5 - Discussion on the South Boulder Creek Flood Mapping Study Climatology and Hydrology ResultsCITYOFBOULDER
WATER RESOURCES ADVISORY BOARD
AGENDAITEM
MEETING DATE: November 15, 2004
AGENDA TITLE: Staff Presentation and WRAB Discussion of the South Boulder Creek
Flood Mapping Study Climatology and Hydrology.
PRESENTER/S: Ned Williams, Director of Public Works for Utilities
Alan Taylor, Flood Management Program Manager
EXECUTIVE SUMMARY:
Staff is presenting the climatology and hydrology results from the South Boulder Creek Flood
Mapping Study to the Water Resources Advisory Board (WRAB) for review and consideration.
This item will be presented for a public hearing before the WRAB in December 2004 with a
request for recommendation to the City Council.
The WRAB review will help determine if the climatology and hydrology results are acceptable
for submission to the Federal Emergency Management Agency (FEMA) for approval and
adoption. The climatology and hydrology results aze critical to the flood mapping study, and
must be accepted by FEMA to allow completion of the hydraulic analysis for delineating
floodplain boundaries. Community understanding of the study is an important obj ective for
developing public confidence and local acceptance. A primary goal of the technical study is
ensuring that results are scientifically based and defensible.
The climatology and hydrology results differ from those of previous studies. For one, previous
studies did not include a detailed dimatological analysis. The development of the design storm
in the climatological analysis represents a significant advancement in the science of storm and
rainfall evaluation, and is based on physical climatic conditions in the basin. This approach more
accurately represents storm conditions which may occw.
The hydrological analysis incorporates three separate approaches: statistical flood frequency,
paleo-flood and rainfall-runoff modeling analyses. The three methodologies were used to provide
a comparison and cross-reference for validation of the hydrologic results to provide a baseline
for defining a level of confidence in the study. This approach also represents advances in the
science of hydrologic studies, and is the first time multiple methods and paleo-flood analyses
have been incorporated into a local flood mapping study. The analysis was very successful and
demonstrated consistent findings to conoborate the design storm hydrology.
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The South Boulder Creek Flood Mapping Study climatology and hydrology studies aze the most
advanced and comprehensive analyses ever conducted for the basin and floodplain. Staff is
confident that the study provides an objective, highly advanced and detailed technical analysis
that is scientifically defensible. Staff recommends WRAB acceptance of the climatology and
hydrology results.
Fiscal Impacts:
Budgetary: The South Boulder Creek Flood Mapping Study project funding totaling
$735,000 is in place. If additional technical analyses and public process activities beyond
the contracted scope of work aze required, a five to ten percent increase in funding may
be needed.
Staff Time: This project is part of staff's annua] work plan.
Other Impacts:
Economic: Refer to community impacts below.
Community: Following the completion of the South Boulder Creek Flood Mapping Study
and FEMA adoption of the floodplain mapping, residents and property owners affected
by the floodplain may be required to purchase flood insurance for mortgaged structures
and will be subject to local floodplain regulations.
Other Board and Commission feedback: None to date.
Public feedback: The basis for the flood mapping study was developed by the Hydrology
Advisory Panel (HAP) following input provided by the Independent Review Panel (IRP) and
Citizen Advisory Group (CAG). Through these committees, the public has been extensively
involved in setting the framework for performing a comprehensive, objective, advanced and
scientifically based flood mapping study.
The flood mapping study team has worked extensively with a Peer Review Evaluation Panel
(PREP) to obtain technical feedback. The PREP was convened to offer expert oversight by local
resident professionals in water resources, and has been an excellent representative of the public.
The PREP has offered valuable expertise and insight that has helped to shape the technical
research and analysis that has been conducted.
Four public meetings have been held since the project kick-off in January 2004. All meetings
have included public participation to receive comments and answer questions. In addition, the
study Web site, wwwsouthbouldercreekmapp9nr.net, has maintained all study information
online and available for public review and comment. The study has also included questionnaires
mailed to 5,000 addresses and presented at meetings to request information about people's
experiences with flooding. Over 100 responses have been received and used in helping to assess
the South Boulder Creek floodplain.
In general, critical public comments about the climatology and hydrology studies have discussed
issues about FEMA approval of the floodplain model used for the study, the impacts of
goundwater and soil moisture on flood flows, the accuracy of stream gage records, hydrologic
model calibration in the lower watershed, placement of the design storm, impacts of Gross
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Reservoir and the CU-South berm, the reduction in design storm depths and flood volumes, and
what impacts the new study will have on the public.
Staff recommendation: None for this meeting. Staff will recommend WRAB acceptance of the
climatology and hydrology results at the next WRAB meeting.
Analysis:
The South Boulder Creek Flood Mapping Study is well underway and past the halfway mark.
The climatology (determining the amount of rainfall that is expected) and baseline hydrology
(determining the amount of flood runoff that is expected) have been developed and staff is
seeking local acceptance of the results prior to submitting to FEMA for federal approval and
adoption. The climatology and hydrology results will then be used to develop the final hydraulic
analysis (determining where flood waters will flow) and risk assessment. Initial hydraulic study
modeling is under development, and the study team has produced partial results that delineate
flooding for proposed 100-yeaz and 500-year storm conditions.
The climatology and hydrology elements of the flood mapping study involved much more
research and analysis than found in previous South Boulder Creek floodplain studies. This
expanded level of effort follows the recommendations made by the public (including the IItP and
HAP) and endorsed by City Council that called for an adequately funded, highly detailed
technical study that accurately defines the flood problem. A high level of technical analysis was
also deemed important in developing community confidence in and local acceptance of the
results. To assist the process, the PREP, comprised of local residents with expertise in water
resources, was assembled to represent the public and ensure that the study was objective,
thorough, technically accurate, and scientifically based.
The South Boulder Creek basin is very large and has several unusual characteristics. Due to these
conditions the standazd practices that are generally applied to floodplains found in earlier studies
were not considered sufficient in realistically defining the flood problem. For the purposes of this
study, the in-depth update and analysis of climatologic conditions found in the area more
accurately reflect the physical characteristics of major rainfall events seen in the foothills region.
The basin is also affected by transitions from mountains to plains, directional changes in the
canyon bottom, a wide variety of surface terrain, a dispersed valley flow regime across alluvial
lands, and the influence of Gross Reservoir and numerous imgation diversions along the creek.
As a result the study includes:
• A detailed analysis of the main creek channel and the overall floodplain using one and two
dimensional computer modeling, which offers a significantly greater level of detail and
accuracy;
• The development of a design storm specific to the South Boulder Creek basin that is derived
using actual Front Range climate conditions. This approach has improved our understanding
of local storm activity;
• The re-creation of historic storms that caused creek flooding in 1938 and 1969;
• The transposition of well-known significant rainfall events, including the 1976 Big
Thompson and 1997 Fort Collins storms, to provide insights into local flooding impacts if
such storms had happened here;
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A two dimensional "spatial flow" analysis that shows a very fine level of detail about site
specific flooding at locations throughout the basin; and
Analysis of modern day flooding impacts using topography developed in 2003 that reflects
significant changes in the landscape since previous floods (US 36 interchange and Foothills
Parkway, Viele Channel major drainageway, extensive neighborhood development north of
South Boulder Road, and the results of gravel mining in the azea.).
The flood mapping study has incorporated more data about the South $oulder Creek basin than
has ever been gathered before and includes critical physical evidence from past floods. We aze
learning much about the climate and environment that affects South Boulder Creek and what that
means for flooding. Conclusions from the climatology and hydrology study indicate that:
1. The South Boulder Creek floodplain will impact more properties than identified on the
adopted 1986 floodplain mapping. Areas west and east of the Foothills Pazkway corridor in
the "west valley" will be affected by flooding. Detailed mapping of these impacts will be
presented with the hydraulic analysis.
2. South Boulder Creek is subject to flooding (and has been flooded) by both eazly season
general storms (wide-spread steady rainfall with possible snow melt occurring in the spring)
and later season thunderstorms (intense rainfall occumng during summer monsoon
conditions).
The most well-known South Boulder Creek floods of record (September 1938 and May
1969) were more significant than perhaps realized. The 1938 flood involved a rare hurricane
driven thunderstorm that produced rainfall amounts seen an average of once every 500 years.
The 1969 flood involved a 5-day general storm that produced rainfall exceeding 100-year
levels, much of which was fortunately captured in Gross Reservoir. Experiencing two
significant storms in 65 years may seem unusual but we are finding that such occurrences are
notunprecedented.
4. The peak flood flow rates for storm events based on annual probabilities, such as a storm
having a one-percent chance ofoccurring in any given year (mare commonly called the100-
year flood), are about 30 percent lower than earlier studies projected. Study results found that
100-year flood flows are around 4,000 cubic feet per second (cfs) at U.S. 36 compazed with
about 6,000 cFs in the 1986 floodplain study.
5. The three sepazate scientific approaches used to study the hydrology (statistical flood
frequency, paleo-flood and rainfall-runoff analysis) demonstrated consistent findings to
corroborate the design storm hydrology.
6. Storm events greater than the 100-yeaz frequency can and will occur on South Boulder
Creek. While the 100-year probability event is usually applied to floodplain regulations,
floodplain management and mitigation should prepare for and consider a wider range of
storm events from all sizes including the 500-year storm and 1938 flood of record.
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7. The full value of this technically expanded and more expensive flood mapping study may be
realized in a much greater understanding of floods affecting South Boulder Creek and in
reduced funding needs that may be required under a more cost-effective approach to
floodplain management and mitigation for the future.
The Technical Memorandum for climatology and hydrology, along with technical support
materials that the staff is presenting, provide full details about the flood mapping study. Staff is
requesting that the WRAB members review the detailed study results over the next month and
prepare a recommendation for City Council regazding the adequacy and acceptability of the
climatology and hydrology. This recommendation will be used to direct the next steps in the
review process before making a formal submittal to FEMA for approval and adoption of the
climatology and hydrology results.
Summary of options:
The current flood mapping study is the most extensive ever developed for South Boulder Creek,
and is the first involving a detailed analysis of the basin specific climatology. Development of a
basin-specific design storm and applying the sepazate approaches for determining hydrology
(statistical flood frequency, paleo-food and rainfall-runoff analysis) adds a scientific element that
far exceeds standard practices for floodplain studies. The study team is confident in the scientifiC
approach that was applied, and believes the results accurately reflect the climatology and
hydrology of the South Boulder Creek basin.
WRAB options for review and consideration include a recommendation to:
• Accept the study findings as presented;
• Request technical changes in the analyses based on scientific principles; or
• Reject the study and continue the use of the U.S. Army Corps of Engineers' hydrology
referenced in the adopted 1986 floodplain study.
Attachments:
There is one attachment containing the South Boulder Creek Flood Mapping Study -
Climato[ogy and Watershed Hydro[ogy Task Memos, prepared by HDR Engineering, Inc.
Technical support material used in preparing the task memorandums is accessible on the South
Boulder Creek Flood Mapping Study Web site at www.southbouldercreekmappinQ.net, under
"Technical Study Information" and "Task 2- Climatology" or "Task 3- Watershed Hydrology."
In addition, all public process activities including public meeting PowerPoint presentations,
meeting minutes, PREP comments and follow-up letters aze available for review on the Web site.
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