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<br />Flood Hazard Area Delineation <br />For Lower Box Elder Creek Watershed <br /> <br />TABLE 3-3 <br />Summary of Precipitation Area Adjustments <br /> <br />Watershed Area Storm Duration Area Adjustments Design Points <br />(souare miles)' (hours) Needed? Where Adiustment Applied' <br />0-10 2 No -all tributaries <br /> -upper 10 square miles ofBG, LH, UH <br />10-20 3 Yes -below upper 10 square miles ofBG, LH, UH <br /> main stems <br />50 and greater 6 Yes -all of BE main stem <br /> -last design point ofBG, 00' <br /> <br />1 There are no watersheds between 20 square miles and 50 square miles in size. <br />2 See watershed designation/abbreviation in Table 3-1 c <br />, Only used to develop hydro graph to be used as input to BE modeL <br /> <br />3.4 CUHP Modeling <br /> <br />CUHP models were developed for each of four watersheds, two development conditions (existing and <br />future), four storm events (10-, 50-, 100- and 500-year) and three precipitation area adjustments (no <br />adjustment, 10- to 20-square-mile adjustment and 50-square-mile and greater adjustment). This necessitated <br />preparation of 80 CUHP input files for the study areac Each CUHP computer input file contained <br />information for all sub-watersheds within one of the four major watersheds. The CUHP input files <br />contained rainfall information (as described earlier) and basin characteristics of area, length, length to <br />centroid, slope, imperviousness, time of concentration, retention storage losses and infiltration losses. The <br />CUHP input parameters for each sub-watershed are included in Appendix G <br /> <br />The sub-watershed area, length and length to centroid were measured using a geographic information <br />system (GIS) to determine the values for the delineated sub-watersheds. The imperviousness of each sub- <br />watershed (as shown in Appendices 0 and E) was overlaid on the watershed delineation (Appendix B)c The <br />overall percentage imperviousness for any sub-watershed was a weighted average of the different <br />percentage imperviousnesses within that sub-watershedc The hydrologic soil groupings in the study area <br />(Appendix C) were overlaid on the watershed delineation. The infiltration parameters (initial infiltration, <br />final infiltration and decay coefficient) assigned to each sub-watershed were based upon a weighted average <br />ofthe soil grouping within that sub-watershed. <br /> <br />Based upon guidance provided in the UDFCD Criteria Manual surface retention losses are OA inches for <br />existing condition pervious areas, 035 inches for future condition pervious areas and 0.05 inches for <br /> <br />. . <br />ImpervIOUS areas. <br /> <br />The time of concentration, as applied in the rational formula, was input for sub-watersheds less than 50 <br />acres in size (future condition) and 90 acres in size (existing condition). The time of concentration for <br />existing conditions and future conditions where the percentage imperviousness is less than 15 percent were <br />based upon equations 3-2 and 3-3 of the UDFCD manual's runoff chapter. The time of concentration for <br />future basin conditions where imperviousness exceeds 15 percent was based upon equation 3-4 of the <br />Runoff chapter in the Urban Storm Drainage Criteria Manual (UDFCD, 1969). <br /> <br />3.5 UDSWM Modeling <br /> <br />One UDSWM95 model was developed for each of the Bear Gulch, Upper Hayesmount and Lower <br />Hayesmount tributariesc UDSWM modeling of the Box Elder Creek main stem included preparation of 12 <br />UDSWM95 input files. The Box Elder Creek main stem models accepted inflow hydro graphs from the <br />basin upstream of the study area, the Bear Gulch tributary and the Upper Hayesmount tributaryc Input <br />hydro graphs for the portion of the basin lying upstream of the study area were developed from Appendix 0 <br />of the CH2M-Hill (1994) studyc The input hydrographs for the upper portion of the basin are included in <br />Appendix K. The inflow hydrographs were input to the UDSWM model using a design point with a <br />designated inflow (i.ec, JK = -1 in card group 3.1 of the input data)c The time and associated flow rate used <br />as input in card group 3.IA are shown in Appendix K. <br /> <br />The UDSWM95 conveyance elements are shown in Appendix H. The conveyance elements include <br />streams, pipes, design points (no routing) and one detention basin (located on DIA property at the upper end <br />of Lower Hayesmount Creek). A connectivity diagram of each of the UDSWM models is included 10 <br />Appendix L A different set of schematics has been prepared for each of the four major watershedsc A <br />modeling schematic was developed for each of the tributaries and the main stem of Box Elder Creek, Bear <br />Gulch, Lower Hayesmount Creek and Upper Hayesmount Creek as shown in Appendix I. The input <br />parameters for each of the conveyance elements arc included in Appendix Jc <br /> <br />Page 6 <br /> <br />991-134cOOO <br /> <br />Wright Water Engineers, Incc <br />