HomeMy WebLinkAboutCOUNCIL - AGENDA ITEM - 08/28/2007 - RAINFALL STANDARD AND THE CANAL IMPORTATION PONDS DATE: August 28, 2007 WORK SESSION ITEM
STAFF: Jim Hibbard FORT COLLINS CITY COUNCIL
SUBJECT FOR DISCUSSION
The Rainfall Standard and the Canal Importation Ponds and Outfall Project.
GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED
1. Should staff prepare more detailed information on the rainfall standard for the October 23,
2007 Work Session?
• Yes— staff will prepare more detailed information for the October work session.
• No—the City will keep the current rainfall standard.
If the answer to the above question is yes, the second question is:
2. Should construction of the Canal Importation and Ponds Outfall/Red Fox Meadows
Restoration and Irnprovement Project (CIPO)be delayed?
BACKGROUND
The Rainfall Standard
The current rainfall standard of 3.67-inches in 2-hours was adopted by City Council in 1999 after
over a year of study and public outreach. The analysis was based on nationally accepted procedures
and principals for such studies and used actual rainfall data collected from official weather stations
recognized by the National Weather Service. Although there was obviously emotion involved during
the adoption process, the adoption of the recommend standard was scientifically, not emotionally,
based. A copy of the August 16, 2007 staff memorandum describing the process leading up to the
adoption of the current rainfall standard is Attachment 1.
From the beginning of the rainfall frequency study to the adoption of the city-wide stormwater
master plan in 2004,and the final adoption of the new FEMA floodplains in 2006,the entire process
took nine years and cost over $4 million. During that effort, City Council has been intimately
involved in guiding the program,adopting new and revising existing regulations,setting and revising
financing plans, changing City Code, approving master plans, adopting floodplains, and providing
other policy guidance to the stormwater program on more than 15 separate occasions.
The Federal Emergency Management Agency(FEMA)and the Colorado Water Conservation Board
(CWCB)use the runoff from a 100-year storm for floodplain mapping. Both of these agencies have
regulations that govern the criteria of the City. Because of this, these agencies must approve the
standard used to map FEMA floodplains. Larimer County, Colorado State University and others
have adopted the rainfall standard based on the City's Study.
August 28, 2007 Page 2
The August 2,2007 Storm Event—A Wake Up Call
On August 2, 2007 a significant storm event occurred in Fort Collins, almost ten years to the day
after the flood of 1997. A small storm came in from the south and intensified as it approached
southern Fort Collins. It tracked from southwest to northeast,with the most intense rainfall falling
over the western portion of the Fossil Creek drainage basin. The maximum rainfall recorded at three
rain gages in the Fossil Creek Drainage exceeded our 100-year rainfall standard of 3.67-inches in
two hours.These gages recorded 4.88-inches,3.86-inches,and 3.78-inches in two hours. Along the
rest of the storm's main path,recorded rainfall ranged from 2.8 inches to 3.4 inches,corresponding
to frequencies of between 25 and 50-year. Rainfall amounts on the north side of the city
corresponded to less than a 10-year event. Rainfall amounts in the southeast part of the city,outside
the main stone path, corresponded to between a 10 and 25-year event. The recurrence intervals for
two hour rainfall from representative rain gages across the City are shown on Attachment 2.
There were numerous flooded roads and intersections throughout town. Examples include Shields
and Elizabeth, Taft Hill Road south of Prospect, College and Prospect, College and Drake, Fossil
Ridge Drive near College Avenue and Fossil Creek Parkway. In spite of the reports of flooding to
numerous homes and businesses,the City was very fortunate. Of all the drainage basins in the City,
Fossil Creek is the best prepared to withstand a large storm event. Had this storm been just two
miles north, there would have been significant damages in the Canal Importation Basin and other
parts of the City. As it was,the absence of irrigation water in the irrigation canals and the track of
the storm helped minimize damages.
Field observations also indicate that the newly installed Oak Street and Howes Street Outfalls
prevented serious damages in downtown Fort Collins. Portions of the Dry Creek Basin and the
Cooper Slough Basin along East Mulberry Street in Latimer County were not so fortunate. Mobile
homes, hotels and businesses in this flood-prone area were hit hard.
The message brought by this storm is that the rainfall intensities of the current 100-year design storm
do happen. The short 10-years of relative quiet since the flood of 1997 should not lull us into
complacency about flood protection.
Climate Change
Over 35 years ago, when the current NOAA Atlas (the source of the old rainfall standard) was
published, the concept of climate change was not even on the horizon. In 1999, when the current
rainfall standard was adopted, the effects of climate change were being discussed, and some
scientists were discussing the possible result could be more intense swings in weather conditions and
higher intensity storms. Today,climate change is a household phrase. The City of Fort Collins joins
Keene, New Hampshire, Miami-Dade, Florida and Homer, Alaska as the "pilot" cities for the
program led by ICLEI-Local Governments for Sustainability and funded by the National Oceanic and
Atmospheric Administration (NOAA) that helps prepare communities for climate change impacts
by employing adaptation strategies. Most scientists are predicting more extreme storm events,but
the means of quantifying the effect are still elusive. With this as a back drop, policy makers must
assess if this is the right time to consider reducing rainfall standards and delaying projects that
provide protection from flash flooding.
August 28, 2007 Page 3
Rainfall Standard Summary and Staff Recommendation
The rainfall standard is just one of many factors that guide the stormwater management program. It
was revised with much thought and care about the appropriate data and methods to use. Returning
to the original NOAA standard ignores important information. Restudying rainfall in order to add
a few additional years of data will not result in a return to the old rainfall and, in the professional
opinion of staff, will not likely result in a significantly different number. Revising the standard,
remapping the floodplains,and revising the stormwater master plan will take significant time,effort,
and resources from numerous government agencies. Based on the information presented to date
concerning the rainfall standard,does City Council want staff to continue to prepare for the October
23, 2007 work session? If so, staff will contact the other government agencies involved and bring
back more detailed information for City Council's consideration. If not, the City will stay with the
current rainfall standard.
THE CANAL IMPORTATION PONDS & OUTFALL/RED FOX MEADOWS NATURAL
AREA RESTORATION AND ENHANCEMENT PROJECT
Utilities and Natural Areas are ready to begin construction of the CIPO/Red Fox Meadows
Restoration and Improvement Project. This project was approved in 2004 as apart of the city-wide
stormwater master plan. The first installment of funding was included in the 2007 budget and design
is near completion.
Project Overview
The project includes the following elements:
• Creation of a new detention area at Taft Hill Road and Glenmoor Drive,
• Modification to the outlet structure at the existing Plum Detention Area, at Skyline and
Elizabeth,
• Expansion of the Avery Park Pond and modification of its outlet structure,
• Expansion of the Fairbrooke Detention Area, southwest corner of Prospect and Taft Hill,
• Relocation and enhancement of the outdoor classroom in the Fairbrooke Detention Area,
• Creation of a new detention area and wetlands, southeast corner of Prospect & Taft Hill
(Kane Detention Area),
• Expansion of the existing Red Fox Meadows Detention Area,
• Creation of water quality areas in Fairbrooke and Red Fox Meadows Detention Areas,
• Approximately 5,000 feet of 24 inch through 102 inch stone sewer pipe and associated
inlets, and
• Restoration and enhancements to Red Fox Meadows Natural/Stormwater Detention Area
The components of the overall design are shown on Attachment 3.
Special Considerations for Red Fox Meadows Natural/Stormwater Detention Area
The project team is aware of the interest and sensitivity to the detention areas and natural area
enhancements at Red Fox Meadows.
August 28, 2007 Page 4
The project team has incorporated the follow elements into the design:
• Gentle slopes on the banks of the new Kane detention area and the expanded Red Fox
Meadows detention area,
• Peninsulas and varied bank shapes to provide a natural look,
• A natural, meandering stream to carry low flow runoff and ground water,
• A small pond or riffle areas in the Kane detention area for improved wildlife habitat,
• Irrigation canals in current locations to minimize loss of riparian habitat,
• Removal of the existing flume over the Latimer County No. 2 Canal,
• Removal of the existing flume over the New Mercer Canal,
• A minimum of new exposed concrete structures,
• Special designs, treatments and landscaping to buffer necessary structures and minimize
potential for graffiti,
• Inlet trash collection devices upstream of Red Fox Meadows,
• Provisions for water quality improvement,
• Removal of Russian olive and small caliber Siberian elm trees,
• Larger caliper Siberian elms removed after collaboration with Natural Areas and Forestry
staff,
• High quality stands of native trees and shrubs preserved where possible,
• Existing fox dens preserved when possible;recreate dens in an isolated area of site when not
possible,
• Native species for re-vegetation,
• Clumps of trees and shrubs throughout newly constructed areas for future wildlife habitat,
• A temporary irrigation system to speed establishment of native grasses, shrubs, and trees,
• A new gravel parking area along Longworth Road for 7 cars, 1 handicap space and 1 bus,
• Educational kiosk, vault toilet, bike rack, trash receptacles near parking lot,
• A crushed rock trail system with access from parking area to Red Fox Meadows Detention
Area,
• An access from the south (via Stuart), including pedestrian bridges over irrigation canals,
• An access from the south from Promenade Way.
• A detached slightly meandering sidewalk along the east side of Taft Hill Road, and
• A wetlands education area near the parking area for use by school groups.
The design elements for Red Fox Meadows Natural Area/Stormwater Detention Area are shown on
Attachment 4.
March 13, 2007 City Council Work Session
The project was the subject of a City Council work session on March 13,2007. The March 13,2007
work session materials provide a more detailed review of the project, including definition of the
problem,past City Council policy direction,and the proposed solution. To help bring new Council
members up to speed, and reacquaint other Council member with the project, a copy of the March
13, 2007 Work Session Agenda Item Summary is included as Attachment 5. A DVD of the March
13, 2007 staff presentation is also in the Council packet.
August 28, 2007 Page 5
Public Outreach
Public outreach on this project has been extensive. The project mailing list contains over 3,500
addresses for newsletters and project updates. The project team has received input from a public
open house, meetings with neighborhood groups, the Land Conservation and Stewardship Board,
City Council, and hundreds of individual contacts. Numerous changes to the project have resulted
from this input. The project team is scheduled to present the final design to the Land Conservation
and Stewardship Board in September.
Why Fairbrooke and Red Fox Meadows Detention Areas Need to be Enlarged
In general, the city-wide results of increasing the rainfall standard in 1999 were wider floodplains,
larger detention ponds, and larger conveyance facilities. The effect on each basin, however, was
dependent on basin specific conditions. The CIPO Project includes expansion of two existing
detention areas, Fairbrooke and Red Fox Meadows. Previous statements by staff indicated the
reason these detention ponds were being enlarged was due to the change in rainfall standards. While
this was generally true across the city,the presence of three irrigation canals in the Canal Importation
Basin makes that statement inaccurate for that specific basin.
The previous Canal Importation Basin Master Plan,completed in 1980,made numerous assumptions
about the behavior of the irrigations ditches during a flood event. General capacity 1 g g p y analyses of the
canals were performed, using the modeling technology available at that time. The canals intercept
storm runoff from the sub-basins above them and convey that runoff until their capacity is exceeded,
at which point the canals spill to the east. The network of flow paths,basins, irrigation canals, and
their spills is a complex system. The 2004 Master Plan studied the basin in much more detail
compared to the 1980 Master Plan. The very flat energy slopes in the canals, the presence of
significant backwater and/or reversed flow effects, the multiple incoming hydrographs, and the
existing spills made steady state modeling inadequate. Consequently,a modern unsteady state flow
model, not available 20 years ago, was used to complete the hydraulic analysis of the three canals.
The result was the canals did not behave as assumed in the 1980 Master Plan. There is much more
water entering the Fairbrooke Channel and the Red Fox Meadows detention area than previously
predicted. This change in modeling technology was a significant reason for the changes required to
the Fairbrook and Red Fox Meadows Detention Areas. Secondly, the 1980 Master Plan did not
adequately predict the effect of the project on and adequately protect downstream Spring Creek
properties. Modern stormwater practices require that projects do not make downstream flooding
worse. This also required the size of the Fairbrooke and Red Fox Meadows Detention Areas to
increase.
An Estimate of the Specific Impacts from the Change in Rainfall Standard
In order to isolate the effect of the rainfall change on the CIPO project,the project team prepared an
alternative of the project that only provides a 50-year level of protection,which is very close to the
old rainfall standard of 2.89-inches in two hours. The resulting decrease in project scope was not
significant. For example,the inlet pipe into the Red Fox Meadows Detention area was reduced from
102-inches in diameter to 90-inches in diameter. Other pipe sizes were similarly reduced one size.
The costs for installing a 90-inch pipe instead of a 102-inch pipe are almost identical. The average
depth of the Red Fox Meadows Detention Area was reduced by 2 feet, reducing the volume of
August 28, 2007 Page 6
excavated soil by about 30,000 cubic yards. Factoring inflation into account, a comparative cost
estimate of the 50-year alternative after a one year delay was$430,000 more than the estimated cost
of the current project starting this year.
Canal Importation & Ponds Outfall Summary and Staff Recommendation
The primary reason the Fairbrooke and Red Fox Meadows Detention Areas need to be enlarged is
due to better modeling technology and adequately protecting downstream properties. The change
in the rainfall standard in 1999 also impacted the size of these facilities,but to a lesser extent. An
alternative to delay the project a year and provide a lower level of protection does not save money
due to economies of scale and inflation. A year or more delay in the project also postpones flood
protection for a neighborhood that has sustained frequent flood damages and delays the
restoration/enhancement of the Red Fox Meadows Natural Area. Staff believes that if Council
chooses to revisit the rainfall standard, the technical analysis will not likely result in a significantly
different number. For all these reasons, it is staffs professional recommendation that the Canal
Importation Ponds and Outfall/Red Fox Meadows Natural Area Restoration and Enhancement
project proceed as designed.
ATTACHMENTS
1. August 16, 2007 Staff Report on 100-Year Design Rainfall.
2. Two Hour Rainfall Amounts for August 2, 2007.
3. The Canal Importation&Ponds Outfall Project Overview.
4. Red Fox Meadows Natural Area/Stormwater Detention Area Design Elements.
5. March 13, 2007 Work Session Agenda Item Summary.
6. Power Point presentation.
Utilities Attachment 1
- � electric • stormwater • wastewater• water
i
1
City of Fort Collins
MEMORANDUM
DATE: August 16, 2007
TO: Mayor and City Council Members
Water Board
THRU: Darin Atteberry, City Manager ��—
Brian Janonis, Interim Utilities General Manager 1p
FROM: Jim Hibbard,Water Engineering and Field Operations Manager
Susan Duba Hayes, Senior Stormwater Engineer,Master Planning
RE: 100-Year Design Rainfall
Recently,concern has been expressed about the City's 100-year rainfall standard. The intent of
this overview is to give City Council and Water Board the history of the current standard and
associated technical background. Utility staff is scheduled to come to a Council Work Session in
October to present detailed information about the standard and to get direction from City
Council.
Definitions
100-year storm
The 100-year storm is an often-used term, and sometimes misunderstood. The 100-year storm
has a precise technical meaning based on the statistical study of multi-year data gathered from
official weather stations: "the amount of rainfall (e.g. 3.67 inches) over a specified length of time
(e.g. 2-hours)that has a 1% chance of being equaled or exceeded in any given year."
Duration
An important part of the definition is the length of time (duration) of the storm. The storm
duration we use is 2-hours, typical of the intense rainstorms that occur in Fort Collins and cause
flash flooding. A 100-year storm can be determined for any duration,for example, 1-hour, 6-
hours and 24-hours. The total amount (depth) of rainfall changes for each duration.
Storm Freauencv
Just as there is a"family"of 100-year storms for varying durations,there is also a"family"of
storms associated with a single duration, such as 2-hours. These are called storm frequencies,
such as the 2-year, 5-year, 25-year, 50-year, 100-year, 500-year,etc. Each frequency has a
different risk of occurring and a different total rainfall amount. The 2-year event has a higher
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chance of occurring, but a lower rainfall depth. The 500-year event has a low chance of
occurring, but a higher rainfall depth.
It is important to note the rainfall amounts for each frequency are not linear. The 50-year storm
is not one-half of the 100-year storm and the 25-year storm is not one-fourth of the 100-year
storm. For example, the City's 100-year storm is 3.67 inches over 2-hours, the 50-year storm is
2.91 inches and the 25-year storm is 2.31 inches. This also applies to the flows. The runoff from
a 50-year storm is not one-half of the I00-year runoff, etc.
Distribution
It is not just the rainfall amount and duration of the storm that determines the amount of runoff,
but also the distribution of the rain. This distribution describes how the rain falls during the
storm's duration, usually in 5-minute intervals for a 2-hour storm. A storm with early high
intensities can create large runoff peaks that can overwhelm the drainage system. A storm with
uniform intensity results in lower runoff peaks that can be routed by the drainage system with
less damage. As with the choice of storm duration, the distribution is chosen to reflect atypical
intense storm for the region.
Runoff Model
Finally, to estimate the runoff from a storm, a rainfall distribution is input into a runoff model.
This "design storm" results in an estimated runoff based on assumptions made in the model
about the physical characteristics of the basin. Rarely does a storm exactly match the design
storm; therefore, the runoff will not match the estimate from the runoff model. For instance, the
August 2, 2007 storm had lower and more uniform intensities than assumed in the model,
therefore the runoff was lower than the model predicted. However, we have to plan for
something, so the 100-year 2-hour storm provides protection for a wide variety of storms of
varying intensity and duration.
Occurrence of 100 year storms
The "amount of rainfall" represents total rainfall at a given location. In any given year, many
heavy storms occur. It is normal, over a state the size of Colorado, to experience numerous
storms each year that locally equal or exceed the rainfall associated with the 100-year storm.
According to the State Climatologist, there are between one hundred and three hundred 100-year
storms somewhere in the state of Colorado in a typical year.
This does not mean if we use the 100-year storm for design we will be safe from a flood for 100
years. For example, over a period of 30-years, a drainage facility designed for a 100-year storm
still has a 25% chance of being exceeded by larger events.
History and Background
As described above, a design storm based upon the rainfall standard is used to calculate runoff
flows. The Federal Emergency Management Agency (FEMA) and the Colorado Water
Conservation Board (CWCB) use the runoff from a 100-year storm for floodplain mapping.
2
Both of these agencies have regulations that govern the criteria of the City. Because of this, these
agencies must approve the standard used to map FEMA floodplains.
In the late 1970's the Fort Collins City Council adopted the policy to use the 100-year storm for
planning and design of City storm drainage facilities. This means the runoff associated with the
rainfall standard is used by the City to map City floodplains, guide new development, and design
major drainage facilities. In 2004, during the update of all the drainage master plans, Council
confirmed its support of providing I00-year protection where feasible.
When the Utility was established in the late 1970s, the 100-year rainfall was determined from the
National Oceanic and Atmospheric Administration (NOAA) Atlas 2, published in 1973 and
based on data gathered up to 1969. This Federal agency conducts precipitation frequency studies
for the United States guided by standard engineering and statistical practices: The City of Fort
Collins originally adopted a 100-year design storm of 2.89 inches over a two-hour period. After
the 1997 Spring Creek flood, this standard was criticized by citizens for being outdated and too
low. The City embarked on a study to re-evaluate the 100-year rainfall standard.
1999 Rainfall Study Process and Recommendation
A Consulting Engineering firm with expertise in developing rainfall standards was retained to
perform the analysis. The study was conducted from February to August 1998 and was guided by
a technical advisory group, the Precipitation Study Task Force. Members represented 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 A.
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:
• use only the CSU gage data vs. use data from three Front Range gages (Boulder, Longmont
and CSU) in order to increase the period of record;
• use a higher value for the 1997 storm than actually recorded at the gage; and
• apply a non-uniform rainfall rate over the City (use a higher rainfall on the west side of the
City).
The final recommendation of the Task Force was to adopt a 100-year design storm of 3.67 inches
over two hours. This was based on an analysis that:
• uses data from the three Front Range gages,
• uses the actual 1997 recorded data from the CSU gage (3.78 inches over 2-hours), and
• applies a uniform rainfall rate over the entire City since the qualitative data did not indicate
storms on the west side of the City are more intense.
Consensus on the recommendation was not reached among the Task Force members. The
minority opinion was presented to both the Water Board and Council.
3
The minority recommendation of the Task Force was to adopt a 100-year design storm of 4.37
inches over two hours, based on only the CSU gage record (including the actual recorded 1997
value of 3.78-inches in 2 hours). The minority believed analysis shows Fort Collins is an
extreme within the region and experiences preferred storm tracks, therefore the Boulder and
Longmont gages are not representative. Another minority recommendation of the Task Force
was to adopt a higher rainfall, 5.5 inches over two hours, for use on the west side of the City,
specifically west of Taft Hill Road. This value is calculated by inserting the estimated rainfall
value from the center of the 1997 storm (6.5 inches over 2-hours) into the CSU data set, rather
than use the actual recorded value at the CSU gage.
The final Task Force recommendation letter for both the majority and minority opinions is
attached as Exhibit B.
Impacts of Increased Rainfall
In general when rainfall is increased, runoff increases and floodplains are wider. This moves
development that is not subject to floodplain regulations further away from stream corridors.
Drainage facilities built in developed areas, to solve existing problems, are sized to provide 100-
year protection when benefits outweigh costs. Therefore, 100-year protection is not always
provided. The impacts of higher runoff are basin specific and dependent on the amount of
detention in the basin compared to the amount of conveyance. Cost increases are due to
additional construction and the need for more land. This is very site-specific based on the
constraints of the site.
Newly developing land is required to provide protection for the 100-year design storm. Higher
runoff requires construction of larger drainage facilities in new developments. The main impact
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.
Public Outreach
The rainfall study and its results were presented to the public over several months. The Water
Board considered the issue and made a recommendation. There was an open house, newspaper
articles, and information given to applicants during Conceptual Review meetings. Local
consultants were provided with the technical information needed to design with the 3.67"
rainfall.
Upon adoption of the rainfall standard, the City embarked on a major update to the Stormwater
Basin Master Plans. It took over a year to map the revised floodplains and another year to
formulate the revised project recommendations. At each step, there was public outreach and
comment. In addition, Council adopted interim Floodplain Regulations for the newly mapped
floodplains while a full review and revision of the Floodplain Regulations occurred. This effort
also involved significant public outreach.
4
Overall, from the start of the rainfall study to adoption of the revised Basin Master Plans in 2004,
the effort took over six years. Adoption of the affected FEMA floodplain maps took even longer.
The revised FEMA maps became effective in December 2006; almost nine years after the start of
the rainfall study.
Summary
The rainfall standard is just one of many factors that guide our stormwater management program.
It was revised with much thought and care about the appropriate data and methods to use.
5
Exhibit A
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 Larimer 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.
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 Liou is a Hydrologist with the Community Mitigation Programs Branch of Region VIII 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
WRC 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 Sveiusson 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.
Exhibit B
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
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 Vill, 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"
Qualitative 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 /rorfrBr6wneg, CW
Rex Burns, Larimer County Lisa n, Lari Co y
Nolan Doesken, CSU Atmospheric Dave Frick, Ayres and Associates
Sciences Dept.
p �
hn Morris, CSU 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.
Reasonina
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 Oil 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
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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.
co" (a- )A
Duane Boes, CSU Statistics Dept. . Don Heyse, Citizen
Bob Davidson, Fort Collins Planning and Zoning Board
7
Attachment 2
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Attachment 5
DATE: March 13, 2007 WORK SESSION ITEM
STAFF: Jim Hibbard FORT COLLINS CITY COUNCIL
SUBJECT FOR DISCUSSION
Canal Importation Ponds and Outfall Project.
GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED
The purpose of this work session item is to provide Council with information on the Canal
Importation Ponds and Outfall (CIPO)drainage improvement project.
BACKGROUND
Staff is ready to begin design of the CIPO drainage improvement project. This project will impact
several neighborhoods in west/central Fort Collins in different ways. These impacts will generally
be excavation of new and existing detention areas,construction of large diameter pipelines in public
right-of-way and construction/restoration in the Red Fox Meadows Natural/Storm Water Detention
Area. As staff begins public outreach and design of this project, Council will likely receive
questions from citizens.
Defmition of the Problem
According to the Colorado Climate Center and the U.S. Geological Survey, on average, there are
about 150 100-year rainfall events in Colorado every year. At any given location, there is a 1%
chance of a 100-year storm in any given year. According to the Colorado Water Conservation Board,
that means that over the life of a typical 30-year mortgage,a property in the 100-year floodplain has
a 26%chance of experiencing a 100-year flood and only a 1-2%chance of a fire.
The Canal Importation Drainage Basin spans nearly five square miles in west-central Fort Collins.
Three major irrigation canals traverse the basin from north to south and impact drainage in the basin.
The canals intercept runoff traveling through the basin and transport it out of the basin,and they also
spill runoff into the basin when their capacity is exceeded. Drainage patterns in this basin are
extremely complicated. During storms,water from as far west as the foothills flows into the three
irrigation canals until they fill up and spill where the banks are low. Floodwaters travel east until
they reach the Colorado State University campus and Old Town area, and eventually the Poudre
River.
The basin has experienced flooding many times due to its urbanized nature and because development
took place with little or no detention. Existing storm sewers are sized for very small rainfall events,
sometimes less that a 2-year storm. Additionally, the loss of the natural drainage channels due to
farming and urbanization assure future flooding potential.
March 13, 2007 Page 2
In September, 1938, over four and a half inches of rain fell in 48 hours west of town and caused
widespread damage. More flooding occurred in August 1951,when over six inches of rain fell in
27 hours,with slightly more than one inch falling in 15 minutes. Runoff caused the irrigation canals
to overflow, with the water flooding the Colorado A&M college campus. During the morning of
August 13, 1974,the basin received nearly four inches of rain. The hardest hit area was near Avery
Park. In 1992, two and a half inches of rain fell in little over an hour. Twenty-five houses in the
basin reported damages,including both basements and main floors. Flooding in July 1997 caused
significant damage throughout the Canal Importation Basin. CSU suffered$100 million in damages
to buildings and property in the worst flooding ever seen in the area. A storm in April 1999 also
caused some minor flooding damage to homes in the basin. It does not take a large storm to cause
flooding in this basin. The regulatory 100-year floodplain for a portion of the Canal Importation
Basin is shown on Attachment 1.
Council Policy Direction
The City's Natural Areas, Storm Drainage, and Water Quality programs have a strong record of
collaboration in the joint acquisition of land and the design ofprojects that meet multiple community
needs. The following guidance provided by City Council indicates how the programs overlap and
therefore must collaborate.
City Code Section 26-492 states that in addition to protecting the health,safety,property,and welfare
of the city,the purpose of the Stormwater Program includes pollution reduction and enhancement
of the environment. City Council Resolution 2004-092 adopted the mission of the Natural Areas
Program;to protect and enhance natural areas and habitats while providing education and recreation.
City Council Resolution 1995-014 directs stormwater staff to view watersheds holistically,protect
habitat, establish stormwater treatment criteria, and establish a program of pollution prevention
education. City Council Resolution 2001-094,The Natural Areas Easement Policy states,
"To the maximum extent feasible,planning for storm drainage, water quality and
natural areas and open lands acquisitions shall continue to be coordinated so that
lands needed for all of these uses are purchased jointly.
Innovative approaches shall continue to be used so that flood control and water
quality facilities are designed, constructed and managed to maintain or enhance
natural area protection values while meeting flood control, stream stability and
water quality needs. "
For these reasons, the Stormwater Program and Natural Areas Program jointly purchased the Red
Fox Meadows Natural/Stormwater Detention Area over the last 16 years. The Natural Areas
Program delayed establishing a vision or making significant enhancements in this area until the
Stormwater Program determined its needs. Stormwater and Natural Areas are collaborating to
ensure the project meets the needs and concerns of both.
Approach to the Problem
City staff has assembled a strong team to tackle the public outreach,design and construction of the
CIPO Project. The project managers will be leading a team of stormwater and natural areas staff,
March 13, 2007 Page 3
design consultants,specialty consultants,communications specialists,and a contractor with aproven
performance record and customer service attitude.
The project team is focused on the importance of keeping citizens informed and involved in plans
for final design and construction. As a group,including Utilities and Natural Areas,staff has worked
hard to develop a good understanding of the proposed project area and its current uses,including its
role in stormwater detention and as a neighborhood natural area.
Canal Importation Basin Stormwater Master Plan
The Canal Importation Basin stormwater master plan was adopted by City Council in 2001 and
reaffirmed in the City-Wide Stormwater Master Plan, adopted in 2004. The master plan called for
$51.5 million in flood control improvements in order to avoid an estimated $125 million of direct
flood damage over the next 50 years. Included in the master plan are improvements to stream habitat
and riparian vegetation,enhancing or expanding wetland areas,maintaining and connecting wildlife
travel corridors and incorporating water quality. To date $15.3 million of the estimated $51.5
million in capital projects have been completed, for an actual cost of$9.8 million. The master plan
estimate of the CIPO Project is$21.5 million. After completion of the CIPO Project,there will be
$14.7 million in remaining projects in this basin.
The master plan considered and evaluated over 60 alternatives for detention and conveyance in the
Canal Importation Basin. In general, these alternatives were considered: (1) level of protection—
providing 100-year flood protection throughout the basin versus providing a lower level of flood
protection;(2)balancing detention and conveyance requirements—maximizing regional and on-site
detention within the basin versus providing less detention along with larger conveyance facilities;
and (3) protecting and enhancing habitat areas. Seven specific criteria were considered for
evaluating the conceptual alternatives; these criteria were as follows:
(1) provide flood protection for homes and buildings;
(2) reduce flood hazards along streets and road crossings;
(3) minimize the exportation of runoff to downstream drainage basins;
(4) minimize adverse impacts to existing habitat areas;
(5) enhancelexpand habitat areas and water quality opportunities;
(6) cost of improvements;
(7) maintenance requirements.
After each successive round of evaluation, more detail was added to the remaining alternatives.
Based on the evaluation and recommendations made by City Utilities staff,other City departments,
the technical consultant, and other sources, a recommended plan of improvements was selected.
Proposed Conceptual Solution
The CIPO Project is one element of the improvements in the Canal Importation Basin. As such,it
must work in concert with existing and other proposed improvements in the Canal Importation
March 13, 2007 Page 4
Basin. The conceptual design of the C1PO Project,shown on Attachment 2,includes the following
elements:
• Creation of a new detention area at Taft Hill Road and Glenmoor Drive,
• Modification to the outlet structure at the existing Plum Detention Area, at Skyline and
Elizabeth,
• Expansion of the Avery Park Pond and modification of its outlet structure,
• Expansion of the Fairbrooke Detention Area,southwest comer of Prospect and Taft Hill,
• Relocation of the outdoor classroom in the Fairbrooke Detention Area,
• Creation of a new detention area and wetlands, southeast corner of Prospect and Taft Hill
(Kane Detention Area),
• Expansion of the existing Red Fox Meadows Detention Area,
• Creation of water quality areas in Fairbrooke and Red Fox Meadows Detention Areas,
Approximately 5,000 feet of 24 inch through 102 inch storm sewer pipe and associated
inlets, and
• Restoration and enhancements to Red Fox Meadows Natural/Stormwater Detention Area
Special Considerations for Red Fox Meadows Natural/Stormwater Detention Area
The project team is aware of the interest and sensitivity to the design of the detention areas and
natural area enhancements at Red Fox Meadows. In preparation for public outreach,several design
concepts have been created. An overall design concept diagram is Attachment 3. A conceptual
rendering is Attachment 4. A conceptual slope design concept is Attachment 5. A conceptual
detention area design concept is Attachment 6.
The project team has incorporated the follow design elements into the concept drawings:
• Gentle slopes on the banks of the new Kane detention area and the expanded Red Fox
Meadows detention area,
• Peninsulas and varied bank shapes to provide a natural look,
• A natural, meandering stream to carry low flow runoff and ground water,
• A small pond or riffle areas in the Kane detention area for improved wildlife habitat,
• Irrigation canals in current locations to minimize loss of riparian habitat,
• Removal of the existing flume over the Larimer County No. 2 Canal,
• A minimum of new exposed concrete structures,
• Special treatments and landscaping to buffer necessary structures and minimize potential for
graffiti,
• Removal of Russian olive and small caliber Siberian elm trees,
• Larger caliper Siberian elms removed after collaboration with Natural Areas and Forestry
staff,
• High quality stands of native trees and shrubs preserved where possible,
• Existing fox dens preserved when possible;recreate dens in an isolated area of site when not
possible,
• Native species for re-vegetation,
• Clumps of trees and shrubs throughout newly constructed areas for future wildlife habitat,
• A temporary irrigation system to speed establishment of native grasses, shrubs, and trees,
March 13, 2007 Page 5
• A new gravel parking area along Longworth Road for 8-10 cars and 1-2 busses,
• Educational kiosk, vault toilet, bike rack, trash receptacles near parking lot,
• A rock trail system with access from parking area to Red Fox Meadows Detention Area,
• An access from the south(via Stuart), including pedestrian bridges over irrigation canals,
• A detached slightly meandering sidewalk along the east side of Taft Hill Road,
• A wetlands education area near the parking area for use by school groups, and
• Relocation and enhancement of the outdoor classroom at Fairbrooke detention area.
Public Communication, Education and Outreach
The public communication effort will be extensive and will include staff from the Utilities Customer
Communications Department and Natural Resources. Outreach will begin in March 2007.
The outline for the public communication, education and outreach plan is listed below.
Three areas of focus:
• Project need and benefits
• Construction impacts for businesses and neighborhoods affected by the project
• Natural Area impacts and restoration
Outreach Goals:
• Inform community about the project
• Help gather input for design planning
• Provide ongoing communication support for the project
Education Goals:
• Inform community about how stormwater drainage and water quality work in the
basin
• Provide information about the natural area affected, including habitat,wildlife and
wetlands
• Help interpret how both construction and restoration will impact the existing area
Communication Plan:
• Project newsletter
• Open House(s)
• Neighborhood/HOA presentations
• City Boards and Commissions
• Community/neighborhood tours
• Project phone line
• Targeted mailings and staff outreach when needed
• Continuously updated web site
March 13, 2007 Page 6-
ATTACHMENTS
1. Existing Canal Importation Floodplain
2. Overall Conceptual Design
3. Red Fox Meadows Conceptual Design
4. Red Fox Meadows Conceptual Rendering
5. Red Fox Meadows Slope Design Concept
6. Red Fox Meadows Detention Area Design Concept
7. PowerPoint Presentation
Rainfall Design Standard
Use of Design Standard
• Predict runoff
• Map floodplains
• Use as criteria for private development
• Design major public drainage facilities
— New
— Retrofit when cost effective
1
1998 Precipitation
Frequency Study
Study Objectives
• Respond to criticism from citizens and
elected officials
• Have a solid foundation for the future
• Incorporate additional rainfall data
• Be acceptable to regulatory agencies
• More specific to Fort Collins than NOAA Atlas
2
Precipitation Frequency Study
• Performed by WRC Engineering
• Guided by Precipitation Task Force
• Scientific exploration of data analysis
Precipitation Task Force
• Federal Emergency Management Agency
• Colorado Water Conservation Board
• Larimer County
• Colorado State University
. Statistics
. Atmospheric Science
. Environmental Engineering
. Facilities
3
Precipitation Task Force ( continued )
• Assistant State Climatologist
• Water Board
• Planning and Zoning Board
• Local Consultant
• Citizens
• Staff
1998 Precipitation Frequency Study
• Actual rain gauge data from
— Fort Collins
— Longmont
— Boulder
• General Extreme Value ( GEV ) methodology
— Statistical method to fit a line to data
4
Selection of Rain Gauge Sites
• National Weather Service gauges
• Minimum of 15 years of data
• Hourly data recorded
• Meteorologically similar
• Topographically similar
Design Standard
• Recommended by majority of task force
• Frequency — 100 year
• Duration — 2 hours
• Depth — 3 . 67 inches
5
Adopting Agencies
• City of Fort Collins
• Federal Emergency Management Agency
• Colorado Water Conservation Board
• Larimer County
• Colorado State University
• Town of Wellington
Timeline
• Began rainfall study — 1998
• Adopted design standard — 1999
• Remapped floodplains — 2000 to 2003
• Updated master plans — 2002 to 2004
• FEMA adoption of floodplains — 2006
• Nine years
• Over $4 million
6
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August 2 Storm Analysis
• Design standard exceeded at three
rain gauges
• Most intense rain was in Fossil Creek
• 25 - to 50 -year rainfall in other areas
• Flooded streets , homes and businesses
• Mitigating circumstances
Summary
• It does rain very hard in Fort Collins .
• The current rainfall standard is scientifically , not
emotionally , based .
• A new study will not result in a significantly
different number .
• A reduction in the rainfall standard will reduce
flood protection for citizens of Fort Collins .
• Climate change is a wild card .
12
Staff Recommendation
Do not restudy the rainfall standard .
October 23 Work Session ?
If Council chooses to continue discussion of the
rainfall standard , staff will :
• Contact regulatory and other government
agencies for input , and
• Prepare costs and timelines .
13
4W LDrainage
OPO Improvement
Project
Canal Importation Ponds & Outfall
N
Areas
CWof re t Coffins C.tyof Fmt Collins
utHires
Natural Areas Easement Policy
City Council Resolution 2001 - 94
. . . To the maximum extent feasible , planning for
storm drainage , water quality and natural areas
and open land acquisitions shall continue to be
coordinated so that lands needed for all of these
uses are purchased jointly .
14
Current Floodplain
EwSling Canal Importation
Floodplain
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Contributing Factors
• Historic streams obliterated by irrigation
canals , agriculture and urbanization
• Urbanization without detention
• History of multiple floods
• No outlet to the Poudre River
• Neighborhood vitality issue
15
Master Plan Status
• Master plan estimate — $ 51 , 500 , 000 *
• Projects completed — $ 15 , 300 , 000 *
• CIPO estimate — $ 211500 , 000 *
• Projects remaining — $ 14 , 700 , 000 *
• Actual cost of completed projects — $ 9 , 800 , 000
*2001 Master Plan estimate dollars , not actual costs
Project Overview
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Post Project Goal
CIPO Post Project
Floodplain
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1
Our Approach to the Problem
As a group , including Utilities and Natural
Areas staff, we have worked hard to develop a
good understanding of the proposed project
area and its current uses — including its role in
stormwater detention and as a neighborhood
natural area .
17
Design Rendering
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Detentiontt
Fairbrooke and Red Fox Meadows
Changes • 1980 master plan thatrequire
largerdetention
Unsteady state flow modeling of canals
Modern stormwater practices — do not make
downstream • • • worse
50 -Year Level of Protection Alternative
• About 2 . 9 inches in 2 hours
• Reduces pipe size from 102 - inch to 90 - inch
• Excavation reduced by 30 , 000 cubic yards
— Depth of Red Fox Meadows reduced by 2 feet
(average )
• One year delay
• Cost increased by $430 , 000
Summary
• Majority of impact is due to modern modeling
and stormwater practices .
• A change in the design standard ( lower level of
protection ) does not have significant impact on
project or save money due to economies of
scale and inflation .
• Delay in project schedule delays flood protection
and environmental benefits of restoration .
20
Staff Recommendation
• Proceed with Canal Importation & Ponds Outfall
Project and the Red Fox Meadows restoration .
Questions for City Council
• Should staff prepare more detailed
information on the rainfall standard for an
October 23 Work Session ?
If yes ,
• Should construction of the Canal
Importation Ponds & Outfall project and
the Red Fox Meadows restoration
project be delayed ?
21