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67. <br />.. . <br />... _ .. _ <br />- CHAP1~f.R 5 r EROSION A~II SLDI\fEYf YIELD 191 <br />K=.4'_ <br />~. ~.,--~ ; ~ Estimates o! K have been made by the Soil Conservation Se,.icc based or. cx- <br />-- ~-~:~- pe rience and soil classification (or subsoils (or most areas of the United States. The <br />- local state StS office shoulJ be consulted for further information. <br />/~~- T' <br />,_,,~ ~ Length Slope Factor, IS <br />~.~~ <br />'~ ~ Uniform Slopes. The plots used (or the original erosion studies were 6 feet <br />_~ <br />___ (1.8 m) aide and 7~.6 feet (...I m) Icng on an approxunate slope o(9 percent. In <br />_ the LISLE. L'te LS factor is even a value o(unity (or these conditions. The LS factor <br />-- is the ratio of soil lou at any length and slope relative to 7..6 feet and 9 percent. <br />Slope length hu been dcfined (Smith and Wixhmeier. 1957) as the distance <br />from the point o(origm of overland tlou~ until the point that the slope decreases such <br />that deposition oaurs or until the Clow entea a defined channel. Based on data (or <br />-, slopes betu~ren 3 and '_0 percent, and lengths up to -500 feet, \1~ischmeier and Sntitlt <br />-~ ~ (1965) proposed that the LS factor could be ~+~~'n by <br />~_( a )m (43021 ~30s X043) (5.6) <br />7:b 6.613 <br />~:. o(hich clay subsoils. where a is slope length, x is sin e. ? is slope angle, and m is an exponent dependent <br />upon slope given by <br />slope < 3S~ m = .3 <br />slope = 45° m = .4 <br />''`, results are scope > SSA m = .5 <br />sucso:l using [he <br />Using these values a LS nomograph has been developed by the Soil Conservation Ser- <br />vice (1977) as shown in Figure 5.10. The dashed lines are for projections beyond tliC <br />range of experimental data. <br />Subsoil <br />For steep slopes, the National Coopera[ive Hig}tway Research Prc^_ram (1976) <br />'a'0 recommends that the LS factor be modified in accordance with [he cqua:ion <br />10.0 <br />3"G L$=( z )m (43021 +302 ~Od3 )( 10.000 ) (S.7) <br />430 12.6 6.613 10.000 *11OOx1r <br />1'0 and that m be set at .6 (or slopes greater than 10 percent. Thu alters the LS factor in <br />0'y equation (5.6) by the ratio (,l cc 1 ~ (10000/(10000+(IOOxIr))for shoe slopes (n=73' ). <br />0'~ For a slope of '_0 pcrcen[ 1 :~=11 °) and a length of 300 feet. this ratio is I.1 I. For a <br />- slope of 50 percent (c=_F°) and a leng[h of 300 fret, the into a 0.9'_. Since neither <br />- equation (5.6) or (5.71 are venlied for slopes greater than ~G percent, they must br <br />used with can[ion. ~icyer (19741 proposes that die exponent m should increase wuh <br />slope length and steepness consistent with equation ($.7 ). P,escarch on long steep <br />slopes is desperately needed to clan(y tltts issue. <br />Irrep lar Slopes. Several possibili[ics of slope shape are shown in figure 5.11. <br />The concave and tom~ex shapes have the same avrmgc slope but the affects o(slopr <br />and length on sedunent production will be diflrrent. Other factors bring equal, the <br />convex slope will hate the highest sediment ,,^.rod uchon to the channel bC'.1u5r the <br /> <br />