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:r- <br /> �, , 116 Transit Mix Concrete Co. <br /> b. Site Curve Numbers <br /> The modeled runoff volume is a function of the curve number (CN), watershed area, and storm <br /> size, with the soil curve numbers used to define the runoff from soils in the model. Runoff curve <br /> numbers are typically determined from soil survey hydrologic soils group classifications and <br /> vegetation cover and condition. Soil Survey classifies soil types into four hydrologic group <br /> ratings: A, B, C or D, with A-type soils having very high infiltration rates and D-type soils having <br /> very low infiltration rates. The area, curve number (CN), and the time of concentration were <br /> determined for each of the subwatersheds and entered into the SEDCAD model. A curve <br /> number of 98 was assigned to the open pit areas based on the impervious nature of the bare <br /> rock. A CN of 91 was used to represent the runoff response from overburden, fines and topsoil <br /> stockpile areas, reflecting the bare conditions and assuming a C hydrologic soil type (Schwab, <br /> 2005). This soil type assumes the stockpiles will be constructed with blasted overburden and <br /> have a sandy texture. <br /> Aerial photo interpretation suggests the native lands have forested or grassland vegetation. The <br /> western portion of the site area is predominately forested with a combination of Douglas <br /> Fir/Ponderosa Pine and the eastern portion is generally a community of sagebrush- <br /> grass/mountain mahogany. Based on vegetative and soil type mapping, as well as aerial <br /> photograph review, weighted curve numbers were developed for the various watersheds. The <br /> curve number values were based on tables from TR-55. As there are no values for Fir/Pine <br /> forests representative of the western portion of the site, it was assumed that Oak-Aspen <br /> community would be similar. Based on a D-type soil for the fir/pine forests, a curve number of <br /> 63 was assigned to all undisturbed subwatersheds for the western portion of the site. For the <br /> lower ranch in the eastern portion of the drainage, a weighted curve number of 68 was used <br /> based on the B-, C-, and D-hydrologic soil groups in that portion of the drainage area. For the <br /> portions of the stockpiles that have been reclaimed, a CN of 71 was selected based on a <br /> meadow with C-type soils. A medium hydrograph response class was designated all the <br /> subwatersheds. A weighted curve number was calculated for subwatersheds that were <br /> comprised of a mix of these cover conditions. Curve numbers are summarized in Table G-3. <br /> Table G-3 <br /> Curve Number Summary <br /> Cover Curve Curve Number Reasoning <br /> Number <br /> Open pit 98 Impermeable bare rock <br /> Overburden, fines, topsoil stockpile 91 Bare cover with C type soil <br /> Fir/Pine 63 Forest with D type soil <br /> Sage-grass/Mountain mahogany 68 Weighted by HSG B, C, and D <br /> areas <br /> Reclaimed stockpile 71 Meadow with C type soil <br /> Hitch Rack Ranch Quarry Permit Application M-2016-010 <br /> September 15, 2016 G-13 <br />