Laserfiche WebLink
TABLE 1 (Cont.) <br />• DESCRIPTION 10 r-24hr 25 r-24hr 100 r-24hr <br />Pea Inflow Rate 2.57-cfs 3.36-cfs 5.95-c s <br />Peak Discharge Rate 0.20-cfs 0.02-cfs 2.54-cfs <br />Basin Trap Efficiency 100% --- --- <br />Detention of Flow (Sediment) 49.85-hrs --- --- <br />Detention of Hydrograph Centers 16.48-hrs --- --- <br />Input consists of precipitation amount (inches), watershed area (acres), <br />time of concentration (hours), runoff curve number (CN), representative <br />particle-size distributions of soils in the area, sedimentology <br />information, spillway information, etc. The drainage area for Sediment <br />Pond 004 was determined from a 1" = 400' topographic map (see Exhibit <br />13-4). <br />In some instances, the mining will cause the drainage area to change <br />during the life of the sediment structure. When the pit moves into the <br />watershed, runoff will be intercepted by the pit and diverted away from <br />the structure. In these cases, a conservative procedure was used; the <br />structure was analyzed for the largest anticipated drainage area that <br />. will contribute runoff to the structure. This condition usually occurs <br />at the start of mining and again during the reclamation period. <br />SCS Curve Numbers (CN) and "k" Factor <br />SCS curve numbers were estimated for each subwatershed based on the <br />cover type, percent vegetation cover, hydrologic conditions, and <br />hydrologic soil type. Several sources were used to obtain this data: <br />1. Cover Type - Aerial photographs of the mine site were used to <br />identify the existing cover type. Maps delineating the <br />proposed mining plan were used to identify areas that will be <br />disturbed by mining. Three general categories of cover type <br />were used: undisturbed, disturbed, and reclaimed. Further <br />sub-classifications were made in each category as shown in <br />Table 2. <br />The cover type (and the tributary drainage area) for some <br />structures will vary throughout the life of the structure as <br />mining and subsequent reclamation occurs. For these cases, <br />• the anticipated worst condition is assumed for the hydrologic <br />analysis. <br />3 <br />