HomeMy WebLinkAboutCOUNCIL - AGENDA ITEM - 03/16/1999 - SECOND READING OF ORDINANCE NO. 42, 1999, AMENDING AGENDA ITEM SUMMARY ITEM NUMBER: 31
DATE: March 16, 1999
FORT COLLINS CITY COUNCIL STAFF• Mike Smith/
Bob Smith/Susan Hayes
SUBJECT :
Second Reading of Ordinance No. 42, 1999, Amending the "Storm Drainage Design Criteria and
Construction Standards" by Revising the Rainfall Intensity Duration Curve Graph.
RECOMMENDATION:
Staff recommends adoption of the Ordinance on Second Reading.
EXECUTIVE SUMMARY:
Ordinance No. 42, 1999,which was adopted 4-1 on March 2, 1999,modifies 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.
• On First Reading Councilmember Bertschy offered two amendments to the Ordinance that were
included in the adoption of the Ordinance. The first amendment involved additional language
requiring the City to establish and operate a City-wide network of precipitation and stream flow
gages,and to use this information in decision making. The second amendment involved additional
language ensuring the Utilities General Manager has the authority to enhance drainage basin master
plans to provide for additional flood protection. These amendments are included in Sections 3 and
4 of the attached'Ordinance.4
Also added to the Ordinance between First and Second readings is a new Section 5. This new
Section clarifies that Section 1 of the Ordinance,which modifies the City's current family of design
storms, does not apply to any land development for which an application for preliminary or final
subdivision plan approval,preliminary or final P.U.D.plan approval, or project development plan
or final plan approval has been filed with the City prior to March 26, 1999,whether filed under the
City's Land Use Code or under prior law.
• 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 ofthe"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, I0-year,25-year and 50-year),and in doing so established aPrecipitation 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 storm frequencies,to be applied uniformly over the entire City; and
WHEREAS, the Colorado Water Conservation Board, Larimer 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 4.37 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"); and
WHEREAS, the City has also determined that in order to provide better flood protection
within the City that a City-wide network of monitoring devices be established, maintained and
operated by the City to measure and record precipitation and stream flow data; and
WHEREAS,this data would be used,without limitation,for emergency response,emergency
preparedness,planning and design of stormwater facilities in the City's master drainage plans, and
for the development of future storm drainage design criteria and construction standards; and
WHEREAS,Council also desires that this precipitation and stream flow data be used by the
Director under Code Section 26-543 as one bfthe factors the,,Diiector may use to make decisions
concerning enhancements to the City's master drainage plan so that such enhancements do not result
in any diminishment in flood protection to the public.
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.
Section2. That the Storm Drainage Design Criteria and Construction Standards,as amended
by Resolution 9144 and by this Ordinance,are hereby reaffirmed,readopted and reenacted by the
Council.
Y.1!
Section 3. That Section 26-543 (b)-oftlie Cod6ofthe CitiX
Fott,C011ins shall be amended to read as follows:
(b) The above master drainage plans are on file in the office of the
City Clerk. The city may adopt additional master drainage plans by
reference and declare them to be a part of this Article and copies of
such master drainage plans shall be on file in the office of the City
Clerk. TheAny of these plans may be modified by the Director, for
the sole purpose of eninuicing sack pianenhancement, provided that
such enhancement does not diminish the general purpose and specific
objectives of the adnpte&plan, wrd-does not diminish the ability of
• the plan to address the disposition of stormwater runoff in the
applicable basin, and4oes not result in a diminishment of-the flood
protectiaprovz�public in the plan; and if so modified, the
modification shall be filed in the office of the City Clerk. In
considering an&l iaknig any enliancem "�, o a master.-drainage plan,
the Direid r shal usesuch�reliable data and information that is
availabl'e 'to th '�Director, :,including„:without°`:limitation,. the
precipitation and.stream flow=data collected by the-zity through its
monitoringprograM established in Section26-548.,
Section 4. That a new cods Section 26-548 be added to Chapter 26 of the Code of the
'it Yof For Y; , 'ns`to read as follows:
Seca8! ) ectgitaft4 nd streamflowmonitoring
Subject',isgany need ' ropnatibn, th city shalll establish, maintain and
ope a cityvgccde.net E omomtoring devicesthakutlY,measure andeeord
prectlsttahon an�am3 The.data obtained from such network shill be
use v tkioutlirnitatio "ergency response,emerge ncyprepared.hess,pl tiing
an gnofsfiarmwatex faci3ihes inthe'city's master drainage plans,and fdr;the
ti a it @
de v "° orient oture sCaEmpyater design criteria and construction standards.
• Sec 5 P,Iiatthe dientstatheStom�rDramage-Li'signCnteria ;yonstiucbon
,Standard, fort ctt veshalLnot app}".to an ,land developme r,?-whiclKan
ti atio re o visioa,planappioval�prellmin Eor U.D plan
peal, ' ect opine o'r final'plan app�dval hasian fileth ity pnot to
a 26,, ed �Citq"s Land Use Code ottmder pnor law v knov¢ruas
foie Transits„n and ise,Re
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
ITEM NUMBER: 31
AGENDA ITEM SUMMARY
DATE: March 2, 1999
FORT COLLINS CITY COUNCIL Mike Smith/
STAF
ob Smith/Susan Hayes
SUBJECT:
First Reading of Ordinance No. 42, 1999, Amending the "Storm Drainage Design Criteria and
Construction Standards" by Revising the Rainfall Intensity Duration Curve Graph.
RECOMMENDATION:
Staff recommends adoption o the Ordinance orcFirstReadin`°
INANCIAL EVIPACT:
Assessing the specific economic impacts on-basin wick master planned facilities is not possible
without conducting full mast '"plan updates. FIbweveerr�we can make several generalizations can
be made about the effects of ceasing rainfall.
Increasing the rainfall criterion to 3.67 inches affects basins with detention ponds more than basins
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
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%
to 35%. .
Newly developing land is reqCedrayide proteclton forth, 100-year design storm, regardless
of the benefit-cost ratio. In general,impacts due to increased rainfall would be similar to the basin
impacts. The main impact of using the 3.67 inch rainfall criterion is 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. Two case studies showed estimated increase to the cost of a typical lot to be 0.5%.
BACKGROUND:
This Ordinance will modify the current family of design storms used by the City of Fort Collins,
1p 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.
Council discussed this issue at its January 12 and February 23, 1999 study sessions.
DATE: March 2, 1999 2 ITEM NUMBER: 31
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 reco `k ende&Council adoptlan it,rease in the 100-year design rainfall
criterion from 2.89 inches to 3_ 7 inche, applied uniformly or 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.
i ,
The current 100-year value used by tl city a de from the National Oceanic and
Atmospheric Administration AA)Atlas 2„published in 1 ,F3. NOAA typically conducts the )
precipitation frequency studies OO e United`States'guided by s6dard engineering and statistical
practices. Based on data up to 1969, the City adopted a 100-year design storm 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 storm:
• 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. bas on from @SU uge 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 storm 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: March 2, 1999 3 ITEM NUMBER: 31
iPrior 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
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 4.37 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"Wculatecruno f flows,map floodplains,and design
drainage facilities in new subdivisions and alreddy&veIbpe as. Increasing the design rainfall
criterion to 3.67 inches, or evete higher 4.3nches, will nprevent 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 runolf'°£iNs,,ass'umting it a`6asiiM fufty developed. The master plans
must show the overall bents to the tasm outwq he the cost of the improvements. In
some basins, 100-year protection,can.not b proanded sin the benefits of the improvements
needed to contain 100-year flowdo not oweigh""the costs. owever,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 Stomr"P3tsinage gn C' 'a r incb�orate additional items such as
analyzing the impacts of la ger story event lYel id". design of new developments and
provide information to futie home ayvners potential oblems, requiring controlled spill
locations on irrigation ditches thaYrun tiro newdeveloprribnts,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,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: March 2, 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, we site infdfmatkn, d oreIne�vspaper articles. This education
outreach will
ll include common ssens com ee such ow to grade lots to prevent local
flooding, ow to protect v bles in ease o fl• d cautio about what to do and not do such
as driving in water) during a flood.
• Flood proofmg and Flood insurance:the City continues to provide floodproofmg 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 in§ur�nce g lic"folderso a 20% discount on their insurance
premium.
• Maintenance of the stormwa' r syst a ify continues o 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 ssatiiy Aso but will, in conjunction with all the
other components of the sto ater pro z op damage and loss of life during all
storm events.
:y