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<br /> <br />- 6 - <br /> <br />encroached upon soils suitable for grasses. In the latter instance <br />eradication precedes revegetation. Fine textured soils which may be <br />in the high erosion potential classification are more favorable for <br />: this purpose since they retain moi~ture in the shallow root zone. <br />. Greater ground cover density is achieved by replacing brush and small <br />trees with grasses. In arid and semi-arid areas seeding has in some <br />cases been effective on low hazard topographic sites. Its effective- <br />ness for reducing erosion on high hazard sites in these climatic environ- <br />ments has not been established. It is recommended for sub-humid and <br />humid climatic environments, high ~nd moderate hazard site conditions <br />and moderate hazard soils, particularly where quick cover protection is <br />needed following a brush or forest fire. <br /> <br />Tab'le I lists some of the more specific management and treatment measures <br />for erosion and sediment control under various site conditions. Climatic <br />environments are listed first, being the key to the success or effectiveness <br />of vegetation which is intimately related to all land treatment measures. <br /> <br />Structural Measures for Erosion and Sediment Control <br /> <br />Strpctural measures as those described 'below have met with more uniform <br />eff~ctiveness than land treatment measures. Achievement of the purpose for <br />whi'ch they were designed is not dependent upon nature. Their design, con- <br />struction and maintenance have a variable flexibility to meet demands of <br />the local situation. <br /> <br />The structural measures as defined are primarily intended for use where <br />chapnel erosion and sedimentation are the major problems. Debris basins <br />are: constructed to prevent sediment, usually coarse textured, from entering <br />a dbwnstream reach where damages may occur because of its accumulation. The <br />degree of control over the sediment problem depends upon the available cap- <br />aci:ty relative to the sediment yield apd on the stability of the channel <br />downstream. The latter must be able to resist scour where the erosion <br />potential is renewed by debris retention. <br /> <br />Reservoirs usually provide storage capacity for sediment likely to enter <br />the reservoir during the project life in addition to the capacity needed <br />for: the design flood. Sediment storage is a secondary purpose unless the <br />dam~ite is chosen so as to reduce stress on a downstream eroding channel. <br />In the Pacific Southwest where valley trenching in fine grained alluvium is <br />com~on, erosion and sediment transport is frequently limited only by the <br />magnitude of the discharge. Reduction of discharge by controlled release <br />above an extended reach of valley trenching can have a substantial influence <br />on channel erosion and sediment'yield. <br /> <br />Drop structures are widely used in dissected alluvial channels and mountain <br />chapnels to prevent continued unraveling of the bottoms and sides. They are <br />