1710 W MARINE VIEW DR 2024-01-03

BMP C131: Gradient Terraces Purpose Gradient terraces reduce erosion damage by intercepting surface runoff and conveying it to a stable outlet at a non -erosive velocity. Conditions of Use Gradient terraces are normally limited to bare land having a water erosion problem. They should not be constructed on deep sands or on soils that are too stony, steep, or shallow to permit practical and economical installation and maintenance. Gradient terraces may only be used where suitable outlets are or will be made available. Design and Installation Specifications . The maximum vertical spacing of gradient terraces should be determined by the following method: VI = (0.8)s + y Where: VI = vertical interval in feet s = land rise per 100 feet, expressed in feet y = a soil and cover variable with values from 1.0 to 4.0 Values of "y" are influenced by soil erodibility and cover practices. The lower values are applic- able to erosive soils where little to no residue is left on the surface. The higher value is applic- able only to erosion -resistant soils where a large amount of residue (1'/z tons of straw/acre equivalent) is on the surface. . The minimum constructed cross-section should meet the design dimensions. . The top of the constructed ridge should not be lower at any point than the design elevation plus the specified overfill for settlement. The opening at the outlet end of the terrace should have a cross section equal to that specified for the terrace channel. . Channel grades may be either uniform or variable with a maximum grade of 0.6 feet per 100 feet length (0.6%). For short distances, terrace grades may be increased to improve align- ment. The channel velocity should not exceed that which is nonerosive for the soil type. . All gradient terraces should have adequate outlets. Such an outlet may be a grassed water- way, vegetated area, or tile outlet. In all cases the outlet must convey runoff from the terrace or terrace system to a point where the outflow will not cause damage. Vegetative cover and energy dissipators should be used in the outlet channel. . The design elevation of the water surface of the terrace should not be lower than the design elevation of the water surface in the outlet at their junction, when both are operating at design 2019 Stormwater Management Manual for Western Washington Volume 11 - Chapter 3 - Page 310