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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. AGENDA ITEM #_5_PAGE_1_ 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 AGENDA ITEM #_5_PAGE_2_ 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; AGENDA ITEM #_5_PAGE_3_ 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. AGENDA ITEM #_5_PAGE_4_ 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. AGENDA ITEM #_5_PAGE_5_