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Senn By: Paul R. �cnroeaer 0.1 Apr '94 11 :40 P9 2 of 3 <br /> Adjustment of Curve Number for Surface Slope <br /> A regression equation was developed to adjust the AMC-II curve <br /> number for surface slope conditions. The regression was developed <br /> based on kinematic wave theory where the travel time of runoff from <br /> the top of a slope to the bottom of the slope is computed as follows: <br /> L2 1/3 1.49l-2�3 (30) <br /> nas <br /> (1 — I)1/3 S n <br /> where <br /> t" = runoff travel time (time of concentration), minutes <br /> i In = steady-state rainfall intensity (rate), inches/hour <br /> I = steady-state infiltration rate, inches/hour <br /> L = slope length, feet <br /> S = surface slope, dimensionless <br /> n = Manning's roughness coefficient, dimensionless <br /> A decrease in travel time results in less infiltration because less time is <br /> available for infiltration to occur. <br /> I Using the KINEROS kinematic runoff and erosion model (Woolhiser, <br /> Smith, and Goodrich, 1990), hundreds of runoff estimates were generated <br /> using different combinations of soil texture class, level of vegetation, <br /> slope, slope length, and rainfall depth, duration and temporal <br /> distribution. Using these estimates, the curve number that would yield <br /> the estimated runoff was calculated from the rainfall depth and the <br /> runoff estimate. These curve numbers were regressed with the slope <br /> length, surface slope and the curve number that would be generated for <br /> the soil texture, and level of vegetation placed at a mild slope. The <br /> four soil textures used included loamy sand, sandy loam, loam, and <br /> clayey loam as specified by saturated hydraulic conductivity, capillary <br /> L drive, porosity, and maximum relative saturation. Two levels of <br /> vegetation were described--a good stand of grass (bluegrass sod) and a <br /> poor stand of grass (clipped range). Slopes of 0.04, 0.10, 0.20, 0.35, and <br /> 0.50 ft/ft and slope lengths of 50, 100, 250, and 500 ft were used. <br /> Rainfalls of 1.1 inches, 1-hour duration and 2nd quartile Huff <br /> distribution, and of 3.8 inches, 6-hour duration and balanced <br /> distribution were modeled. <br /> The resulting regression equation used for adjusting the AMC-II <br /> curve number computed for default soils and vegetation placed at mild <br /> slopes, CN,,,, is: <br /> L.2 CNII0.81 (31) <br /> CNg = 100 - (100 - CNIO - <br /> S <br /> 27 <br />