Laserfiche WebLink
<br /> <br />- 8 - <br /> <br />(1) identify the major source or cause of sediment; i.e., land use, upland <br />erosion, channel erosion, by referring to columns G, H, and I in the table <br />on !'Factors Influencing Sediment Yield in the Pacific Southwest"; (2) from <br />thel aforementioned table, extract the topographic and soils characteristics <br />listed in columns E and B for upland erosion areas; (3) determine from <br />Fig~re 1 the climatic environments for the area on the broad basis of arid, <br />semi-arid, sub-humid, and humid. If the treatments listed in Table I are <br />checked as appropriate for each of the variables of climate, soils and topo- <br />graphy for the area considered, the treatment would likely reduce erosion <br />in the area. <br /> <br />Tho~e areas which may be identified geographically as "cliffs" or "badl~nds" <br />should not be considered suitable for land treatment measures. All areas <br />are, affected by geologic erosion, the amount depending upon the geologic, <br />topographic and climatic conditions peculiar to the site. This is a "back- <br />ground" rate of erosion unaffected by man's level of use either directly or <br />indirectly. High geologic erosion sites are characterized by an arid en- <br />vironment and/or by periods of exceptionally heavy rainfall and runoff. <br />Detached or easily dispersed soils on very steep slopes in an unfavorable <br />climate furnish an unstable medium for vegetative growth. In humid or sub- <br />hum~d areas landslides and land slips may be the characteristic expression <br />of geologic erosion, although land use can be a contributing or even major <br />cause. <br /> <br />Management measures applied alone are termed an extensive treatment. When <br />these are combined with land treatment measures, they are termed intensive <br />treatment. (See Table 2). Whether or not extensive and intensive measures <br />are recommended depends on treatments indicated as appropriate on Table I <br />and on other possibly limiting factors, including economics. <br /> <br />Eva1uation of Structural Measures <br /> <br />The: scope and method of evaluating structural measures is similar to that <br />for. treatment of the land in that off-site and on-site benefits may accrue <br />by application of some of the measures. For example, prevention of continued <br />bank erosion by a stabilization structure can reduce the sediment yield as <br />well as prevent the loss of more land along the bank. On the other hand, <br />land treatment measures usually apply to broad areas whereas structural <br />measures for the purposes herein described are placed in stream systems where <br />spe~ific sites may be involved. Evaluation is thus more on a specific site <br />bas~s. <br /> <br />Debris basins are designed for specific purposes. These may include pre- <br />vention of land destruction or deterioration by overwash, reduction of <br />cleanup costs, prevention of channel aggradation, and resulting overbank <br />flooding. A reduction in sediment yield based on debris basin construction <br />is justified only when coarse sediment is the major constituent. <br />