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Copper Spur - Spillway Adequacy Memo <br />DAMID: 350xxx <br />DATE: December 21, 2017 <br />Page 2 of 3 <br />Spillway Adequacy Evaluation <br />A spillway for a No Public Hazard (NPH), minor sized dam must convey the inflow <br />design flood (IDF) from the 25 -year, 24-hour rainstorm event'. The IDF resulting from <br />a 25 -year, 24-hour storm was determined by preparing a simple, single basin model <br />using HEC -HMS software. Development of model parameters is discussed in greater <br />detail below. <br />Meteorology <br />A total rainfall depth of 2.08 inches and storm distribution was sourced from the <br />National Oceanic and Atmospheric Administration (NOAA) point precipitation <br />frequency data server. The ten percent exceedance, first quartile temporal <br />distribution was selected to model the storm. <br />Basin Response <br />A soils characterization report was generated from the National Resource <br />Conservation Service (NRCS) web soil survey. The 4.06 square mile basin exhibits <br />predominantly sandy loam soils, which promote infiltration and tend to minimize <br />runoff. Colorado Dam safety offers a spreadsheet whereby soils loss may be calculated <br />using the Green and Ampt method. The resulting model parameters are as follows: <br />Initial Content <br />0.21 <br />Saturated Content <br />0.46 <br />Suction (in) <br />4.0 <br />Ksat (in/hr) <br />0.35 <br />Percent Impervious <br />negligible <br />Unit Hydropraph <br />Colorado Dam Safety offers a spreadsheet wherein rainfall excess is converted to <br />runoff by the Clark Unit Hydrograph method. A roughness value of 0.25 was assumed, <br />which is within the range recommended by Colorado Dam Safety. Additional basin <br />parameters of drainage area, slope and longest flow path were sourced from United <br />States Geological Survey Stream Stats. The resulting unit hydrograph was of 30 <br />duration and 10 minute time step. <br />3 Per Rule 5.9.1.4.7 <br />