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are 25 year, 24 hour storm flows or 100 year, 24 hour storm flows depending on the size of • <br />the structure. The spillway for MBn3 was sized based on the 25 year, 24 hour storm while f .' <br />both MBnl and MBr2 were sized in accordance with the requirements of the State <br />Engineer. D: ainage basins for the spillway sizing are shown on the Hydrologic Data Sheet <br />for each pond. <br />MBrl and MBi*2 have been classified in the United States $ureau of Reclamation <br />(USBR) Hazard Assumption "B" category. Accordingly, the probable maximum precipi- <br />tation inflow design flood has been reduced to the "B" assumption. This assumption is <br />justified in that the largest storm drainage/sediment control pond has a storage volume of <br />less than 15 acre/feet. Furthermore, the potential for loss of life is minimal as immediate <br />downstream households are limited in number and could be evacuated quickly. In addition, <br />the impact on the Gunnison River is minimal since the energy from such a small volume of <br />water would be quickly dissipated. <br />The pond storage volumes were determined using the 10 year, 24 hour storm and a <br />sediment inflow of 0.1 acre/feet of sediment per acre of disturbed area. The 10 year, 24 <br />hour storm was adopted in accordance with OSM regulations. The 0.1 acre/feet of • <br />sediment per acre of disturbed area requirement was used in lieu of the Universal Soil , , <br />Loss Equation (USLE) because the USLE is not directly applicable to this site. The USLE f'-~' <br />is not recommended when overland flow lengths are long and basins are steep, as is the <br />case with this site. <br />A system of ditches and pipes has been designed to collect runoff from the 10 year, <br />24 hour storm event and direct it to the runoff ponds. Drainage basins tributary to these <br />ditches and pipes are shown on Pages 9 and 20 of the Plan Set. Ditches are shown on Page <br />11 and pipes are shown on Page 8. <br />The hydrologic parameters for the minor basins are summarized on Table 3. This <br />table gives the area, length, drop in elevation, curve number. rainfall. runoff, time of <br />concentration, peak flows, volumes of runoff, and volumes of sediment for each minor <br />drainage basin. <br />Combined ditch flows are summarized for the system as they progress toward the <br />runoff ponds. These are shown in Table 4 of this report. Peak flows and volumes passing <br />through the pipes are tabulated in Table 5. A summary of inflows and volumes for each <br />pond is presented in Table 6. Table 7 lists the capacities and hydraulic characteristics of• <br />all pipes specified for the minor drainage system. ~s' <br />