<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
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