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• Hydrologic and Sedimentology Results <br />Results of the analysis are presented for existing conditions throughout the site, <br />conditions during the key period of block yeazs 6 to 7, and post-reclamation conditions. <br />Existing Conditions. Table 2 is a summary of peak flow, runoff volumes, and sediment loads <br />for the 10-yeaz, 25-year, and 100-year storm events for existing conditions. The table is divided <br />into each of the ten arroyos with values presented for the various structures (that is, design <br />points) shown on Figure 1. Also shown is the approximate station of the arroyo where the design <br />point is located and a description of the location, where appropriate. <br />For instance, in Arroyo "A" at arroyo station 3550, the structure number is number 5 and <br />the flow direction is from sub-basin BS to B3, where the rail road spur crosses the arroyo. The <br />total drainage area at this point is 450 acres, the 100-year peak flow is 815 cubic feet per second <br />(cfs), and the corresponding sediment load is 1,546 tons. Values are also shown for the 25-and <br />10-year storm events. <br />A regression analysis correlating unit discharge (cfs/acre) to azea in acres was performed <br />for the 100-yeaz event. The values correlate well (R2 of 0.7) and vary from 2.5 cfs/acre for small <br />watersheds to 1.0 cfs/acre for larger watersheds (see Part A of the Calculation Appendix). A <br />similar analysis was also performed for the sediment loads (tons of sediment versus acres) for the <br />10-year storm event and was also found to be well correlated (R2 = 0.7). The sediment loads <br />ranged from less than 50 tons from 50 acres to almost 1,200 tons for 700 acres for the 10-year <br />• storm (average of 750 tons/mi2). This unit load per squaze mile, which converts to an annual <br />load of 915 tons/mil, compares reasonably well to reported annual loads for agricultural land <br />uses (Novotny 1981). <br />For preliminary design of surface water management facilities (culverts, arroyos, <br />sediment, and detention ponds), the values in Table 2 were used. This approach is considered <br />conservative because under post-reclamation condition, the sub-basins will be flatter and the <br />slopes less variable, resulting in lower peak runoff rates and lower sediment loads during storm <br />events. <br />Critical Mining Period Analysis. A detailed analysis of storm runoff and sediment loads <br />was also prepazed for the critical mining period which results in 5-years of back fill under <br />vazious stages of reclamation and one yeaz of blasting, stockpiling, and resource handling. The <br />unit dischazges for this condition varied from 2.5- to 3.0 cfs/acre for the 100-year storm event, a <br />narrow range likely due to the smaller azea. The sediment loads for the 10-year event ranged <br />from 100-tons for the 5-yeaz mining period to over 700 tons for the I S-year mining period. This <br />range converts to an annual load of 1,000 to 5,000 tons/mil, indicating the potential impact of <br />bare soil on sediment loads. Whereas the sediment loads can be substantially higher than <br />existing conditions, the in-pit, sediment pond can be readily enlarged to accommodate both <br />sediment loads and storm runoff. <br />• <br />SWMP BC June 28 2002.doc, 06/28/02 <br />