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C}'CC - .Cedimenr Yield Demon.crrnrion Eckman Park Area 41 <br />• 2.0 METHODOLOGY AND ASSUMPTIONS <br />The required hydrology, sedimentology, and hydraulic calculations for Area 41, and the comparable adjacent <br />pastureland areas were performed utilizing the SEDCAD+ computer model developed by Civil Software Design. <br />SEDCAD+ is a third generation hydrologic and hydraulic calculation model designed for use on IBM~ompazible <br />computers. <br />2.1 HYDROLOGY <br />The SEDCAD+ model calculates runoff and peak flow via a numerical modeling technique based on user inputs of <br />runoff bazin information and design storm. Design storm inputs include rainfall amount and duration, selection of <br />a rainfall distribution, and convolution increment. Hydrographs are developed on a subwatershed basis with the <br />inputs of area, time of concentration, SCS curve number, and selection of one of three dimensionless double <br />triangle unit hydrograph shapes. This information is further discussed in the SEDCAD+ Users Manual (Schwab <br />and Warner, 1987) and in the SEDCAD+ -nputs discussion that follows. Routing of hydrographs as appropriate <br />is accomplished by Muskingum's methods. <br />Specific hydrologic output values calculated by the SEDCAD+ model are as follows: <br />• Peak Flows (cfs) <br />• Storm Runoff (inches) <br />• Runoff volume (acre-feet) <br />• • Total Storm Hydrograph <br />Inputs to the hydrology component of the SEDCAD+ computer model include: <br />• Precipitation Distribution <br />• Storrs Duration <br />• Return Period/Precipitation <br />• Hydrograph Response Shape <br />• Time of Concentration <br />• Curve Number <br />Precipitation Distribution. A precipitation distribution is input to model the runoff hydrograph. SEDCAD+ <br />allows the user to choose between the SCS Tvpe 1 or Tvpe 11 storms. The SCS Type II storm distribution was <br />selected for this demonstration. <br />Storm Duration. A storm duration of 24 hours was used for comparison purposes. <br />Return PeriodlPrecipitation. A precipitation amount is required for the appropriate return period. For the <br />CYCC Eckman Park areas, the following precipitation value .vas used: <br />10-year, 24-hour event: 1.8 inches <br />. The precipitation value vas obtained from the Precipitation Frequency Atlas of the Western United States, <br />Volumc 3, Colorado (NOAA, 1973). <br />• c~nbmimnmrntalt~'Anrhondrcl`~rral Ldoc <br />