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1 <br />1 <br />'P, <br />BMP C131: Gradient Terraces <br />Purpose <br />Gradient terraces reduce erosion damage by intercepting surface runoff and conducting <br />it to a stable outlet at a non -erosive velocity. <br />Conditions of Use <br />• Gradient terraces normally are limited to denuded land having a water erosion <br />problem. They should not be constructed on deep sands or on soils that are too <br />stony, steep, or shallow to permit practical and economical installation and main- <br />tenance. Gradient terraces may be used only where suitable outlets are or will be <br />made available. See Figure II-4.1.6 Gradient Terraces (p.309) for gradient terraces. <br />Design and Installation Specifications <br />. The maximum vertical spacing of gradient terraces should be determined by the fol- <br />lowing method: <br />Where: <br />VI = vertical interval in feet <br />s = land rise per 100 feet, expressed in feet <br />y = a soil and cover variable with values from 1.0 to 4.0 <br />Values of "y" are influenced by soil erodibility and cover practices. The lower val- <br />ues are applicable to erosive soils where little to no residue is left on the surface. <br />The higher value is applicable only to erosion -resistant soils where a large amount <br />of residue (1'/2 tons of straw/acre equivalent) is on the surface. <br />. The minimum constructed cross-section should meet the design dimensions. <br />. The top of the constructed ridge should not be lower at any point than the design <br />elevation plus the specified overfill for settlement. The opening at the outlet end of <br />the terrace should have a cross section equal to that specified for the terrace chan- <br />nel. <br />. Channel grades may be either uniform or variable with a maximum grade of 0.6 <br />feet per 100 feet length (0.6%). For short distances, terrace grades may be <br />increased to improve alignment. The channel velocity should not exceed that <br />which is nonerosive for the soil type. <br />. All gradient terraces should have adequate outlets. Such an outlet may be a <br />grassed waterway, vegetated area, or tile outlet. In all cases the outlet must convey <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 307 <br />