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<br />3.5 Soils Information <br /> <br />UDFCD outlines a procedure to develop various soil loss coefficients for use with the CUHP model. These <br />variables include pervious and impervious retention, infiltration rate, decay coefficient, and final infiltration <br />rate, <br /> <br />Surface depression losses were estimated using Table RO-6 in the District's Criteria Manual, Volume 1 <br />(Reference 3), For impervious areas, the recommended value of 0,1 inch (associated with large paved areas <br />or flat roofs) was used basinwide, For pervious areas, Table RO-6 suggests a value of 0.4 inch for wooded <br />areas and open fields and 0,35 inch for lawn grass, The former is typically associated with rural and <br />undeveloped areas, and thus it was used for pervious areas in the upper and central watersheds, The latter is <br />generally associated with urban development and thus was used for pervious areas in the lower watershed, <br /> <br />CUHP uses Horton's equation to estimate hydrologic losses due to soil infiltration, It was noted in previous <br />studies of this watershed that the entire basin consists of soils classified as NRCS Type C and D, Table RO- <br />7 in the Criteria Manual lists an initial infiltration rate of 3,0 inches per hour, a final infiltration rate of 0,5 <br />inches per hour, and a decay coefficient of 0,0018 for both C and D type soils, These coefficients were used <br />throughout the entire watershed, and are shown in Tables 1 through 3 of Appendix B, <br /> <br />3.6 Rainfall <br /> <br />Subsequent to the development of the Phase A study, the rainfall section in the District's Manual was <br />completely revised to reflect more current data used to develop rainfall patterns, The project sponsors <br />agreed to use this new rainfall information in the development of this model. While the new rainfall data <br />varies from rainfall patterns used in previously published reports, it was assumed that this would more <br />accurately reflect hydrologic response, and the model would be calibrated accordingly, <br /> <br />To more accurately reflect the quickly changing rainfall patterns caused by orographic impacts in the <br />Ralston Creek basin, the watershed was divided into three rainfall regions - western (upper), central, and <br />eastern (lower), The western (upper) region consists only of the upper, mountainous region of the Ralston <br />Creek watershed, The central region consists of the middle reaches of the Ralston Creek watershed and the <br />highest regions of the Van Bibber watershed, The eastern (lower) region consists of the lower reaches of <br />the Ralston and Van Bibber watersheds, and the entire Leyden Creek watershed, All areas below the <br />reservoirs are in the eastern region, <br /> <br />Using section comers as reference points. the delineated watersheds were superimposed onto the depth- <br />duration-frequency figures given in the District's Criteria Manual (Figures RA-I through RA-12 in the <br />Manual), Point rainfall depths were then estimated for the lO-year, 50-year, and 100-year events for I-hour <br />and 6-hour durations for the western, central, and eastern regions, These values are presented in Table 4 of <br />Appendix B, Point rainfall depths for the 500-year event were estimated using log-probability plots, <br /> <br />Because the Ralston Creek watershed is large, rainfall patterns needed to be adjusted according to areal <br />correction factors, There were a number of different scenarios, which could produce peak events at various <br />locations in the watershed, An uncorrected rainfall pattern is valid anywhere in the watershed with total <br />contributing drainage area less than 10 square miles, A basinwide correction factor for 92 square miles <br />could produce results valid anywhere in the basin, However, additional considerations must be made for <br />localized analysis, For example, a more intense storm falling just on the eastern region (which is generally <br /> <br />urban in nature) could produce higher flow rates at downstream design points than a less intense storm <br />covering the entire watershed, In addition, to analyze operations of the reservoirs located on the Ralston <br />Creek mainstem, a rainfall pattern considering a storm raining on areas contributing just to these reservoirs <br />was needed, <br /> <br />As such, a total of five correction scenarios were created and modeled: <br /> <br />1. Uncorrected - For design points with total contributing drainage area less than 10 square miles (2- <br />hour rainfall event) <br />2, 92 square mile correction - Considered a storm event falling uniformly over the entire basin (6-hour <br />rainfall event) <br />3, 49 square mile correction - Considered a storm event in the areas above ArvadaIBlunn Reservoir <br />only to analyze its operations (6-hour rainfall event) <br />4, 40 square mile correction - Considered a storm event in the eastern region of all three watersheds <br />only (6-hour rainfall event) <br />5, 10-20 square mile correction - Used for reaches of Leyden and Van Bibber Creeks with total <br />drainage area greater than 10 square miles (3-hour rainfall event) <br /> <br />For any design point of interest, results from two models should be considered, The reported flow rate <br />would be the larger of the 92 square mile correction model and the appropriate other model for the location <br />of interest. <br /> <br />Rainfall patterns were developed using spreadsheet templates developed by UDFCD during the update of its <br />Criteria Manual. These spreadsheets only considered areas up to 75 square miles, so the 92 square mile <br />correction hyetograph was developed separately using methods outlined in the Criteria Manual. It should <br />also be noted that the spreadsheets do not automaticall y develop 500- year rainfall. However, the same <br />procedure to develop the 100-year pattern is used, but with 500-year point depths replacing the 100-year <br />point depths, <br /> <br />To summarize, all five rainfall patterns were utilized for the eastern region and three patterns (uncorrected, <br />48 square mile correction, and 92 square mile correction) were utilized for the central and western regions, <br />In addition, the 10-20 square mile correction was developed for the central region because it included a <br />portion of Van Bibber Creek, This means a total of 12 rainfall patterns were developed for each design <br />event, resulting in a total of 48 rainfall patterns developed and used in this study, <br /> <br />These rainfall patterns jU'e shown in tabular form in Table 5 of Appendix B, <br /> <br />3.7 SWMM Routing Element Characteristics <br /> <br />Using the same techniques described earlier for the CUHP variables, a number of variables were calculated <br />for use in the hydrologic routing process, These variables include: <br /> <br />. Bottom width (ft) <br />. Left and right side slopes (z: I) <br />. Main channel depth (ft) <br />· Longitudinal slope of stream invert(ftlft) <br />. Reach length (ft) <br />. Manning's 'n' roughness coefficient <br /> <br />5 <br />