2012-02-28_REVISION - M1980244 (73)

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SOIL TYPES AND RUNOFF CURVE NUMBERS As is required to use the TR 55 method, the surface runoff characteristics of the drainage area were determined based on the natural and "artificial" soil types in Arequa Gulch. Three soil categories were developed and are designated "original ground," "roads and dumps," and "fill slopes." Original ground, as demonstrated over the past years by the limited runoff, has a relatively low runoff coefficient. Fill slopes, as has been evidenced on site by the lack of runoff off from the slopes, also have relatively low runoff coefficients. Haul roads, which comprise the third category, have high runoff coefficients for most precipitation events as a result of compaction. These three categories are further described below. Original Ground: Soil Type B (Moderately Low Runoff Potential - e.g., mostly sandy soils and loess). Consists primarily of 4" to 12" of coarse topsoil covered with grass, brush, or trees and underlain by a loose, rocky sandy soil. Viewed as either thin vegetative stand with limited cover (if rangeland or meadow) or with good vegetative cover (if forest land). This category also includes revegetated ground. (Surface area characterized in this manner_will initially diminish, but then will reappear as reclaimed surfaces are developed.) Roads and Stock Soil Type D (High runoff potential e.g., shallow soils pile Surfaces: with nearly impermeable subhorizons). Consists of haul roads and horizontal surfaces of overburden storage area. Compacted by haul trucks. (Surface area in this category will generally diminish with time.) Fill Slopes: Coefficient: Wooded 66 Grasses 71 Coefficient: 90 Soil Type A (Low runoff potential e.g., deep sands with little silt and clay). Consists of segregated coarse rock overburden faces with high infiltration rate. No runoff is observed from these slopes. (Surface area in this category will remain more or less constant, until completion of the storage area, and will then decrease as the overburden faces are graded and reclaimed.) Coefficient: 50 These coefficients, upon delineation of the area to which they apply in any one subbasin, were developed into a composite coefficient for each subbasin, based on relative areas, to form a single curve number for each the drainage subbasins. AREQUA GULCH OVERBURDEN STORAGE AREA CONSTRUCTION SEQUENCE Because the Arequa Gulch Overburden storage area is to be constructed over a number of years, the drainage control plan has been developed to accommodate the changes that will occur over time. Five points in time were used to assess and modify the drainage controls. These "configurations" of the storage area are presented in five of the six plan views accompanying this documentation (see Attachments). The five plans show approximations of the topographic configurations that would likely develop as the storage area is built. This allows delineation of the drainage areas that will develop and incorporates the changes in surface type as the undisturbed land is disturbed and then revegetated. Field- adaptation of the drainage facilities depicted will occur, given the need to be responsive to changes in the construction or construction 3