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2.0 Prevention <br />P -e erosion problems can be prevented by controlling three factors: 1) rooftop <br />c+ . 2) surface runoff from lawns and yards; and 3) seepage of ground water at <br />of steep slopes and at the base of retaining walls. <br />Roofs should be guttered and the drainage directed, as much as possible, toward <br />the street. Where grading permits, rooftop drainage from the rear of the house can <br />be carried to the street in shallow subsurface drains. In special cases, drains can <br />carry rooftop drainage down slope. Subsurface drains should be six-inch diameter <br />corrugated polyethylene unperforated tubing. <br />Surface runoff from lawns, yards, or unguttered portions of roofs must be <br />controlie,j to minimize water volume and velocity at any single point. Landscaping <br />should be modified to prevent channelling of surface water. Low walls cr berms an <br />be constructed along the :.ope crest. Water ponded behind the walls must be <br />drained through subsurface storm sewers. Eight -inch diameter corrugated <br />polyethylene linogrforated tubing can handle drainage from approximately 10,000 <br />square feet if it slopes greater than 1 on 2 (50% grade). Inlets must be protected <br />from trash, leaves, etc. Outlets must discharge oni:o riprap. Vents must be placed in <br />the line where there are significant changes in grade. <br />Many slope failures are caused by excessive pressure exerted by ground water at <br />the toes of slopes. Subsurface drains running along the contour at the toes of slopes <br />and behind retaining walls will relieve the excess pressure and prevent slumping. <br />Standards for subsurface drains are given above. <br />3.0 Seeding and Mulching <br />It is important to develop a good vegetative cover cn steep slopes as cuick:y as <br />posssible. The work to be performed for seeding disturCed areas includes: <br />