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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> 3.2.15 BMP C131: Gradient Terraces � <br /> Purpose <br /> Gradicnt turaces reduce erosion damagc by intercepting surface runoffand wnducting it to a stable outlet <br /> at a non-erosive velocity. <br /> Conditions o/Use <br /> Gradient tertaces nocmally are limited to denuded land having a water erosion problem. They should not <br /> be constructed on deep sands or on soils that arc too stony,steep,or shellow to pertnit practical and <br /> economical installation and maintenance. Gradient tertaas may be uscd only where suitable outlets are <br /> or will be mede availablc. See DCSS Dcawing#203 for gradient terraces. <br /> Design andlnstallation SpeciCcations <br /> 1. The maximum spacing of gredient teaaces shall be detcrtnined by the following method: <br /> VI =(0.8)s+Y <br /> Wherc: <br /> VI = vcrtica( interval in feet <br /> S = land rise p�r t00 feet,expressed in feet <br /> Y = a soil and cover variable with values(rom l.0 to 4.0 <br /> Values of"y"are influeoced by soil erodibility and cover practices. The lower values are <br /> applicable to erosive sails where little to no residue is left on the surface. i'he higher value is <br /> appliceble only to erosion-resistant soils where a lazge amount of residuc(l.5 tons of straw/acrc <br /> equivalent)is on the surface. <br /> 2. The minimum conshvcted cross-section should meet the design dimensions. <br /> 3. The top of lhe consWcted ridge should not be lower at any point than the dcsign elevation plus <br /> the specified ove�ll for settlement. The opening at the outlet end of the tertace should have a <br /> cross-section equal to that speci6cd for the tetrace channel. <br /> 4. Channel grades may bc cither unifocm or variable with a maximum gradc of 0.6 feet per 100[eet <br /> length. For short distances,tecrace grades may be increased to improve alignment. 'fhe channel <br /> velocity should not cxcced that which is non-erosive for the soil typc with the plaoned treatment. <br /> 5. All gradient:erraces should have adequate outlets. Such an outlet may be a grassed watenvay, <br /> vegetated area,or tile outlet In al(cases the oudet must convey runoff from the tertace or terracc <br /> system to a point where the outtlow will not cause damagc. Vegetativc cover should bc used in <br /> the outlet channel. <br /> 6. The design clevation of thc water sudace of thc rerrace should not be lower than the design <br /> cicvation of the watcr surface in the outlet al thcirjunction,when both arc operating at design <br /> noW. <br /> 7. Vertical spacing determined by the ahov:methods roay be increased as much as 0.5 feet or <br /> 10 pereent,whichever is greater,to provide beucr alignment ar location,to avoid obstacles,to <br /> adjust for equipment size, or to rcach a satisCactory autlet. <br /> 8. The drainage area above the tap should not excecd the arca that would be drained by a tecracc <br /> with nortnal spacing. <br /> 9. The terrace should have enou�h capacity to handle the peak runoff expected from a 2-yeer, <br /> 24-hour design stortn without averiopping. <br /> Volume 11—Construction Stormwater Pollution Prevenfion <br /> 3-27 <br />