Laserfiche WebLink
<br />" <br /> <br />N <br />00 <br />QO <br />'I <br /> <br />3 <br /> <br />on a discrete hydrologic unit with a defined outlet or yield point. <br />It is proposed that such a geomorphic unit PSIAC rating be used in <br />conjunction with a sediment delivery ratio procedure. A, sediment <br />delivery ratio (SDR) is generally defined as a percentage of the <br />total gross estimated sediment yield that is transported. toa <br />watershed outlet or major stream channel. The SCS National <br />Engineering Handbook (USDA, SCS, NEH) defines sediment delivery <br />ratio theory and describes techniques for estimating a sediment <br />delivery ratio. The PSIAC sediment yield should be routed to a <br />perennial flowing stream channel or appropr~ate delivery point. <br /> <br />Modification. 5: It is proposed that the PSIAC sediment yield <br />procedure may be used to model salt yield from rangeland caused by <br />accelerated erosion. The salt is a natural constituent of some <br />soils and is associated with rock fragments as a cementation agent <br />within sedimentary rock. Salt is also derived directly from the <br />weathering of limestone, halite, evaporite and gypsum rock <br />'formations. The salt content is defined as the percentage by <br />weight' of salt contained within a soil and associated rock <br />fragments. Some judgment must be exercised in establishing the <br />salt content of a soil that is actively becoming sediment because <br />large limestone or lime rich sedimentary rock fragments may not <br />completely break down in transport to yield the full percentage of <br />sal t content. . <br /> <br />The salt content of a soil can be obtained by direct sampling and <br />,chemical aniilysis in a laboratory or by estimatingthesal t content <br />from previous published analyses of comparable soils. ;J:t should be <br />noted that SCS soil survey reports contain some soil salt data. <br />This data often refers only to the solution prone soil salt content <br />and does not take into account the salt content of rock fragments <br />that can be yielded by a combination of mechanical and chemical <br />weathering processes that occur in the, stream transport ,of <br />sediment. ' , <br /> <br />The salt delivery percentage is not always a direct relationship <br />with the sediment delivery percentage. In the Colorado Plateau,as <br />well as in the intermountain basin region, streams will trap <br />sediment in channel bar and bank deposits. In some very active <br />streams with dominant mechanical weathering characteristics the, <br />salt content of stream sediments is yielded long before the <br />sediment is transported out of the watershed and may even be <br />yielded before the sediment is trapped in deposition within the <br />watershed. The decision of what percentage of salt is contained in <br />a sediment yield load and how much of that percentage is yielded is <br />a matter of a combination of empirical analyses and judgement based <br />on experience and observed stream transport characteristics. <br /> <br />Modification 6: It is proposed that the numerical sediment yield <br />classification system be reversed to reflect that the highest <br />sediment yield be given the highest category rating. The present <br />rating of low numerical rating for high sediment yield class has <br />