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The impacts Of increased runoff in the receiving watersheds is not <br />• <br />considered as being too severe--r_onsidering the potential damage that <br />could occur by doing nothing. The massive bedrock that outcrops in both <br />watersheds will prevent excessive down cutting. In addition, extremely <br />high concentrations of sediment already exist in the runoff produced <br />under natural conditions. The impact from the introduction of comparatively <br />small quantities of additional sediment should not be detrimental to the <br />flow regime of Stevens Gulch. <br />Sediment Retention Dams <br />An estimated water balance as determined by a computerized hydrologic <br />• <br /> <br />simulation model (Leaf, 1977) for: (a) the watershed prior to disturbance, <br />and (b; conditions as they presently exist at Orchard Valley Mine is <br />summarized in Tables 1 and 2. This analysis is the basis for determining <br />the quantity of direct runoff to he expected from a 10-year, 24-hour <br />storm event, as required by EPA guidelines. <br />Storm Runoff Prediction: Rationale <br />The equation used for computing direct runoff is <br />Q= P+Rc <br />where P = storm rainfall in inches, and <br />s <br />Rc = the soil water recharge requirement, <br />in inches. <br />The theoretical basis or equation [1] is discussed in Part Q. <br /> <br />