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<br />4.0 - HYDROLOGY AND HYDRAULICS <br />This section provides hydrologic and hydraulic design <br />criteria that has been used to size the surface water <br />drainage channels. <br />4.1 Precipitation Event <br />For sizing the perimeter and temporary collection channels, <br />100 year and 10 year, 24-hour precipitation events have been <br />used. Precipitation depths of 2.9 inches and 1.8 inches have <br />been obtained from the NOAA Atlas (2) for the 100 year, and <br />10 year, 24-hour events, respectively. <br />4.2 Run-off Curve Number <br />The run-off curve numbers used in the design calculations are <br />as follows: <br />Refuse Pile Area: CN 88 <br />Undisturbed Area: CN = 71 (Mountain Brush) <br />• <br />It is noted that for the temporary collection and perimeter <br />drainage channels, the weighted run-off curve number will <br />13 <br />