decision about this item at the October meeting
when she will have further information for them to consider.
UPDATE CITY RrTDE STORTv^'IATER FINANCING PLAN - CITY COUNCIL FIRST
BEADJNQi
Paul Clopper reported that last week the City Council passed, on first reading, by a vote of 6-0,
approval of the Water Board's recommendation made two months ago, to adopt City-wide financing,
as opposed to the basin-by-basin approach, to bring the capital stotmtwater improvements schedule
forward to a 15-year build-out period."There was a concern about development paying its fair share
and what proportion of the capital improvements that we are now accelerating, are going to be borne
by developers versus existing citizens," he said. "Several of the Council members echo a concern on
that issue. That is something staff will be looking at and bringing to the Board.
Dave Agee said staffis hoping to come to the Board in October with a proposal relating to that issue.
He emphasized that the Stormwater Utility does charge development fees and those are updated
periodically, and it is time to update them again. "The methodology we use is to review the program
to determine if development has paid its own way." Staff does that by comparing the ratio of
undeveloped land at the time a master plan was adopted, to the total land in a particular basin. "We
compare that to fees generated for new development, both from monthly capital and development
fees and the percentage of revenue that generates over time to see if development is in fact paying
its own way," he explained. "After looking at that we would present some adjustments, if they are
needed, that we could use in each basin."
Mr. Agee provided a Stormwater New Development Fee Work Plan for September 21, 1998 through
January 19, 1999. "According to the schedule, might you have a summary by the next meeting that
would describe how that is done for Stormwater capital improvement projects?" Mr. Clopper asked.
He mentioned that former Water Board members, prior to combining the Storm Drainage and Water
Boards, have not heard that. "We hope we can get this done within this time frame,"Mr. Agee said.
"Our plan is to bring it back to the Board," he said.
Dave Frick wondered if there was any logic in considering a uniform development fee rather than
basin-by-basin.Mr. Agee replied that with different topography in different basins, it makes sense that
those fees would not be uniform city-wide. "The City attorney was extremely nervous about that
versus what we just did. They want to make a really close tie to development impact fees," Mike
Smith related. "When we suggested early on that we could do that, they said they were okay as far
as monthly capital and O&M fees were concerned,but the impact fees made them nervous. They like
that close tie, but their concern doesn't mean they wouldn't do it,"Mr. Smith added. "Aren't some
PRECIPITATION FREQUENCY ANALYSIS
SUMMARY OF AVAILABLE DATA FOR ANALYSIS
FIGURE 1
Year 1900 10 20 30 40 50 60 70 80 90 2000
Current
m mmENOAA 2
Now XI/
Study
Daily
Data Must be converted to 24-hr. data.
010/m//;
v
Hourly Not avai able Data needed for our study.
Data
o rn
z •� i
.o
Future
NOAA NOAA will use available digitized data.
Study
EXHIBIT B
TASK FORCE
• Precipitation Task Force Members
Summary Resumes
Dr. Duane Boes is a professor in the Statistics Department at Colorado State University. His
expertise is in stochastic modeling and time series analysis of geophysical phenomena, statistical
inference, reservoir and storage theory.
Tom Browning is a Professional Engineer who works for the Flood Control and Floodplain
Management Section of the Colorado Water Conservation Board (CWCB). The CWCB is a
state agency that must approve the technical accuracy of local floodplain studies before a
community can regulate to them. The CWCB also administers loan and grant programs that the
City participates in.
Rex Burns is a Professional Engineer with Latimer County in Fort Collins, Colorado. He has
over 25 years of experience with Larimer County pertaining to hydrology and stormwater
engineering. He is a Project Engineer and Floodplain Administrator with the Floodplain and
Improvement Districts section. The City and County work closely together in the basin master
plan studies and the implementation of improvements identified in the master plans. Common
drainage criteria are also applied within the Urban Growth Area.
Bob Davidson works at Hach Chemical Company. He is a member of the City of Fort Collins
Planning and Zoning Board.
M Lisa Dunn is a Project Engineer with Larimer County in Fort Collins, Colorado in the
Floodplain and Improvement Districts section. She has a Civil Engineering degree from
Colorado State University and has her Engineer-in-Training certificate.
Nolan Doesken is a Research Associate with the Atmospheric Science Department at Colorado
State University. He is also the Assistant State Climatologist with expertise in Colorado climate
conditions and historical perspectives,western weather patterns, and national weather policies
and weather agencies. He is widely recognized as an expert in local and regional Colorado
climatological systems.
Dave Frick is the Regional Vice President for Ayres Associates in Fort Collins, Colorado. He is
a Professional Engineer and has a Ph.D. from Colorado State University. He has conducted
numerous hydraulic and hydrologic studies pertaining to the Flood of July 1997 and prepared
several stormwater master plans for the City of Fort Collins. He is also a member of the Fort
Collins Water Board.
Susan Duba Hayes is the Senior Stormwater Engineer for Master Planning with the Water Field
Operations and Planning Division of the Utilities for the City of Fort Collins. She is the Project
Manager for the Precipitation Task Force. She has over 11 years experience in Stormwater
design, review, and management for the City of Fort Collins. She has her Professional
Engineering license.
Don Heyse is a citizen representative from the Fairbrooke Heights neighborhood. His home and
neighborhood experienced severe flooding during the Flood of July 1997.
Marsha Hilmes is the Floodplain Administrator with the Water Field Operations and Planning
Division of the Utilities for the City of Fort Collins. She has experience with flood mitigation,
early warning systems, and fluvial systems.
John Lion is a Hydrologist with the Community Mitigation Programs Branch of Region VIE of
the Federal Emergency Management Agency (FEMA). He has his Professional Engineers
license. The City of Fort Collins participates in the National Flood Insurance Program,
therefore, FEMA must review and approve all local floodplain designations.
John Morris is a Professional Engineer III with the Facilities and Utilities Services Departments
at Colorado State University. He represents Colorado State University and has experience in
flood mitigation (due to the flooding from July 1997). He is also a member of the Fort Collins
Water Board.
Dr. Tom Sanders is an Associate Professor with the Civil Engineering Department at Colorado
State University. He has a Professional Engineering license. He is the Coordinator of the Civil
Engineering Department's Environmental Engineering Division including environmental
engineering, hydrology, and water resources. He is also a member of City of Fort Collins Water
Board.
Bob Smith is the Water Planning Manager with the Water Field Operations and Planning
Division of the Utilities for the City of Fort Collins. He has over 20 years experience with City
stormwater management, design, and development. He has his Professional Engineering license.
Technical Support
N0'RC Engineering in Denver, Colorado and Nevada is the consulting company hired by the
City of Fort Collins to assist in the study and analysis of the City's precipitation. WRC has been
working on the revision of the City's Storm Drainage Design Criteria and Construction
Standards manual. WRC has experience in the development of precipitation frequency studies
and modeling criteria for the State of Nevada and for the Denver area(?).
Oli Sveinsson is a graduate student in the Statistics Department at Colorado State University.
His knowledge of Generalized Extreme Value methodology, L-moments, and statistical analysis,
has enabled him to assist WRC and the City of Fort Collins in this study and analysis of
precipitation frequency.
MEMORANDUM
DATE: October 22, 1998
TO: Fort Collins Water Board
Mike Smith, General Utilities Manager
FROM: Precipitation Frequency Study Task Force
SUBJECT: Adoption of Revised Rainfall Rates for Design of Storm
Drainage Facilities
Background and Purpose
In July 1997, the City of Fort Collins experienced an extreme rainfall which
caused severe flooding, resulting in the loss of five lives and significant property
damage. At the time, the City was in the process of revising the Storm Drainage
Design Criteria and Construction Standards. In the aftermath of the storm, the
City decided to include in the criteria update a reevaluation of the amount of
rainfall used for mapping 100-year floodplains and designing storm drainage
facilities.
The purpose of the study was to provide the City with a statistical analysis which
assessed the impact of additional rainfall data, gathered. over the last 30 years,
on the rainfall amounts currently being used for drainage design. In addition,
feedback after the storm indicated some people felt it rained more often and
harder on the west side of town. A qualitative precipitation study, using data
from weather watcher reports from the largest storms in the Fort Collins area
over the last fourteen years, was completed to investigate this question.
Task Force
This study was performed with the guidance of a technical advisory committee.
The members of this committee represent a cross section of regulatory
agencies, (Federal, State, County and City), technical expertise, (CSU, private
consultants) and private citizens. The committee was composed of the following
individuals:
Susan Hayes City of Fort Collins Utilities (Project Coordinator)
Robert Smith City of Fort Collins Utilities
Marsha Hilmes City of Fort Collins Utilities
1
Duane Boes CSU Statistics Department
Tom Browning Colorado Water Conservation Board
Rex Burns Larimer County Engineering Department
Lisa Dunn Larimer County Engineering Department
Bob Davidson City of Fort Collins Planning and Zoning Board
Nolan Doesken CSU Atmospheric Sciences Department
Dave Frick Ayres and Associates, Inc.
Don Heyse Citizen Representative
John Liou FEMA Region VIII, Denver, Colorado
John Morris CSU Facilities Department
Dr. Tom Sanders CSU Civil Engineering Department
Technical work was provided by W RC Engineering, and Mr. Oil Sveinsson, a
graduate student at CSU.
Study Results
Current Criteria
The City of Fort Collins and Larimer County currently use a design storm with a
total rainfall of 2.89 inches falling over a two hour period. This is referred to as
the 100-year event (a storm with a one percent chance of being equaled or
exceeded in any given year). The shorter 2-hour duration storm is used for
design purposes since it is considered to be the most typical event experienced
here.
Statistical Study
The Generalized Extreme Value statistical method was chosen because it
provides the best fit to the available data. It will also correspond well with the
method used by NOAA when they update the statewide NOAA Atlas 2 in several
years.
Both an "at-site" and regional analysis were completed for comparison.
• Results of the statistical study are summarized below:
Current Regional At-Gage
Criteria Analysis Analysis
Rainfall Amount for
100-year, 2-hour event 2.89" 3.67" 4.37"
Oualitative Rainfall Pattern Study
The data used for this study included weather watcher reports for twenty-eight
major storm events from June 1983 to August 1997. The data was considered in
a variety of ways to compare the rainfall on the western portion of the city to the
2
. eastern portion. Elevation characteristics were also looked at since generally
ground elevations are higher on the west side of the city. The following
conclusions were reached from the qualitative study:
• There appears to be a trend towards larger precipitation amounts occurring in
the region west of the center of the city (approximately College Avenue). The
difference was as much as 19% for the storm totals. It appears it rains more
often and longer on the westside, but it is not clear whether it rains heavier for
shorter duration's, such as the 2-hour event we use for design. Those
reported storms, which produced more rain on the west side, were longer
duration storms. There was no data available to draw conclusions for short
duration (one hour or less) storms. The slope of terrain can trigger storms,
but it is not certain where this occurs and if it really affects high intensity,
short duration storms. In this study, there was not a significant difference in
the amount of rainfall due to the elevation of the weather watcher stations.
Final Recommendation
Based upon the study results the Task Force recommendations are:
1. Adopt a rainfall amount for the 100-year, 2-hour event of 3.67". This amount
is a reasonable value, which takes into account the storm of 1997 but
. balances the loss of data from the first half of the century with other regional
data. The Task Force recognizes this value may be higher than the future
NOAA study will recommend, however, it is believed this study is more
accurate for Fort Collins.
2. Adopt a uniform rainfall amount across Fort Collins. The Task Force did not
believe the qualitative rainfall pattern study showed significant difference
between rainfall values for the shorter duration storms to warrant a varied
rainfall rate across the city.
3. Actively pursue installation of additional recording rain gage sites to provide
information in the future to revise the design storm data and possibly adjust
for different areas of the city.
Reasoning
Significant discussion about difficult issues took place during the course of this
study. Included below is a summary of those discussions and the reasoning
behind adoption of the final recommendation.
• Regional vs. "At-Gage" Analysis: much discussion was held about the
appropriateness of using an "at-gage" analysis rather than a regional
analysis. A regional analysis is typically conducted in a rainfall study in order
to increase the amount of data available for analysis. Data from similar sites
has the effect of "extending the period of record", giving greater confidence in
the results. This will be the approach used by NOAA in the future. However,
the task force recognized that the front range area creates a topographic
• barrier that could result in more extreme events than elsewhere in eastern
3
Colorado. It was also recognized that even though an additional 30 years of
hourly data had been collected, the data gathered in the first half of the
century was no longer available from NOAA. Therefore, the results of
analyzing the available 2-hour rainfall data would not provide any additional
years of data than were used in the original NOAA Atlas 2 study. Due to the
unique nature of front range locations, however, the region defined for this
study was limited to Fort Collins, Longmont, and Boulder (locations of long-
term hourly data along the front range). The future NOAA study will most
likely be based on a larger region which will include locations with
characteristics not consistent with the front range.
1997 Storm: because the 1997 storm was so large it has a significant impact
on the statistical study, bringing the final values up considerably. Discussion
was held about how to incorporate the actual storm value into the data set.
Opinions ranged from treating it as an "outlier" and throwing it out, to using
the maximum recorded storm value. It was agreed to use the actual recorded
value at the CSU rain gage in the data set. Another discussion focused on
using the actual CSU gage data or data from closer to the storm center.
Since the data from closer to the storm center was only one value and was
not representative of a long term record at a given gage site the accepted
standards of engineering practice would dictate using data from the long term
gage site rather than creating a non-homogenous data set
• Climate change: the question was raised about the effects of climate change
creating more severe storms in recent years. This question was addressed
by analyzing 1-day storm magnitude and frequency for Fort Collins, Boulder,
Waterdale (Loveland), Longmont and Greeley. This analysis concluded that
when analyzing the 1898-1948 period and comparing it to the 1948-1998
period no significant shift in storm magnitudes has occurred at any of the long
term gaging sites in the region.
• Fort Collins is unique: another question was raised about Fort Collins being
unique among other front range communities in terms of more frequent and
more severe storms that have occurred. Again when looking at the Fort
Collins, Boulder, Waterdale, Longmont and Greeley gages the data showed a
slight trend for more severe storms closer to the foothills (Boulder, Waterdale,
Fort Collins) compared to Longmont and Greeley. No significant differences
were found in the data among the front range sites with the exception of the
1997 storm data which did affect the Fort Collins data.
• Consistency with adjacent areas: the regional analysis will provide more
consistent data when dealing with drainage basins that cross jurisdictional
lines. It is also more consistent with (although higher than) preliminary data
that is being developed by NOAA for the remainder of the state.
• Rainfall patterns: there needs to be more recording rain gage information
obtained from several sites in Fort Collins to develop good quantitative data
related to locations around town. The CSU gage is located centrally (east to
west) in Fort Collins; therefore it represents a good average of the area until
more quantitative data can be obtained.
4
Bob Smith, rt Collins Utilities Marsha Hilmes, Fort Collins Utilities
J n Liou, FEMA Region VIII om Br ni g, CW
Rex Burns, Larimer County Lisa n, Lan Co y
Nolan Doesken, CSU Atmospheric Dave Frick, Ayres and Associates
khn
ces Dept.
P
Morris, CS Facilities Dept. Dr. Tom Sanders, CSU Civil
Engineering Department
Susan Hayes, Fort Collins Utflities
5
Minority Opinion
Consensus on the final recommendation was not reached among the Task Force
members. Because of the variety of issues to be resolved and the complexity of
the subject it was agreed the minority opinion of the Task Force would be
presented for the Board's information.
Minority Recommendation
1. Adopt the GEV analysis results from the "at gage" analysis: 4.37 inches over
2-hours for the 100-year storm. We believe analysis shows Fort Collins is an
extreme within the region. The majority of the largest regional storms occur in
Fort Collins because the City experiences preferred storm tracks.
2. Adopt a higher rainfall amount for use on the west side of the City.
Anticipating greater rainfall on the west side, we should apply a greater design
storm for that area, specifically 5.5 inches for the 2-hour 100-year storm. This
should be applied west of the CSU campus, and in particular west of Taft Hill
Road.
Reasoning
It is our opinion several independent studies have shown that it does in fact rain
more on the west side of Fort Collins than on the east side. These studies
include WRC's 1998 study, data analysis by Oli Sveinsson in 1998, and the
Colorado Climate Center's 1998 program of rainfall data collection (CoCoRAHS).
Atmospheric physics and the study of storm cloud dynamics offer good
explanations for this phenomenon. Briefly, as an upslope storm moves across
the city, it is forced upward by the rising elevation of the foothills, and the
resultant cooling increases the rainfall. The foothills also tend to trap the storm,
increasing the duration.
Rainfall maps prepared by the Colorado Climate Center after the 1997 flood
show that the storms of July 27 and 28, 1997 dumped peak accumulations of 12
or more inches within a broad swath stretching over 8 miles long from north to
south along the west edge of the city.
Data from weather radar and private rain gauges surveyed by the Colorado
Climate Center indicate a 2-hour peak of 6.5 inches the night of the flood south
of Drake at Overland Trail. There were reports of possibly greater values farther
north, closer to Prospect and Taft Hill.
The CSU campus rain gauge recorded a 2-hour maximum of 3.8 inches on
July 28, 1997. However, the bulk of the flooding was caused by overland
water flows coming downhill from the west, where the rainfall was much greater.
There is a historic pattern of heavy storms on the west side of town causing
6
flooding on the CSU campus. This was observed in the 1938 and 1951 floods,
as well as the 1997 flood.
We lack long-term data for the west side. However, we can still make
a reasonable estimate. Recent data, historical accounts and atmospheric
physics lead us to believe that the rainfall has been consistently greater on the
west side over the last 100 years. Therefore, we believe we can transpose the
CSU gauge data to the west side without risk of overestimating rainfall, and
incorporate the 6.5 inch 2-hour peak into our calculations. This yields a 5.5 inch
2-hour 100-year storm for the west side of Fort Collins.
Our climate is warming. This has been documented both locally and globally.
We believe we are moving into a period of warmer, wetter weather.
National studies have shown a long-term increase in the number and severity of
rainstorms. We need to prepare for a future that will present us with floods of
greater frequency and severity than those we have experienced in the first half of
this century.
�U_OALAI� ( �
Duane Boes, CSU Statistics Dept. . Don Heyse, Citizen
Bob Davidson, Fort Collins Planning and Zoning Board
EXHIBIT C
IMPACTS
M
EXHIBIT C
BASIN IMPACTS
• PEAK FLOWS AND VOLUME
Assessing the exact impact of increasing the rainfall criteria is difficult to do. Each basin
is unique and, depending on the existing drainage facilities and those proposed in the
master plan,will respond differently from another basin. Based on our analysis,
however, several generalizations can be made about the effect of increasing rainfall.
We looked at six basins within the City. The recommended 3.67" rainfall was run
through the current models. We also ran the 4.37" rainfall for comparison purposes.
Rainfall distributions, with the peak intensity at 35 minutes, were run through Canal
Importation, Dry Creek, Foothills, Old Town, Spring Creek and West Vine basins.
In all basins, the difference in runoff between the current 2.89" and the 3.67" distribution
is not as significant as the difference between the 3.67" and 4.37" distribution. This may
be attributed to the difference in peak intensities; the 3.67" peak is 9 in/hr (similar to the
2.89"), whereas, the 4.37" peak is 12 in/hr. Thus, there is a greater peak at the height
of the storm, 35 minutes.
The increased rainfall affects basins with detention more than basins without detention.
This effect is due to the increase in rainfall volume from 2.89" to 3.67"and from 2.89" to
4.37". As the rainfall volume increases the storage required to detain runoff increases.
If the storage is not available to detain the additional runoff volume, then the
downstream discharge rapidly increases as detention ponds overtop.
The peak discharge for undetained basins is primarily influenced by the peak rainfall
• intensity of the design storm. Peak discharges from undetained areas did not
significantly increase from the current 2.89" design storm to the 3.67" design storm
because the peak rainfall intensity did not change. Peak rainfall intensity is calculated
from the 1-hour rainfall depth and this study did not show an increase in this value from
the original NOAA study. However, the peak discharges increased from the current
2.89" design storm to the 4.37" design storm because the peak rainfall intensity
increased (because the 1-hour event also increased).
Summary information regarding peak flows and pond volume for the six basins are
shown in Tables 1 through 7. Basin maps identifying locations of flow and volume
information are on Figures 1 through 6.
MASTER PLANNED DRAINAGE FACILITIES
We were also asked to look at the financial impact of increased rainfall on the
implementation of our master plans. Again, exact impacts are difficult to define unless a
full master plan update is completed. However, we have attempted to assess some
typical financial impacts on existing and proposed master planned facilities.
As stated before, the greatest impact of an increase to 3.67" is on the volume of
detention needed in a basin. Therefore, it is the basins which have existing detention or
proposed detention which will be the most impacted financially. We chose two typical
sites to analyze financial impact:
The Rodeo Arena detention pond in the Canal Importation basin. This pond
is located west of Overland Trail between Drake Road and Prospect Road.
The Taft Hill Road detention pond on Spring Creek.
1
Rodeo Arena Pond
This pond serves to detain a large area west of Overland Trail Road. Currently the
existing road embankment provides some inadvertent detention. To control flooding
downstream additional detention was proposed in the master plan. Based on 2.89" of
rainfall, approximately 34 ac-ft of storage was proposed. Based on 3.67" of rainfall, 45
ac-ft of storage is required.
This increase of 11 ac-ft would cost approximately$200,000. The bulk of this cost is for
additional excavation, however, an additional five acres of land would also be required.
The cost estimate for the 34 ac-ft pond is approximately $1.4 million. An increase of
$200,000 represents an increase of 14% to the project cost.
Based on 4.37" of rainfall, 59 ac-ft of storage is required. Comparing proportionally, the
cost of an additional 25 ac-ft would be approximately $475,000, an increase of 34% to
the original project estimate. This assumes the pond area doesn't encroach into known
bedrock in the area, which could increase the cost considerably. Realistically, the pond
would be sized based on the best combination of volume on site and facilities
downstream to carry the increased flows. This would require a full master plan update.
Taft Hill Road Pond
This is an online detention pond on Spring Creek created by the embankment of Taft
Hill Road. The master plan calls for additional detention to control the flows
downstream. Since downstream road and channel improvements are already in place it
was assumed additional detention would have to be provided to meet downstream flow
restrictions. Based on 2.89" of rainfall, approximately 69 ac-ft of storage was required.
Based on 3.67" of rainfall, approximately 112 ac-ft of storage is required. This was
accomplished by raising the road embankment and purchasing additional ROW for
ponding of more water.
The cost of the increased volume is approximately $673,000. In this case the cost of
ROW is almost half of the project cost due to the value of existing structures, however,
the actual construction costs are only a small portion of the total cost. The cost estimate
for the 69 ac-ft pond is $1.9 million. An increase of$673,000 represents an increase of
35% to the project cost.
Based on a rainfall of 4.37", approximately 218 ac-ft of storage is required. Given the
physical constraints of this site, it would not be possible to obtain this much storage.
Approximately 40 ac-ft of additional volume could be obtained. In this case, the master
plan would be reanalyzed and optimized to determine the best alternative. Final
improvements could include retrofitting downstream improvements or mapping a
residual floodplain. Final costs would be based on the best alternative to handle the
flows.
In summary, for projects where ROW is available and inexpensive, the cost of adding
additional volume will be lower and have less impact on the funding needs. However,
where ROW is limited and expensive, costs will be higher. Retrofitting existing ponds
will also be a problem, since additional ROW may not be available at all. Where peak
flows increase, channels and storm sewers may need to be retro-fit for the higher flows.
If this is not cost effective, then a larger residual floodplain will be mapped. Until a full
master plan update is completed for each basin the full financial impacts will not be
known.
2
CANAL IMPORTATION BASIN
The majority of the Canal Importation Basin is developed with a portion of the basin improvements
in place. The master plan calls for a variety of drainage improvements with a mix of conveyance
. facilities and regional detention. There are several basin improvements that are not complete at
this time including detention facilities and a parallel ditch system adjacent to the irrigation canals.
The canal importation channel from Taft Hill Road to the Spring Creek is constructed.
The Canal Importation Basin has a mix of undetained and detained areas. The impact of a 3.67"
or 4.37"design storm will be most noticeable for proposed detention ponds and areas directly
downstream of existing detention ponds. Facilities draining undetained areas may not experience
a significant increase in design discharge for a 3.67"design storm, but these facilities would
experience an increased design discharge for a 4.37"design storm.
Existing Facilities
1. The 2.89" model was revised for the 3.67"hyetograph and 4.37"hyetograph.
2. The pond rating curves were modified to reflect overtopping conditions.
3. Bold numbers correspond to locations on attached basin map (see Figure 1).
TABLE 1
Location 2.89" 3.67" 4.37"
(Discharge/Storage) (Discharge/Storage) (Discharge/Storage)
1 - Rodeo Detention 389.5cfs/12.2ac-ft 410.8cfs/12.3ac-ft 710cfs/ 13.9ac-ft
Pond
Pond 353
2 - Brown Farm Pond 125.7cfs/13.8ac-ft 250cfs/15.2ac-ft 610cfs/ 16.3ac-ft
Pond 326
3-Avery Park Pond 608.6cfs/15.Oac-ft 684.8cfs/15.3ac-ft 1008.2cfs/ 16.4ac-ft
Pond 378
4--Canal Importation 481cfs 686cfs 1130cfs
• Channel at Shields St
ICE 42
Master Plan Facilities
1. The only modification to the model was to change the hyetograph for each rainfall event.
2. Sold numbers correspond to locations on attached basin map (see Figure 1).
TABLE 2
Location 2.89" 3.67" 4.37"
(Discharge/Storage) (Discharge/ Storage) (Discharge/Storage)
1- Rodeo Detention 37 cfs/34.2ac-ft 48.6cfs/45ac-ft 64cfs/ 58.9ac-ft
Pond
Pond 353
2 -Brown Farm Pond 8.1 cfs/5.6ac-ft 9cfs/7.2 ac-ft 9.6cfs/9.1 ac-ft
Pond 326
3—Avery Park Pond 543.3cfs/ 14.1 ac-ft 564.5 cfs/ 14.2ac-ft 954 cfs/ 15.7ac-ft
Pond 376
4—Canal Importation 1339 cfs 1402.4 cfs 1755 cfs
Channel at Shields St
CE 42
5 - PV&L Parallel 501.2 cfs 576 cfs 766 cfs
Channel
CE 265
3
DRY CREEK BASIN
The lower Dry Creek Basin is somewhat built-out with very few detention ponds, except for the
Evergreen-Greenbriar basin,which is included in the Dry Creek model. Outside of the Evergreen-
Greenbrier basin, there currently are three existing detention ponds in the basin—one at the
Streets Facility,one at Vanworks, and one at Roselawn Cemetery. The hydrology for the basin
was run for developed conditions without on-site detention and with on-site detention. The results
presented in Table 3 are for developed conditions with on-site detention.
Master Plan Facilities
1. The only modification to the model was to change the hyetograph for each rainfall event.
2. Bold numbers correspond to locations on the attached basin map (see Figure 2).
TABLE 3
Location 2.89" 3.67" 4.37"
Dischar a (Discharge) (Discharge)
1 -from NW corner of 872 cfs 894 cfs 1276 cfs
Lemay&Vine
CE 602
2-above Vine to north 1148 cfs 1199 cfs 1693 cfs
side of airport
CE 603
3-outfall into Poudre 1224 cfs 1450 cfs 1958 cfs
from new Dry Creek
CE 611
4-outfall into Poudre 1030 cfs 1059 cfs 1507 cfs
from old Dry Creek
CE 614
FOOTHILLS BASIN
The Foothills Basin is mostly developed with several large detention ponds within the basin built
including the Woodwest 7th, Meadows East, and Nelson ponds. The Foothills North Tributary
Channel is a master plan improvement which has been completed from Timberline to the FCRID.
The hydrology for the basin was run for developed conditions with regional and on-site detention.
Existing Facilities
1. The only modification to the model was to change the hyetograph for each rainfall event.
2. Bold numbers correspond to locations on the attached basin map (see Figure 3).
TABLE 4
Location 2.89" 3.67" 4.37"
(Discharge/Storage) (Discharge/Storage) (Discharge/Storage)
1 -Woodwest 7 Pond 104 cfs/9 ac-ft 109 cfs/12.8 ac-ft 118 cfs/20.1 ac-ft
CE 15
2- Meadows East Pond 15.8 cfs/7.5 ac-ft 16.9 cfs/10.3 ac-ft 18 cfs/13.9 ac-ft
CE 71
3-Timberline& 428 cfs 491 cfs 554 cfs
Foothills Channel CE 78
4 - FCRID outfall 992 cfs 1051 cfs 1300 cfs
CE 96
5-Westfield Park Pond 2.5 cfs/16.1 ac-ft 11.4 cfs/19.9 ac-ft 22.2 cfs/24.3 ac-ft
CE 102
4
OLD TOWN BASIN
The Old Town Basin is characterized as highly developed with relatively no drainage facilities
designed for the current design storm of 2.89". The Old Town Basin Master Plan calls for a
variety of conveyance improvements (channels, storm sewers, street lowering), and no detention
ponds. None of the drainage improvements specified in the master plan have been constructed
at this time.
In general, the peak discharge for undetained basins, such as the Old Town Basin, is primarily
influenced by the peak rainfall intensity of the design storm. Peak discharges in the Old Town
Basin did not significantly increase from the current 2.89" design storm to the 3.67"design storm
because the peak rainfall intensity did not change. However, the peak discharges increased
approximately 60%from the current 2.89" design storm to the 4.37"design storm because the
peak rainfall intensity increased.
Master Plan Facilities
1. The hyetograph for each rainfall event input into the model.
2. The model with master plan facilities with no flows diverted from Canyon St.was used.
3. Inflow hydrographs from the Canal Importation Basin were updated for the revised rainfall.
4. Bold numbers correspond to locations on attached basin map (see Figure 4).
TABLE 5
Location 2.89" 3.67" 4.37"
(Discharge) (Discharge/Storage) (Discharge/ Storage)
1- Howes Outfall at 537.2 cfs 565.5 cfs 770.5 cfs
Laporte Avenue
CE 92
2— Howes Outfall at 1051 cfs 1084 cfs 1526.1 cfs
Cherry Street
CE 304
3—Locust Street 223 cfs 225 cfs 355 cfs
CE 304
SPRING CREEK BASIN
The Spring Creek Basin is mainly developed with several large detention ponds within the basin
including the CSU Vet pond, Woodwest, BNRR, Taft Hill, and CSSRR. The hydrology for the
basin was run for developed conditions with regional and on-site detention, and inflow
hydrographs from the master plan condition for Canal Importation. The Canal Importation model
was not modified from the original (i.e., none of the ponds or conveyance elements were adjusted
to alleviate surcharging or pond extensions), only the hyetographs were changed. The four inflow
hydrographs from CSU were not modified for the 3.67" rainfall. (As a note,the BNRR pond
embankment elevation is at 4996'with a discharge of 2304 cfs and a storage volume of 362 ac-ft,
according to the EPI 1989 analysis.) The 4.37" hyetograph was not run due to the time needed to
modify the Canal Importation inflow hydrographs.
Master Plan Facilities
1. The only modification to the model was to change the hyetograph for each rainfall event.
2. Bold numbers correspond to locations on the attached basin map (see Figure 5).
TABLE 6
Location L&A 7/98, 2.89" Pond 304 release kept @ Pond 304 release
(Discharge/Storage) 1400 cfs, 2.89" kept @
(Discharge/Storage) 1400 cfs, 3.67-
(Discharge e/Storage)
1 -Taft Hill Pond 1523 cfs/70.6 ac-ft 1400 cfs/69 ac-ft 1400 cfs/112.2 ac-ft
CE 304
2- BNRR Pond 2034 cfs/220 ac-ft 2134 cfs/263 ac-ft 2337 cfs/381.1 ac-ft
CE 303
3 - Stuart— Stover Area 2070 cfs 2170 cfs 2371 cfs
. CE 213
4—Canal Importation 618 cfs 1290 cfs 1365 cfs
confluence CE 319
5
WEST VINE BASIN
The West Vine Basin is partially developed with very few detention ponds. There currently are
three existing detention ponds in the basin—one near the northwest corner of Laporte and
Overland Trail, one at the west end of Hollywood St., and one at the southwest corner of Laporte
and Overland Trail. The hydrology for the basin was run for developed conditions without on-site
detention. The results are presented in Table 7.
Existing Facilities
1. The only modification to the model was to change the hyetograph for each rainfall event.
2. Bold numbers correspond to locations on the attached basin map (see Figure 6).
TABLE 7
Location 2.89" 3.67" 4.37"
(Discharge/Storage) (Dischar a/Stora a) (Discharge/Storage
1 - Hollywood Pond 190 cfs/17.2 ac-ft 254 cfs/19.4 ac-ft 334 cfs/21.8 ac-ft
CE 302
2-Vine Outfall 2116 cfs 2378 cfs 2979 cfs
CE 174
3-Taft Hill just north of 681 cfs 834 cfs 1020 cfs
Laporte
CE 252
4- Soldier Creek at 234 cfs 265 cfs 347 cfs
Laporte CE 38
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EXHIBIT C
LAND DEVELOPMENT IMPACTS
General
In general, impacts on new land development due to increased rainfall would be similar
to the basin impacts.
Issues related to land development criteria are:
Increased Volume for Ponds: the greatest impact of the new rainfall criteria is the need
for more on-site detention. Depending on the physical constraints of the site providing
additional volume may or may not be a significant impact. (See discussion below of
Sear-Brown Group's analysis.)
Off-site Flows: currently new sites are required to safely convey offsite, existing 100-
year flows through the site. Hydrology updates for the basins may result in offsite flows
increasing, especially if upstream detention ponds are overtopping.
Street Capacity: since the peak rainfall intensities for the 3.67" rainfall have not
increased it is not expected the peak flow rates from sites would increase. This means
peak street flows are not expected to change and no increase in on-site infrastructure
such as storm sewers and inlets is expected.
The peak rainfall intensities for the 4.37" rainfall do increase, thereby increasing the
peak flow rates from the site. Increased infrastructure such as storm sewers and inlets
is expected. (See discussion below of Sear-Brown Group's analysis.)
Floodpiain Development: revised hydrology of the basins may show an increase in the
existing condition floodplains due to pond overtopping and increase peak flow rates.
Until the basins are re-evaluated base flood elevations may not be accurate.
Implementation: adopting higher rainfall is a challenge administratively. Issues we
expect to deal with include which sites currently under design will be required to
incorporate the revised rainfall? Previous drainage reports will be outdated and no
longer useful for the design of new, adjacent sites. Current phased developments,
designed under the old criteria, may not have enough volume. Will they be required to
upgrade when a new phase comes in for review? Will old on-site ponds have to be
retrofitted, and by whom? How will we coordinate with Larimer County on developments
in the Urban Growth Area?
Site Analysis
To better assess the impacts on a new development we hired the Sear-Brown Group to
re-design two of their own projects. Both of these sites were modeled using UDSWMM
and have on-site detention
Ridgewood Hills, 2nd Filing
This development, currently under design, is located in the Fossil Creek basin,
specifically, south of Trilby Road between College Avenue and Shields Street. It is
made up of 215 lots on approximately 122 acres. The Fossil Creek basin is
characterized by good topographical relief which influences how facilities are
incorporated into the site. This site has a total of seven on-site detention ponds. The
cost per lot is approximately $32,000.
With the 3.67" rainfall, the required detention volumes increased anywhere from 20 to
33%. Due to the topographical relief of the site, additional detention volume could easily
be gained by raising pond embankments with minimal impact on adjacent lots. One
pond was enlarged by excavating additional volume rather than raising the berm. This
required additional land area, but did not encroach onto existing lots. A storm sewer
connecting two ponds also had to be enlarged. Sear-Brown calculated the total
increase in cost to be approximately$25,200.00,which translates to less than $117.00
per lot.
With the 4.37" rainfall, the required detention volumes increased anywhere from 70 to
93%. Additional volume was gained by raising the berm and expanding the area of the
ponds. The majority of storm sewers and inlets on the site had to be increased in size
due to the higher peak flows. Sear-Brown calculated the total increase in cost to be
approximately $303,200, which translates to $1410.00 per lot.
Stetson Creek First Filing
This site is located in the McClellands portion of the McClellands/Mail Creek basin,
specifically south of Harmony Road between Timberline and County Road 9. It is made
up of 44 lots on approximately 18 acres. This site has two on-site detention ponds
which drain directly into the McClellands Channel. The site is basically flat, which
makes adding additional volume more difficult.
With the 3.67" rainfall only the larger pond required additional volume, a 25% increase.
Additional berming and road embankment regrading were required to gain the additional
volume. A storm sewer also had to be enlarged to take into account a higher tailwater
condition due to the higher ponding limits. The higher ponding limit encroaches slightly
onto the existing lots. This has been taken into account in the cost estimate, however,
this impact may have been easily mitigated during original site layout without additional
cost. Sear-Brown calculated the total increase in cost to be approximately $7,032.00,
which translates to less than $160.00 per lot.
With the 4.37" rainfall, the increase in pond volume is 67 to 75%. Additional pond
volume was gained by raising the berm and adding area to the pond. Both pipe sizes
and inlet sizes increase in order to carry the higher peak flows to the ponds. Sear-
Brown calculated the total increase increase in cost to be approximately $39,400, which
translates to under $790.00 per lot. Mitigation for the 4.37" rainfall would not be as
easily handled in the initial layout as the 3.67" rainfall. Some lot area would be lost to
accommodate the additional volume for the pond.
Though each site will be different, these two examples illustrate the additional costs
associated with implementing a higher rainfall. For smaller sites, with more impervious
surface, the impacts may be greater.
Precipitation Impact Study
For The
City Of Fort Collins
February 1, 1999
•
Prepared by:
The Sear Brown Group
209 South Meldrum
Fort Collins, Colorado 80521
(970) 482-5922
THE
SEAR-BROWN
GROUP
Standards in Excellence
•
Investigation Into Impact Of Increased Rainfall On New Development
1. Background
Recently the City of Fort Collins Storm Water Utility re-examined historic
rainfall data used for the 100-year storm and determined that the amount of
rainfall was significantly more than that which is being used at the present time.
The current design storm is 2.89 inches of rainfall over a 2-hour period based on a
1973 NOAA study. The Sear-Brown Group (SBG)was commissioned to
examine the impacts that could occur to a land development project from a
development engineer's point of view and estimate the additional land and
infrastructure cost on that development. The City of Fort Collins Storm Water
Utility asked SBG to examine two different rainfall depths—(1) 3.67 inches of
rainfall over a 2 hour period and (2)4.37 inches of rainfall over a two hour period.
Sear-Brown assessed the impacts of the change in rainfall by inputting the new
rainfall data into the calculations and computer models of two previously
approved subdivision drainage reports. These subdivisions were Ridgewood Hills
I" and 2nd Filing and Stetson Creek 1S1 Filing.
The City of Fort Collins provided the rainfall data for both the Storm Water
Management Model (S WMM), which is used for sizing the detention ponds, and
the IDF curve, which is used for rational method calculations. The rational
method is used to estimate storm water flow rates, which are then used to size
much of the storm drainage infrastructure such as inlets and pipes.
2. Methodology
Sear Brown focused on two key runoff characteristics,which would cause
impacts to the developments, these are:
1. The increase in peak runoff rate
2. The increase in detention volume.
The peak run-off rate for development is typically calculated from the rational
method. This peak rate is then used to size the majority of storm water
conveyances such as roads, inlets and storm drain pipes. Sear Brown compared
the IDF values provided by the City of Fort Collins to the IDF values that were
typically. The impacts of the change in rainfall data will be discussed in the
following sections.
The detention volume for larger sites is typically calculated using a Storm Water
Management Model (SWMM). SBG input the new rainfall data provided by the
City of Fort Collins Storm Water Utility into two existing and previously
approved SWMM models. The detention pond rating tables were then adjusted in
the SWMM model to maintain the allowable discharge from the pond. It was
found that the adjustment to the out fall discharge rate had no significant effect on
• the detention pond volume for both developments. The impacts caused by the
two different rainfall depths are discussed in the following sections.
The impacts considered were those that would occur during an original design
effort, not as a retrofit to existing subdivisions.
3. Impacts of the 3.67 inch 2-hour Duration Storm
a) Storm Drainage run-off rates
The run off rate, which is calculated by the rational method, for both the 2-
year storm and the 100-year storm does not increase for this particular _
storm. This means that the storm water conveyances, which include streets,
storm inlets and pipes will not need to be increased in length or size.
b) Detention Pond Volume
Depending on the subdivision the increase in detention volumes varied.
The Ridgewood Hills I" and 2nd Filing subdivision and detention volumes
increase in proportion to the increase in rainfall, approximately 27%. This
was an overall increase in detention volume from 17-acre feet to 21.8-acre
feet, increases varied between individual detention ponds.
The detention pond volume increase for Stetson Creek was approximately
18%. This was an overall increase in detention volume from 1.1-acre feet
to 1.3-acre feet, increases varied between individual ponds.
c) Cost Impacts
The cost impacts were primarily due to the increase in detention volume.
Since there is no increase in peak run-off rate for either the 2-year or the
100-year storm, there is no cost impact to the conveyance elements.
However, there is a cost implication for the increase in storage requirement
for each detention Pond (See attached Spreadsheet.) This included the cost
for land take, earthwork and increased pipe size. In general the impact per
lot is low, varying from $117 to $160 per lot. The average lot price is
estimated at $32,000.
Subdivision Detention
Impact Costs
Stetson Creek I" Filing $7,032
Ridgewood Hills 1°"and 2" Filin $25,200
Total Cost $32 232
4. Impacts of the 4.37 inch 2-hour Duration Storm
a) Storm Drainage run-off rates
The run off rate, which is calculated by the rational method for the 2-year
storm does not increase, while the run-off for the 100-year storm does
increase. This means that there are different impacts depending on the
configuration of the development's storm drainage system. For example if
the storm drainage pipe systems were designed to carry only the 2-year
storm there would be no impact to its size. However if the storm drainage
pipe system was designed for the 100-storm, then the pipe sizes and inlet
sizes needed to be increased. In general approximately 50 to 80 percent of
the pipes and inlets needed to be increased.
b) Detention Pond Volume
In this case the increase in detention volumes was uniform. The
subdivision detention pond volumes increased by a greater percentage
(78%) than the proportional increase in the rainfall (51%). In order to
assess the impact for this large increase in the detention volume it was
decided that a combination to increasing the depth of the detention ponds
and taking land from adjacent lots would be the most economical. Sear
Brown then estimated the cost impact for each detention Pond. This
primarily included the cost for land take and earthwork.
ic) Cost Impacts
The cost impacts are due to both the increase in size of the storm drainage
conveyance infrastructure and the increase in detention volume. The
detailed break down of the impact costs are in a spreadsheet in the
appendix. The summary of the impact costs are listed in a table below.
Subdivision Conveyance Detention Total
Impact Costs Impact Costs Impact
Costs
Stetson Creek 1"Filing S8,000 $31,000 $39,000
Rid ewood Hills 1"Filing $33,000 $48,000 $81,000
Ridgewood Hills 2 no Filing $29 000 $303,000 $332,000
Total Cost $70,000 $382 000 1 $452 000
The cost impact due to the increase in detention volume ranges from
$400/lot to $1600/lot. Due to the large increase in detention volume it is
unlikely that this cost could be mitigated by reconfiguring the subdivision
i and the fact that infrastructure sizes are now being increased.
1
5. Other Impacts
Other impacts are primarily off-site impacts. The primary impact is the increased
off-site storm water runoff from upstream detention ponds that are now
undersized due to the new rainfall data and the increased engineering cost to
analyze this off-site flow. Other impacts would be related to the regional drainage
facilities and are not within the scope of this report.
6. Couclusions
The 100-year 3.67 inch 2 hour storm only causes an increase in detention volume
and not an increase in peak run off rate as calculated by the rational method.
The cost impact of revising the rainfall data from 2.97 inches for a 100-year 2-
hour storm to a 3.67 inches for a 100 year 2 hours storm ranges from $117 to
$160 per lot.
The 100-year 4.37 inch 2 hour storm causes both an increase in detention volume
and an increase in peak run off rate as calculated by the rational method.
The cost impact of revising the rainfall data from 2.97 inches for a 100-year 2-
hour storm to a 4.37 inches for a 100 year 2 hours storm ranges from $400 to
$1600 per lot.
All of the above conclusions vary from site to site.
EXHIBIT D
PUBLIC OUTREACH
Utilities
light&power • stormwater • wastewater - water
it of Fort Collins
WE NEED YOUR INPUT
Date: November 16, 1998
To: Developers/Engineering Consultants
From: City of Fort Collins Utilities
Re: Open House to Present Precipitation Study Results
OPEN HOUSE
The City of Fort Collins Utilities is conducting an Open House for consultants, developers, and
the general public to present the results and recommendations from the recently completed
Precipitation Frequency Study. The Open House will be on November 30, 1998 at 700 Wood
Street in the Poudre Conference Room from 4 pm to 6 pm.
The Water Board has already adopted the proposed recommendations. We anticipate
taking this to Council for adoption in December and January.
DETAILED INFORMATION WILL BE PROVIDED AT THE OPEN HOUSE. IF YOU ARE
UNABLE TO ATTEND, BUT WOULD LIKE ADDITIONAL INFORMATION, PLEASE
CONTACT SUSAN HAYES AT 221-6700.
PRECIPITATION STUDY - PURPOSE
• In July 1997, the City of Fort Collins experienced an extreme rainfall event which produced
severe flooding, resulting in the loss of life and significant property damage.
• The purpose of the study was to provide the City with a statistical analysis which assessed
the impact of additional rainfall data, gathered over the last 30 years, on the rainfall amounts
currently being used for drainage design.
• Feedback after the storm indicated some people believed it rained harder and more often on
the west side of town. A qualitative precipitation study was completed to investigate this
question.
RECOMMENDATIONS
• Adopt a rainfall amount for the 100-year, 2-hour event of 3.67". This amount is an
increase from the existing criteria because it takes into account the storm of 1997 but
balances the loss of data from the first half of the century with other regional data. The
current criteria is 2.89" in 2-hours for the 100-year event.
• Adopt a uniform rainfall amount across Fort Collins. The Task Force did not believe the
qualitative rainfall pattern study showed significant difference between rainfall values for the
shorter duration storms to warrant a varied rainfall rate across the city.
i
700 Wood St. • P.O.Box 580 • Fort Collins,CO 80522-0580 - (970)22116700 - FAX(970)221-6619 - FAX(970)221-6593 - TDD(970)224-6003
e-mail: utilities@ci.fort-collins.co.us • www.ci.fort-collins.co.usNTILITIES
Minority Opinion Recommendation -Consensus on the final recommendation was not
reached among theTask Force members.
Adopt the results from the "at gage" analysis: 4.37 inches over 2-hours for the 100-year
storm. The minority believes the majority of the largest regional storms occur in Fort Collins
because the City experiences preferred storm tracks and is therefore an extreme within the
region.
Adopt a higher rainfall amount for use on the west side of the City. Anticipating greater
rainfall on the west side, the minority believes we should apply a greater design storm for
that area, specifically 5.5 inches for the 2-hour 100-year storm.This should be applied west
of the CSU campus, and in particular west of Taft Hill Road.
IMPACTS OF INCREASED RAINFALL
Master Plan Basins-The increased rainfall affects basins with detention more than basins
without detention. This effect is due to the increase in rainfall volume from 2.89" to 3.67". As
the rainfall volume increases, the storage required to detain runoff increases. If the storage is
not available to detain the additional runoff volume, then the downstream discharge rapidly
increases as detention ponds overtop.
New Development- In general, impacts on new land development due to increased rainfall
would be similar to the basin impacts. Peak flows on-site would not go up significantly,
however, additional detention volume would be required. Issues related to land development
criteria are:
Increased Volume for Ponds: the greatest impact of the new rainfall criteria is the need for more
on-site detention. Depending on the physical constraints of the site, providing additional volume
may or may not be a significant impact
Off-site Flows: currently new sites are required to safely convey offsite, existing 100-year off-
site flows through the site. Hydrology updates for the basins may result in offsite flows
increasing, especially if upstream detention ponds are overtopping.
Street Capacity: peak rainfall intensities most likely will not increase, therefore it is not expected
the peak flow rates from sites will increase. This means peak street flows are not expected to
change and no increase in on-site infrastructure, such as storm sewers and inlets, is expected.
Floodplain Development: revised hydrology of the basins may show an increase in the existing
condition floodplains due to pond overtopping. Until the basins are re-evaluated, base flood
elevations may not be accurate.
Implementation: We will be presenting our proposal for implementation of the new rainfall
criteria at the open house. We are currently evaluating various alternatives for applying the
criteria to projects already in the City process and would like input from our customers.
2
•
Open House
City of Fort Collins
Precipitation
Study
•
November 30,
g 1998
g 4 pm to 6 pm
• an
WHY DID WE
STUDY
RAINFALL ?
improve quality of life for citizens by providing cost-
effective drainage facilities based on well-supported rainfall
information.
Last study done by the National Oceanic and
Atmospheric Administration (NOAA) in 1973.
Extreme storm in 1997affects the rainfall statistics.
* Could not wait until NOAA's revised study to be
completed in 3-5 years.
* Needed updated rainfall information to update master
plans and construct high priority projects.
The City was in the process of updating the current
design criteria.
Citp of Fort Collins
CURRENT .�
CRITERIA
City uses 100-year storm as the design standard.
Current design storm rainfall is 2.89" over a 2-hour
period (based on 1973 NOAA study).
The design storm is used to map floodplains and to
design new and retrofitted drainage facilities.
. City of Fort Collins
Rainfall Stud y
Results
Regional Analysis:
Utilized data from Boulder, Longmont and CSU
gages.
- 3.67" over 2 hours (27% increase).
- Peak intensity remains at 9 in/hr.
At-Gage:
- Utilized data from the CSU campus gage only.
w
- 4.37" over 2 hours (51 % increase).
- Peak intensity increases to 12 in/hr.
Rainfall Pattern Analysis:
- Appears to rain more often on west side of the
city.
- More total rainfall appears to occur on the west
side of the city during long duration storms.
- Not sufficient data to determine if short duration,
high intensity storms produce more rain on west
side of the city.
(,tity of Fort Cnllin
TA SK ♦_ '�
FORCE
The City formed an Advisory Task Force consisting of
representatives from regulatory agencies, technical experts, and
private citizens. The Task Force provided advice and guidance on
the technical analysis.
Members:
Susan Hayes City of Fort Collins Utilites
Robert Smith City of Fort Collins Utilites
Marsha Hilmes City of Fort Collins Utilites
Duane Boes CSU Statistics Department
Gfom Browning Colorado Water Conservation Board
Rex Burns Larimer County Engineering Department
Lisa Dunn Larimer County Engineering Department
Bob Davidson City of Fort Collins Planning & Zoning
Nolan Doesken CSU Atmospheric Sciences Department
Dave Frick Ayres and Associates, Inc.
Don Heyse Citizen Representative
John Liou FEMA Region VIII, Denver, Colorado
John Morris CSU Facilities Department
Dr. Tom Sanders CSU Civil Engineering Department
4 VRC, Inc. City of Fort Collins Consultant, tech. support
Oli Sveinsson CSU graduate student, tech. support
Cih•of Forl Coltim
FINAL
RECOMMENDATIONS
Adopt 100-year, 2-hour design
storm of 3.6721
.
* Adopt uniform rainfall across Fort
Collins.
Install additional recording rain
gage sites.
City of Fort Collins
•
� 4 WHY DOES
THE CITY
RECOMMEND
3. 6 7" RA THER
THAN 4. 3 7"?
First 50 years of NOAA data lost; using a
regional analysis increases the amount of data;
thus, more data means better statistical
accuracy.
* 1997 storm rainfall value at CSU was used
"as-is" rather than as an outlier; this is very
reliable data which increased the final value.
Based on past 100-years of data, no
significant shift in storm magnitude has
occurred, which supports a regional analysis.
* Analysis showed it rains more often and
longer on west side of town, however, there is
no data available to draw conclusions about
short duration, high intensity storms used for
design. Installation of rain gages will provide
. this data.
City of Fort Collins
NOT EVERYONE
AGREES WITH THE
CITY
RECOMMENDATION
y
Minority group within the Task Force
disagreed with the 3.673' recommendation; they
recommended using 4.37" and a higher rainfall
amount on west side of city.
* Minority believed topographic effects of the
foothills produce more rainfall on west side of
city, and the city is in a preferred storm track
producing more extreme rainfall events.
City of Fort Collins
IMPACT OF
INCREASED
RAINFALL
* Runoff volume increases.
* Basins with detention affected adversely by
ponds overtopping, thus increasing downstream
flow.
* Basins without detention see very little
increase in peak flows since peak intensity did
not change.
Cost to build some drainage facilities will go
up. Increase will vary depending on the site.
Cih of Fort Collins
z Land
s
Development
Impacts
* Off-site flows may increase.
Street capacity is not expected to change.
Floodplain development - BFEs may not be
accurate.
Implementation issues - retrofit and Larimer
County coordination.
City of Fort Collins
Land
# 4
Development
Impacts
Pertaining to Detention Ponds
* Release rates for ponds may not be impacted .
* Increased volume for ponds.
Ponds maybe deeper.
Additional earthwork costs.
More land needed for ponds.
Possible increase in inlet pipe size due to
higher ponding elevation.
Cib•of Fort Collins
PROPOSED -
I MPL EMEN TA TI O N
3 L�
Currently using 3.67" rainfall for design of City
projects.
Development projects currently under City
review will not require redesign.
* A date will be established after which all
submittals will be required to design with the new
criteria. This date will become effective before
Council approval.
City of Fort Collins
_ WHA T'S
z
NEXT?
x
x
tt
hi
i
An ordinance will be taken to Council in
early January.
Full adoption is expected by January 31 ,
1999.
City of Fort Collins
THANK YOU
FOR YOUR
TIME AND
INTEREST!
We would appreciate it if you would share your
thoughts and concerns with us so we can try to
address them.
Please fill out a comment sheet - either here or
take it home and mail it back to us.
City of Fort Collins
•
SummaryImpactof Costs
Ridgewood Hills First and . Second
Filings
• Total Site Acreage = 122.43 acres
• 215 Lots
• 7 Detention Ponds
• Total Cost Increase for Development = $25,200
• Total Cost Increase per Lot = $117
• Using Lot price of $4.00/square foot
filef Pml Cdlin
Summary of Impact Costs
Stetson Creek First Filing
• Total Site Acreage = 18.29 acres
• 44 Lots
• 2 Detention Ponds
• Total Cost Increase for Development = $7,032
• Total Cost Increase per Lot = $160
• Using Lot price of $4.00/square foot
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City of Fort Collins
Precipitation Study
The City would like to hove your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn: Susan Hayes
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City of Fort Collins
Precipitation Study
The City would like to have your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn:: lSusan Hayes `
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City of Fort Collins
Precipitation Study
The City would like to have your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn: Susan Hayes
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City of Fort Collins
. Precipitation Study
The City would like to have your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn: Susan Hayes
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f�'r
City of Fort Collins
Precipitation Study
The City would like to have your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn: Susan Hayes
\oc& Q��
13(N) c
�c3 a7G 9—
• City of Fort Collins
Precipitation Study
The City would like to have your comments. Please provide them below and leave
here or mail to the address below by December 7, 1998.
City of Fort Collins Utilities
Water Field Operations and Planning
700 Wood Street
Fort Collins, Colorado 80521
Attn; Susan Hayes
oreC hu�
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usan Hayes- Must have 5.5 inch design Page 1
From: <Dkgkkkbk@aol.com>
To: wu.usc(BSMITH)
Date: Mon, Nov 30, 1998 1:19 PM
Subject: Must have 5.5 inch design
Mr. Smith,
My name is Dolores"Ann" Kiley. I own my home at 1813 Corriedale Drive,which
was flooded in July of 1997. Because of illness I will not be able to attend
the open house forum this afternoon, November 30, 1998, but wish to make
known to you my feelings and comments. Plesae print this message and place
with any other written comments you gather at the meeting.
You have in your possession a letter dated 5 November, 1998 from the Brown
Farm Fairbrooke Stormwarer Subcommittee. I won't reiterate its message in this
E-mail, but cannot urge you strongly enough to take to heart the studies and
conclusions given therein. For the Fairbrooke channel, Brown Farm detention
pond, dam and spillway to be designed for any less than the 5.5 inches/2 hour
rainstorm value would be a serious disservice to the citizens of this city,
and most especially to the residents of the west side of Fort Collins,where
we have already had more than that amount of rain and which caused great loss
to so many. It would be foolish to think that a storm of that magnitude (or
greater)cannot happen again.
If you don't think the entire city needs that much protection, at the very
least you are aware that the west side does get heavier rainfall than the rest
of the city and should get consideration of that fact and proper potection as
a result.
I know that you and the other final decision makers on this matter
must consider cost, and builders and developers are most certainly considering
possible lessened profits. Did any of the afore mentioned live in the flooded
areas and suffer terrible financial and personal losses? If not, perhaps you
and/or they don't understand first hand what is like to see storm clouds
gathering and wonder if it is going to happen again. I can only speak for
myself when I say that at age 65, my security and financial investment is
almost entirely in my home.To think that the personal and financial losses of
July of 1997 could happen again to the same or even greater degree because
the proper steps were KNOWN but NOT TAKEN by this city......well, that thought
is unbearable. In the not too distant future I might need the proceeds from
the sale of my house to cover medical care. I would want to be able to say,
"Yes, this house was flooded in 1997, but the city has since taken steps to
better protect this area and it hasn't flooded since."Will I be able to say
that? Or will most of my money be tied up in a home unappealing to buyers
because of flood potential???
As regards the cost, we will ALL pay for the improvements. I think most of us
who suffered losses to the flood know and accept that improved and proper
protection does not come free of cost. Do the builders and developers
concerned want profits at the peril and expense of their neighbors? If they
do, then perhaps they should build and develop in some other city.
Fort Collins is called "The Choice City". It is touted as a great place to
live. Maybe that description is a bit overstated. It is a nice place to live,
and will be a great place to live if its officials and administrators
demonstrate that they consistently and truly consider the safety,welfare and
:usan kayes- Must have 5.5 inch design Page 2
input of its citizenry, consider special needs and act on these accordingly.
. To do less or otherwise rests on the conscience of those who falter in these
responsibilities.
I repeat, please print this and add it to the written comments you may receive
this evening at the open house forum. I regret that I cannot be there this
time.
D.Ann Kiley
1813 Corriedale Drive
Fort collins, Co 80526
(970)224-5876
Dkgkkkbk@aol.com
i
Utilities
light &power stormwater wastewater water
of Fort Collins
December 14, 1998
Mr. and Mrs. William H. Wilson
1836 Corriedale Drive
Fort Collins, Colorado 80526
Re: City of Fort Collins Precipitation Study
Dear Mr. and Mrs. Wilson,
Thank you for your letter received on December 7, 1998. I appreciate the time and effort
you've taken to share your thoughts and opinions.
At this time Council is scheduled to consider the adoption of the increased rainfall early
in 1999. Exact dates have not yet been determined, but I will let Don Heyse, the
Fairbrooke Stormwater Subcommittee representative, know the final schedule. Your
comments will be forwarded to Council and you are welcome to personally provide
comments to Council during the public comment portion of the discussion.
When the revised rainfall has been adopted, final design of the Fairbrooke Heights
detention pond will continue. The City will continue to seek input on the final design from
the Stormwater Subcommittee.
The process of reevaluating our rainfall rate and recommending a higher standard has
been challenging. Not everyone agrees with the City's recommendation of 3.67" over
two hours and we respect that. But the process used to study the issue was open and
fair. Input was obtained from experts in the field and a wide variety of issues were
discussed and analyzed. We believe the increase to the 3.67" rainfall is a reasonable
recommendation based on all the information available, and it will improve the quality of
life of the citizens of Fort Collins.
If you have any further questions please feel free to call me at 416-2233.
Sincerely,
Susan L. DubP.E.
Senior Stormwater Engineer, Master Planning
Cc: Council
John Fischbach, City Manager
Mike Smith, Fort Collins Utilities General Manager
Bob Smith, Water Planning Manager
A Wood St. • P.O.Box 580 • Fort Collins,CO 80522-0580 • (970)221-6700 • FAX(970)221-6619 • FAX(970)221-6593 • TDD(970)224-6003
e-mail: utilitiesrrci.fort-collins.co.us • www.ci.fort-collins.co.us/UTILI77FS
UUl-v0-QU I VL: UJ-Cv All 1 .
t •••• -•_ _ 6b3 bVUMIL
John F. Fischbach
M.Sotith/W.Williaxs
(Please copy Council on response.)
RECEIVED
City of Fort Collins Precipitation Study DEC 0 7 1998
CITY MANAGER
To:
Water Field Operations and Planning
700 Wood Street
Fort Collins,CO 80521
Atta: Sxwm Mayes
From:
Mr.&Mrs. William K Wilson
1936 Corriedale Drive
Fort Collins,CO 80526
F)rJL uu w6 tiVGuiti la5a iG Say tu9i we iiLlfil `Atlas-i SllOte Culunii7YLn3c Sht7eiidbt USBd W$tn yUit
disrupt the normal flow of water that has served this area for a long time. The Fairbrooke cbatrael
was build to hold a very large amount of water. In the twenty-tb=years we have lived here,
before the flood,we have not even had the water come uP to our back yard. This means that it
would have to over its banks,come up another So feet to reach our house,and then rise anothor
foot to possibility come into our house. If the channel was left to its more natural flow there
would have a lot less destruction and the city would not have to spend large amounts of its budget
to corns an obvious problem. I call this an obvious problem because it was presented to the City
and ignored.
Second we would like to support the 5.5 inches in two hours be adapted as the standard for the
100 year storm design. These standards am of particular need on die west side where most of the
heavy storms occur and to compound this they flow togetber to make an even bigger problem.
We would also Mce to say that no building shouts be done where radical changes to the flow of
water need to be done to open up more land for developmcntt especially along the foothills.
Sincerely,
T
irmu1MY•�rr�CM ,- .•ur n
cc:Bob Smith
City Council
Environmental News -4 - Winter 1998
Grassland Restoration Begins ' ,
By Karen Manci .
SThis fall, the City's Natural the past six years due to residential
ources Department began development. The Coyote Ridge
restoring 270 acres of the Coyote grassland restoration will eventually
Ridge Natural Area to native short- replace that lost acreage of habitat
grass prairie. The restoration site is for the prairie dog. `'la %
west of Taft Hill Road and south of Although initial seeding will be
Trilby. Over the next three years, completed in three years, prairie n^_��A„ Amounts
O�*�
native grasses and _wildflowers will specialists caution that patience is RainfaH L' m as
be seeded into fields that, prior to the key to achieving desired results.
City purchase, were in winter It could take up to 10 years or Dnve Stormwater
wheat production. Sowing tech- more before a good mix of native Design
niques will include drill seeding and plant species is thriving on the site
an experimental process involving . Costs for seeding and.weed
goats. management during the first three By Lori Clements-Grote
The goal is to create a diverse years of the project are estimated
grassland capable of supporting at$145,000. The Fort Collins City Council
prairie dogs. In the Fort Collins Karen Aland is a senior environmen- will decide soon whether to revise
area, about 300 acres of prairie dog tal planner with the City of Fort the rainfall amounts used to design
colonies have been destroyed over Collins Natural Resources Department. stormwater projects.
City of Fort Collins Utilities
recently conducted a precipitation
study to see if the rainfall amounts
Get an Engine
� used to design stormwater projects
•; .; were accurate. Based on the study
�-� results, the Water Board approved
3.67 inches
Block Heater; a higher rainfall rate —
(� over two hoes — to use for storm-
Get a Rebate water facility design.
The study is one part of the
Utilities' response to the Flood of
By Aaron Fodge 1997. Citizens have urged the City
The City's December breath- is hard on its engine. An engine to improve the level and tinning of
ing lesson promotes engine block block beater will warm the coolant flood protection improvements. In
heaters. An engine block heater is and oil. Therefore, the engine response to citizen input, the Utilities
an electronic device attached to, Or avoids a long warm-up period, and developed a citywide plan for
inserted into, an engine block When the heater works instantly. financing,prioritizing,building, and
plugged-in, it warms the engine Think Ahead managing stormwater improvements.
block components, relieving engine The results of the study could
Now is the time to install an impact the cost of existingand
stress and greatly reducing air mP
engine block heater. There are five future storm drainage projects.
pollutant.emissions. models: 1) frost plugs, 2) radiator
The City will discount the Master plans beach of the city s
installation and/or purchase of hose heaters, t immersionflid tank 13 stormwater basins must be
circulation heaters, st fluid reservoir revised before the full impact of the
engine black heaters at participating p
heaters, and 5) dip-stick heaters. revised rainfall amounts are known.
automotive service and parts shops. Ask your local auto slap for the
The rebate program, expected to For more information contact
model(s) to fit you car. Susan Hayes, 221-6700 or TDD
trunthrough March, will be funded Ya $4000 EPA grant. Think Responsibly 224-6003.
Think Toasty During "cold start," emissions Lori C/emems-Grote is o marketing
of unburned hydrocarbons (HC), analyst with the City of Fort Collins
When outside temperatures (Continued on Page S) Utilities.
close to freezing, starting a car
Study: Larger drainage gystem i
-.w;:. 1 . iFlCs.3'wi l;.vtt_ r a•CrtKi�S.al'.h.dnsl'faa!NirnrfJ,l n:. • :e M,
wouldn't prevent all flooding : r
By KYLE HENLEY systems which;are built '3,000 homes and business- iral data..
The colorsdoan to cant'2 89 inches of rain= es.
But some members of the
Expanding the stormwa- fall over a two hour period The federal government
(y task force : think: ::the
ister system 'to protect to handle 3.67.'inches in 1973 conducted the last stormwater s should
against an additional V4- during the same time, a 21 such study.The mbst recent be able to hold�ee�greater
�4 inch of rain could keep resi- percent increase: study re-evaluated the
e dents dry in 'flood-prone . City Council will consider numbers using data collect- flows tas wo
sa -an inches
_� parts of Fort Collins, but it the proposal some time in ed over the last'30 years highe�'� two hours and.even
would of
not be a secure de- January. The July 28, 1997,storm,' town, on the west stude'dy
fense against floods such as In September, council ap- largely considered a weath- town, where the, 'study
the one that devastated the proved a 21 percent increase er anomaly, dropped as
showed the most rain falls
city in 1997. in stormwater utility fees to much as 6.5 inches of rain in "We want to pick a num-
A new rainfall study fimd more than $60 million a two-hour period.• ber that really is going to
showed the capacity of the in citywide flood-control Building a system to ban- protect us,"said Don Heyse,
city's stormwater system projects dle such freak storms also a member of the task
needs to be increased to at It could take six to 18' wouldn't be fiscally prudent, for0&
least accommodate an extra months to figure what it will said Dave Frick, a member Residents atten an
3/4-inch of rainwater during cost to upgrade the city's 13 of a task force that oversaw open. house on the ssue
a two hour period:. storm basins based on the the study. ' Monday
"Me higher your rainfall new recommendations of 'The risk of those bigger y agreed.'
number goes to, the greater the precipitation study, events has to be balanced 'This keeps the numbers
your protection," said Bob Smith said. against the cost of designing low,and We not going to be.
Smith, water planning and The city commissioned for those events,"Frick said. adequate," said Vivian
development manager for the study on the heels of the 'But we didn't want this to Souza, whose west Fe
the city. 1997 flood that killed five be an economic number. Collins home sustain%,
The study recommends people,injured 54 and dam- This was about study the more than $30,000 in dam-
the city beef up flood control aged or destroyed nearly (rain)gauges and the histor- age during the 1997 flood.
C.
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EXHIBIT E
WEATHER ARTICLES
COLORADO
.gin: kat�n.
"i'^''` ayi'.!•�1a CLIMATE
nihl
' �iitfiM
:
I...
i�.i•:PiY _
ry e:.-fir ... "ML - AM Y 1996
Volume 19 Number4
Colorado climate Center
;•S'". Dopartment of Atmospheric Seienee��ifs.. ....... .. .. ,`
pT.2ww�:w:
Colorado State Unberslly
fort Collins,CO u057S M nrpon hu been prrp inrd each worth since F�braary J977 viA the aapp�
- 5 ofd+ColoradoAgricatNrai£ryeriu,enrStaoon and A-Corky,afEnri, rim
The I00-Year Storm — Do We Really Know What That Is?
The term"100-year storm"has been used by hydrologists, statistical meaning misuse the term. Our misuse propagates
climatologists and civil engineers for many decades. It has a swiftly as more and more of us are impressed with the attention
precise meaning based on statistical distributions of observed the tam seems to attract
rainfall from official weather stations with many years of
consistent data(,vet another reason why it is so important to So what is the correct meaning of the term"100-year storm?
maintain our cooperative weather stations across Colorado The technical definition is"the greatest amount of rainfall .
and across the nation). The 100-year storm also has many during a specified length of time that has a 1%chance of
brothers,sisters and first cousins. The 50-year,25-year,10,5 occurring in any given year." Interpretted slightly differently,
and 2-year storms,and everything in between,arc also computed the 100-year storm is the heaviest rainfall that should be
from observed data_The amount of precipitation associated with expected to fall sometime in any 100-year period.
a"100-year storm" and its smaller relatives,is very important
in the design and construction of everything from the gutters on A very important element in this definition is the length of time
your house to the sizes of bridges,culverts and storm sewers and during whih the rain falls. The specified length of time is
much more. Many professional engineers in Colorado and known as the storm duration. Tbe.magnitude of rainfall
across the country make their livings applying their knowledge associated with a"100-year storm"is a function of storm ..
•of how water flows and collects during heavy rain to design and duration. The longer the period of time,the greater the
build our nation's infrastructure so that it can withstand heavy precipitation. No surprise there. Here is an example for
rams up to and,for certain structures,exceeding the"100-year Limon,Colorado:
storm."
100-vear rainfall estimates for specified storm durations
A great deal of misunderstanding has developed regarding the
"I00-year storm"and its relatives. Over the course of the past Limon,Colorado (from NOAA Atlas 2, 1973)
20 or 30 years,(or maybe longer—I haven't been around long Storm duration Rainfall amount
enough to know for sure)the"100-year storm"bas gone from 30 minutes 2.16"
being a technical term used only by professionals who have 1 hour 2.73"
strong backgrounds in engineering or statistics to being quite a 6 hours 3.60"
common term used freely in daily conversation by about anyone 24 hours 4.50"
who is interested in the weather—and there's a lot of us in that
category. If you were to eavesdrop down at the neighborhood An additional factor also comes into play that can add a great
coffee shop the morning after a heavy summer thunderstorm deal of confiWon. That is the area associated with the heavy
moved through the area,there's a fair chance you'll hear rainfall. Let's use an example here. It may be fairly common
someone say something like this. '"What did you think of that for a single location or weather station to receive two inches of
storm last right? I heard that up north of Fred's place had a rain in six hours. That may happen once every two.or three
100-year storm." In common usage,it seems to be taking on a years,on average. At the same time, it is extremely rare for
broader meaning. In fact,it's modem definition has become several counties to all get two inches in six hours from the same
"any storm or flood that is bigger than what any local resident storm A large area of several contiguous counties may see ow
remembers,"or even more loosely, "the heaviest rain in a long inch of rain in 24 hours as often as a single station gets three
time." Even some of us who know its technical definition and inches in six hours. Arc you following? Put this all together
Inside this Issue
April 1996 Weathe Summary r ._—_.___.__.._._._._44 Comparative Hating Degree Day Data.—__._.—.____.__..__—_49
May 1996 Wotha summary_.__---_-- --»--45 April 19%Climate Daiz..._.__......__._._....__ 50
Temperature Campvisom- April-May%._.._-._._._ -__.._.46 May 1996 Climate Data.._.._._.-........._...—_—..____._._—._.w 51
April-May 1996 Precipitation Comparison 47 Solar Radiation-April-May 19%,.___.— 52
19%Watc yearPrecipitation_...__._�__._._-_—_�.._..r.48 Sot?Temperature- April-May 19%. —••_ 52
40
and what you discover is that the 100-year storm and all the rain may exceed 3 inches. Deaver's greatest 24-hour storm was
smaller brotbers,sisters and cousins are not a single storm but, back in May 1876 when 6.53"was measured. Y
rather,a whole family of numbers that vary as a function of the ,
duration of the storm(tbe longer the duration,the larger the Engineers,hydrologists and climatologists traditionally have
rainfall totals associated with a given return Period) and also prepared lots and lots of maps for different combinations of
vary depending on the arcs of the storm(the smaller the area, storm durations and return periods based on all the historic
the larger the rainfall total associated with a given return precipitation data available. Here are a set of examples for
Colorado taken from the'Precipitation Frequency Atlas of the
Western United States—NOAA Atlas 2,Volume III "
Now you're saying"Wait a minute,what do you mean by re (Colorado}"While this was published 23 years ago,this
emod 7 Where did that term come from?" The return period remains the single most widely relied-upon source for
refers to the likelihood of occurrence. A storm that only bas a determining design rainfall for specified return periods and
1%chance of occurring in any given year has a return period of storm durations.
100 years,while a storm that has a 50%chance of occurring is
any given year has a 2-year return period. We humans tend to ll IO it 11 I7 13 14
1t 7_
interpret return period to be the likely interval of time between TbR' -_ Ya>G '
storms of that intensity. Therefore.if we received a downpour I:e <
tomorrow that was technically equal to the 50-year storm,we
would rand to breathe a sigh of tclief and say, Whew! Now vve 2 .f ' ;-' !
NL _ _
won't have to worry about getting another storm lice that for 50 t
more years.` in truth,the chances remain the same from we
year to the next that we could get such a storm—2%chance is
any given year. We might have another storm of that ar sir the
very next ye or maybe even the next day,but over a very long to c
period of time(decades to centuries)we would only average one
such storm per 50 years \ 011
},t
The"100-year storm'and an its smaller(and larger)brotherand sister storms are not great mysteries. They(the depth of • - y�''" — __:
rainfall for a given point)can be calculated with considerable
confidence for any location for which many decades of accurate \ tt
historical precipitation data are available. Let's take Denver _. :
and Pueblo,for example. Each location has well over 100 years \ 12 '- ttf*, _
of precipitation records(don't mention the fact that each
weather station has been moved a few times during its history} n
To determine the 100-year storm for a storm duration 0f24-
n
hours,what has been done traditionally is to go through all the
Colorad )s of S-yea 6-tsour precipitation in reniln of an infi for nardr earril
daily precipitation totals for each station and select the greatest
daily total each year. Each of close annual 24-hoar extremes are Colorado
then ranked from lowest to highest Some type of extreme-value
statistical distribution.a simple graphical curve fit,or empirical I! 14 ti 0 20 22_ 24 24 22 72
probabilities arc then applied to relate the rainfall amounts to
particular return periods
412t
You can see that in most years,the maximum one-day eci its-
lion at these locations is between I and 2 inches. Occasionally,
the heaviest rain all year is only about 0.50" but occasionally ^
MAXIMUM OBSERVED 24 HOUR PRECIPITATION
w B —�-- DENVER -•.— PUEBLO �:3 :s _tb `--1t _ __'c• sc�t. �� I t sr,,_
0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.? 0.8 0.9 1 taoptuV"of 5-yea 24-hots prWipintion in tenths of m loch far north
PROBABILf1Y m tort Colorado.
41
121 lb 18 20 22_ 28 26 28 36 some 104,000 square miles,it is possible in any given year that
— i —- — • ha 1%of the slate's area may experience a 100. storm.
- - ,t - -, i ; _ perhaps Y � Year
. That means about 1000 square miles may be impacted by a 100.
28 - year storm in any given year-on average. Due to the common
._- size of Colorado's convective storms,it is easy to imagine that
'a I_=x \ '_ '' °• ` most of these storms would drop their heaviest rains(the ones
that equal or exceed the"I 00-year storm")over areas of 10
24; _- - ' li.+ •• square miles or less. What I'm getting at is that it is totally
8�i•. -
normal,over a state the size of Colorado,to experience many,
many storms each yen that locally equal or exceed the rainfall
" 8.•,21 ? y�� associated with that location's I00-year storm. I do not know
.16 the precisie number,but I could easily be convinced that there
are between one hundred and three hundred 100-year storms
somewhere in the state of Colorado each year. This climatic
reality makes total sense,and yet we are tempted-even those
7. n - ' of us who know exactly how the 100-year storm is defined-to
get all excited and start hollering,"The sky is falling,the sky is
falling,the climate is changing,blab,blah,blah....We had ten
100-year storms last year right in our own counryl The sky is
falling!!...." We get all worked up,because we forget that the
100 ear storm,as we common] apply r to a rat"
., � r,"--• Y o�, Y PP Y�t.P� �•
- -�21� .�-' 283
InM .,¢4;2^ ����\\.t:�; There are so many points in Colorado that OF COURSE we are
going to have lots and lots of 100-year storms.
Isopluvials of lo0-yw 64 precipitation in tenths of an inch for north
central Colorado. If we wanted to try to clear up some confusion(while probably
adding some of our own),we could specifry the"100.year
IB26 11 1 16 20 7 34-•-38 -12-36-_= storm"as the maximum point rainfall with a 1%chance of
,, - T 4— r, - t occurring in any given year anywhere within a specified area-
1 s i' ` perhaps a county,a region,or the entire state. What we would
.� I I find is the 24-hour point rainfall with a 1%chance of occurring
YYY ,�.i-- 7h - �_ i t ! in any given year and onywhe!e in the store of Colorado is
,••w-ea - S 4Bs I I ,�, �' probably somewhere between 12 and 14"-a true deluge. This
'-•` • ; s - is much different than the 100-year 24-hour rainfall amounts for
a+s ; �- _ specified points that range from 2.50"for parts of western
Colorado to more than six inches in southeast Colorado.
' - -�� `.'�. `•''_ - I-._� j I'm going to end here,and you're probably more confused now
21
= _ 1) ! than when you started reading. But this is only the tip of the
i 76'-- '' :.- { `� iceberg. We could get even more confused by comparing 100-
_� c `3ce%.:_3^•._ I s - I year storms with 100-year floods. Logic would tend to make us
r L,.
m-�� •" • �� � :.; •.� _ _! %_ t believe that a ]00.year rainstorm ought to cause a 100.yeat
; i i flood,but due to the amazing complexities of rainfall-runoff
relationships,it doesn't usually work that way unless the
watershed you are looking at is a paved parking lot
Isopluviak of 100-year 14-hour precipitation in tenths of an inch for north
central Colorado.
The important thing to remember when looking at these maps,is
that these are all based on individual point rainfall data from
weather stations scattered across the area. Therefore,the maps
are showing rainfall totals for specified storm durations and
return periods for points. This becomes a major source of
confusion for interpretting reports of very heavy rains. Here
in Colorado,and in many parts of the world,the heaviest
rainfalls tend to fall from summer-time thunderstorms. Some of
these storms drop very heavy rains.but these rains may only
IWect areas of a few square miles or less and may not fall in
cial precipitation gauges. In any given year,many heavy •-
storms occur scattered about the state. Since Colorado covers
42
f
Winter 1995.99
tt
y
r o
univeritT
z
' 2 n .ic T;o r A 1 u m ❑ i and Tr f e n d s
a
_ r r
t .4�
__ FATHER
y t do we really know?
Society of giving
} Sh 's/got game: Unstopp Zle
Becky Hammon r
tv
a
1. yr✓:
I
� 1
1,
E3 c)
. . . what do we really know?
41 DtWid J1'(Yri1tM
TORNADOS RAVAGE NHDb1TST! DROUGHT, FIRES U'RM HAVOC! HURRICANE PUAMIEIS
COAST! NM D MNTEIR COLD SPRING! GLOBAL WARMING COULD NIFL.T POLAR ICECAPS!
Those aren't tabloid lheadlines,but something very much like them appeared in print,and similarstotics
acere broadcast, all across the nation this year. In Tact,for much of the year,weatlici erents dominated our
.national ps)chc. Lcerynne talked about it, complained about it, and wondered about its causes. We all
learned a nchc name El Nino, the demon of global e'eadier systems. Lately, it appears that (It(- threat of El
Nino has ebbed but new reports talk of somethingcalled La Niria.
This has been a terriljiug and suange year for wearer.Hurricane Mitch capped off an already horrific
hurricane season by turning into one of die deadliest storms ever.Latest estimates lime risen to as many as
11,000 dead. In China, the Yangtze River burst from its banks taking more that 400 lives' leasing nearly a
million otlhers without %better. Aberrant weather patterns brought the worst drought in 50 yeais to thc•
southern United States. In Guadalajara, snoic fell for the first time since 1881.
People'arc asking,"Meat s going on?"We talked to some of Colorado State University's weather experts
in the Deparuncnt of-Atinosphedc Science to get some answers.
r ,
jay
Y
l
t
Y
touched down on the Great Plains,it
` might have disturbed plan tedfields,
destroyed sod dwellings or fences,
perhaps even killed livestock or a
homesteader.Chances are,however,
that it formed,destroyed,and dissi- r
Is weather becoming more severe. •,r
It may be,but it appears that nature isjust pated unseen.Today,a touchdown
doing what comes naturally,according to Colo- is likely to affect people and prop•
rado State scientists.That is,climate is always erty,and millions will see the dam-
changing.This maybe due tohumanacdonsor age on television or read about it in i
itcouldbe natural iluctuationsin the climate or newspapers.
a combination ofboth.Regardless of the cause, Steven Rutledge, an atmo-
Richard Johnson,a professor of atmospheric spheric science professor who studies light- may be because there's a lot of media attention
sciences who studies Pacific phenomena such ning, agrees with Johnson. Says Rutledge, "I paid to such phenomena. There are more
as El Nino, believes media fascination with think a big pan of this perception of more people living closer to the coasts,so hurricanes
severe weather may affect our perception. severe weatheris ourability to observe through may seem to be getting worse, but it's the
"Some people say there have been more mass media. The public is more savvy about population density, not the weather, that's
extremes in the weather in the last decade or severe weather." changed."
two,"says Johnson,"but there's no agreement Hurricane expert Michael Montgomery, What about global warming?
an associate professor,has a simple answer to
and certainly no scientific consensus on that. Some scientists suggest that global warm-
Instead,there are more people,they're more the question of whether weather is increasing ing may be causing more severe weather.The
affected, and they're informed much more in severity.No. problem with thatassumption is that there's no
quickly about severe weather.And when bad "I don't think therc'sanydiscernabletrend general agreement on whether the climate
�at^heroccurs,theysuffermorepropertydam• toward an increase in severe weather,"he ex- actually is gettingwarmer,and noagreementat
plains."]n specific areas,severeweather—for-
age. all on whether warming is a by-product of
One hundred years ago, if a tornado nadoes,hail,hurricanes—does occur,yes.This human activity or pan of an asyet-unrecog-
fda odoSWr Uni.rnin winwI M99 7
nized natural cycle of climate change. So what does science know about weather severity and global
The Office of Science an d Technology Policy,which advises the warming?
White House on scientific matters,says the mean global temperature has "The climate models we have now are rudimentary,"VonderHaar
risen about I degree Fahrenheit during the past century and attributes says."We believe the best of them but need to recognize theirweaknesses
it to"the greenhouse effect."That phenomenon occurs when green- —they don't take detailed variations of clouds into effect,for example."If
house gases—carbon dioxidecItief among them—are released intothe greenhouse warming occu rred,it would produce more clouds that coul d
atmosphere by burnin g fossil fuels,for example.These gases trap infra- reflect more sunlight that woul d offset warming...
red radiation that warms the atmosphere.Measurementsofgreenhouse Roger Pielke suggests a historical perspective. No fan of media
gases and temperatures at the earth's surface for tens of thousands of reports ofglo-
years show a close correlation,an d estimates by the federal government, bal warming,
sciendficconferencesconvenedbytheUnitedNadons,andothersources he says, "The
suggest that mean surface-air temperatures could increase bymore than climate could
6 degrees Fahrenheitbytheyear 2100. coolorit could
So?Is that true?
"Ask me in 10 years,"says S
Johnson.Hisworkon climatevan-
zbility and ocean-atmospheric in-
teractions tells him that climate
is incrediblycomplicated.For in-
stance,data suggest strong,west- q
erly bursts of wind can in some Looking HE USGS vegetation
cases produce El Nifio. Says
Johnson,"Thesewindbursts can
,lead to wave disturbances,an up-
pe r ocean surge that can trigger
an El Nino,but the processiscom- ' 3
plex.Vde don't know,at this point,
why some wind bursts trigger El
Nino and some don't"
Rutledge,who investigates
lightning among otherweather phenomena,agrees that the jury is still
out,but he believes lightning might hold a clue.As the planet warms,
,
there should be more lightning since heat helps evaporate water that
forms clouds,and more clouds should mean more lightning.
"If that's the case,"says Rudedge,"lightning could serve asa kind of
global thermometer,but the record(regardingbothclimatewarming
and electrical storms)is so short that we can't predict any trend as of
now."
That doesn't mean there's no need forconcern.Thomas Vonder
Haar,who founded the Cooperative Institute for Research in the Atmo-
sphere at Colorado State,points out thatjust because you can't prove
global warming is going on right now doesn't mean it isn't orwon't
happen.To do that wouldbe to ignore some indisputable facts.
"We've known climate has changed on the planet before and will
again due to natural causes,"says VonderHaar."Now,there are anthro-
pogenic(manmade)influences as well."
3 Umado Sm tfinemry Wt 1998.99
1
y ,
L Digging for evidence
of global warming
` -- A lot of researchers looking at global warming—
humancaused or otherwise—seek answers in the
atmosphere and oceans.At Colorado State,however.
some scientists are more down to earth in their quest
for indications of the consequences of global
changes.
Dennis Ojima(below),senior research scientist
at the Natural Resource Ecology Laboratory,heads a
c project that examines the ecosystem response to
global changes in the Great Plains and the steppes
_ of China and fJlongoka.
"We're looking at the impact of climate
variability on an ecosystem's uptake and release of
carbon dioxide;"he said."We're also trying to
determine how sustainable resources are under
current and future land-use systems:'
-- Ojima is trying to understand what people need
to know in order to use their land wisely and assure
._ . �� themselves of an adequate food supply.Land use,he
•j • ...w ,: . #��,tr ��•�.�: r'- ;.'_• :T:;�j.:.yr,_.••I` noted,can affect how much sunlight is reflected back
•': �� t!• ,!; + •• • ' •�t— Into the atmosphere,how much carbon dioxide is
r •,•'-• - �' —
. r•-• —�� absorbed or released,and how much evaporation
takes place—all processes affecting greenhouse
warming and local climate dynamics.
� � 1