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1 I I I I I . I . I ,I I ,. I I I I J I SECTION 3.0 - HYDROLOGIC ANALYSIS 3.1 General The summary of the flood hydrology for Ralston and Leyden creeks is presented in this section. The summary is based on the detailed hydrology study presented in the RaIstonlLeyden Creeks Hydrology Report by Boyle Engineering Corporation, updated in April 2003 (Reference 2). 3.2 Background Urban Drainage and Flood Control District and the City of Arvada requested that Boyle prepare a hydrology report for Ralston and Leyden Creeks based on updated hydrology. That study, entitled "Ralston/Leyden Creeks Hydrology Report" and dated April 2003, was conceived in order to convert the hydrologic model for the Ralston Creek watershed from MITCA T to CUHP (Reference 1) and-UDSWMM (Refe.rence 5) and to account for reductions to the roO-year floodplain as a result of the construction of and/or enlargement of various reservoirs. Because results from previous studies were used in development of the published Flood Insurance Study (PIS) and associated floodplain mapping, it was necessary to first perform a calibration procedure to insure that results from this study compared well with those previously published. The ArvadaIBlunn Reservoir was not included in previous hydrologic models. Leyden Lake was omitted from the Phase A report, but was considered as a separate model for the Phase B report. However, Leyden Lake was improved in 2000, and no longer operates as represented in the Phase B modeling. For these reasons, in order to determine consistency between the models, it was first necessary to model the Ralston Creek basin without considering effects from Arvada/Blunn Reservoir and Leyden Lake. Ralston Reservoir, which was in place before the Phase A report was published and has not been modified since, remained in the model. Once the new CUHP-UDSWMM model was sufficiently calibrated to previously published results, effects of the new reservoirs were added to the model in order to determine an updated hydrologic response for the watershed. These results were used to determine the extent of the 100-year floodplain, as well as the magnitude of flooding for other events, for the future developed condition. Design rainfall was then prepared for various points in the watershed. Design events considered include the 10-,50-, 100-, and 500-year events. Rainfall patterns for a number of various aerial corrections were developed and used in the analysis. To more accurately represent rainfall, the entire basin was developed into upper, central, and lower regions, with separate rainfall patterns developed for each. 3.3 Subwatershed Delineation The first step in the procedure was to delineate the entire Ralston Creek basin and separate the Ralston, Leyden, and Van Bibber subbasins. These basins were then delineated into subwatersheds. For areas upstream of Arvada/Blunn Reservoir and Leyden Lake, subwatersheds were delineated with a maKimum basin area of five square miles. For areas below these reservoirs, a detailed delineation was performed with maximum basin areas of 130 acres. Because the Van Bibber basin was only included in this study to determine its effects on the Ralston Creek basin (an analysis of the Van Bibber floodplain was not included in this study), it was not subject to the same aerial criteria as the Ralston and Leyden basins. The Van Bibber Creek subbasin delineations were based on previous studies where the subwatershed maximum area limits did not apply. 3.4 Watershed Imperviousness A critical variable for development of subwatershed hydrographs is the percentage of impervious surfaces in each subbasin. Impervious surfaces increase the hydrologic response of a subbasin, and the hydro graphs generated for each subbasin are particularly sensitive to this value. It was understood that any changes relative to Arvada and Jefferson County's planned developments were local rather than regional in nature. For this reason, aggregate impervious percentages developed in previous studies (which utilized much larger subbasin areas) were kept the same. However, a zoning map (Reference 6) provide by the City of Arvada was utilized to determine actual impervious percentages for each individual subwatershed. For each large subwatershed in the 1986 Phase B Study that was subdivided into smaller watersheds for the present study, a weighted average was computed to maintain consistency over larger regions. 3.5 Soils Information UDFCD outlines a procedure to develop various soil loss coefficients for use with the CUHP model. These variables include pervious and impervious retcntion, infiltration rate, decay coefficient, and final infiltration rate. Surface depression losses were estimated using Table RO-6 in the District's Criteria Manual, Volume 1 (Reference 7). For impervious areas, the recommended value of 0.1 inch (associated with large paved areas or flat roofs) was used basinwide. For pervious areas, Table RO-6 suggests a value of 0.4 inches for wooded areas and open fields and 0.35 inches for lawn grass. The former is typically associated with rural and undeveloped areas, and thus it was used for pervious areas in the upper and central watersheds. The latter is generally associated with urban development and thus was used for pervious areas in the lower watershed. CUHP uses Horton's equation to estimate hydrologic losses due to soil infiltration. It was noted in previous studies of this watershed that the entire basin consists of soils classified as SCS Type C and D. Table RO-7 in the Criteria Manual lists an initial infiltration rate of 3.0 inches per hour, a final infiltration rate of 0.5 inches per hour, and a decay coefficient of 0.0018 for both C and D type soils. These coefficients were used throughout the entire watershed. 3.6 Rainfall Subsequent to the development of the Phase A study, the rainfall section in the District's Manual was completely revised to reflect more current data used to develop rainfall patterns. The project sponsors agreed to use this new rainfall information in the development of this model. While the new rainfall data varies from rainfall patterns used in previously published reports, it was assumed that this would more accurately reflect hydrologic response, and the model would be calibrated accordingly. 5