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<br />SECTION 1.0 - INTRODUCTION
<br />
<br />1.1 Authorization
<br />
<br />This study was authorized by the Urban Drainage and Flood Control District (District) and the City of
<br />Arvada, Boyle Engineering Corporation (Boyle) was retained to complete the scope of work summarized in
<br />Subsection 1,2, This hydrology study is comprised of this report, and computer input and output files for
<br />CUHP, Version 1.8.3, and UDSWM2000, Version 1.1.0, This work updates and replaces the previous
<br />studies performed for the Ralston and Leyden Creek watersheds,
<br />
<br />1.2 Purpose and Scope
<br />
<br />Over the past few decades, development has continued to grow in the City of Arvada within the Ralston
<br />Creek watershed, Improvements to existing lakes and reservoirs have reduced flooding in the lower
<br />reaches, The ArvadaIBlunn reservoir has been upgraded from an irrigation reservoir to a stormwater
<br />detention reservoir and recent improvements to the Leyden Lake dam have upgraded it to a CORPS-
<br />approved stormwater control facility, Previous studies for the Ralston Creek watershed used the
<br />Massachusetts Institute of Technology Catchment program (MITCA T) for development of runoff and peak
<br />flow rates, The Federal Emergency Management Agency (FEMA) published a flood insurance study (FIS)
<br />(revised in 1992) based on the results of the MITCAT model.
<br />
<br />There are multiple purposes for preparing this report, First, the District desires to replace the outdated
<br />MITCA T model with the more commonly used CUHP and UDSWM models, The newer models would
<br />provide the District and City with a user-friendly format to input and analyze future changes within the
<br />watershed, Second, since improvements were made to two reservoirs, evaluation of the resulting flows was
<br />the next step, And finally, revised flows from this report will be used in preparing a Flood Hazard Area
<br />Delineation (FHAD) report, The FHAD report will assist governmenl officials from various agencies in
<br />floodplain management for Ralston and Leyden Creeks, Some properties may be removed from the
<br />floodplain due to the revised hydrology and the results of the FHAD would also provide developers with
<br />more accurate floodplain data for the construction of new roadways and developments,
<br />
<br />The hydrologic analysis was completed using the District's Colorado Urban Hydrograph Procedure
<br />(CUHP), Version 1,8,3 (Reference 1) to determine peak flows from over 200 subwatersheds, The peak
<br />flows for the subwatersheds were routed along the existing streams and conveyance elements using the
<br />District's Storm Water Management 2000 model (UDSWM2000), Version 1.1.0 (Reference 2), Because
<br />this project involves a conversion of methods, it is expected that minor discrepancies may exist as a result of
<br />differences in computation regimes between the two modeling applications, However, the results of this
<br />study will be calibrated to the previous results for the 100-year event,
<br />
<br />This study analyzed peak flows for the 10-,50-, 100-, and 500-year events, Results from the previous
<br />analyses for the 100-year event were utilized to calibrate the model to previous studies, Results from the
<br />lO-year event were also available for the Lower Ralston watershed, but these values were not used for
<br />calibration, No results for the 50- and 500-year events were available from previous studies,
<br />
<br />1.3 Watershed Study History
<br />
<br />The Ralston, Leyden, and Van Bibber watersheds have been studied extensively before, Previous studies
<br />were conducted by the U,S, Army Corps of Engineers (1970), Frasier and Gingery (1974), Wright-
<br />McLaughlin Engineers (1976), Gingery and Associates (1983), and Wright Water Engineers (1984),
<br />
<br />Of particular importance was the 1976 study by Wright-McLaughlin Engineers that resulted in Phase A of
<br />the Major Drainageway Plan for the watershed, That study used the MITCAT model to establish flow rates
<br />for all three basins, The model included Ralston Reservoir, but did not include the ArvadaIBlunn Reservoir.
<br />Leyden Lake as it was built at that time was included in the model.
<br />
<br />The 1984 study by Wright Water Engineers resulted in Phase B of the Major Drainageway Plan, That study
<br />was essentially an update of the 1976 study, The MITCAT model was still used, but a more detailed
<br />analysis was performed, The ArvadaIBlunn Reservoir was still not included in the model. At the time,
<br />Leyden Lake was determined to have a significant risk of failure due to an inadequate spillway, For this
<br />reason, its function as a flood control element was in doubt; therefore, two models were analyzed - with and
<br />without Leyden Lake, Flow rates and hydraulics used for the Flood Insurance Study (1989) utilized results
<br />from the Phase B report,
<br />
<br />Because MITCA T is no longer widely used, and in order to conform with District standards on the use of
<br />CUHP/SWMM to model basins in the Denver metropolitan area, it was desired to update this watershed and
<br />convert it to CUHP/SWMM, which is the model used for this current study, Although results from this
<br />model will be calibrated to past results, this model is intended to fully replace the previous working model
<br />when complete,
<br />
<br />1.4 Background Information
<br />
<br />Background information provided by the sponsors was used to develop the hydrologic analysis,
<br />Information provided is identified below:
<br />
<br />. Citv of Arvada Drainage Studv Plan and Profile, City of Arvada, Nov, 1968,
<br />. Flood Control Improvements, Levden Dam. Jefferson Countv. CO, URS Greiner Woodward Clyde
<br />and Muller Engineering Co, Dee, 2000,
<br />. Maior Drainagewav Planning. Lower Ralston. Van Bibber. Levden Creeks. Phase A Report, Wright-
<br />McLaughlin Engineers, May, 1976,
<br />. Citv of Arvada. Official City Map, Unofficial Zoning Map, City of Arvada, Jan, 2001.
<br />. Van Bibber Creek, Flood Protection Project. Plan. Profile, and Underground, Burns & McDonnell,
<br />Jan, 1998,
<br />. Numerous letters prepared by FEMA to various local agencies
<br />. Summary of Hydrologic Evaluations for Additional Water Storage, Arvada Reservoir, Blunn Dam
<br />(letter), URS Corporation, Feb, 2002,
<br />. City of Arvada Drainage Basin Maps, City of Arvada, February, 1985,
<br />. Major Drainagewav Planning, Lower Ralston/Van BibberILevden Creeks, Phase B Report, Wright
<br />Water Engineers, Feb. 1986,
<br />. Urban Storm Drainage Major Drainagewav Planning, Ralston!Levden Creek, Wright-McLaughlin
<br />Engineers, Sept, 1977,
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