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2001 • _2_ 003-2142.6 hydrologic analysis for the 100-year, 24-hour storm was conducted assuming the Bateman Creek detention pond and the Arequa Gulch and Crusher sedimentation ponds and the BMP sump are dry at the beginning of the storm event. The precipitation depth for the 100-year, 24-hour storm event is 3.5 inches and has an assumed SCS Type II distribution. The times of concentration for each contributing sub-basin were developed using SCS methodology. The SCS curve numbers were the same as those used for Amendment 8: - CN=71 for reclaimed slopes and original ground with grass; - CN=66 for reclaimed slopes and original ground with trees; and - CN=90 for stockpiles and roads. The precipitation depth for the 10-year, 24-hour storm event is 2.4 inches and has an assumed SCS Type 1[ distribution. The time of concentration calculation method and curve numbers were the same that were used for the 100-yeaz, 24-hour analysis. The Arequa Gulch sedimentation pond is sized based on two times the 10-year, 24-hour event stormwater runoff from the Arequa Gulch Overburden Storage Area (AGOSA) after reclamation grading has been completed. The extent of the reclamation grading is shown on Drawing 1. The size of the Arequa Gulch sedimentation pond is approximately 9 acre-feet. The pond will be constructed as a subgrade depression into the Arequa Gulch fill with nominal 2.5 horizontal (H): 1 vertical (V) sideslopes. Outflow from the pond will be routed into the Phase IV diversion ditch. The Crusher sedimentation pond is designed to contain two times the 10-year, 24-hour stormwater runoff from the crusher azea. The resulting pond volume is approximately 3.6 acre-feet. The pond will be constructed as a subgrade depression into the Arequa Gulch fill with nominal 2.SH: 1 V sideslopes. Outflow from the pond will be routed via a CSP beneath the haul road and into the Phase IV diversion ditch. i~oo~iax~ziaxenr~+nr+x ooc Golder Associates