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Donald A. Crane <br />Page 5 <br />• have a static safety factor of 1.3 (Section 816.102(b)) or a (vertical <br />separation of 50 feet is specified for valley fills (816.72(e)) or would be 20 <br />feet or less wide and not more than 50 feet apart (Section 715.15(b)(8)). Tlie <br />Contractor's draft report did not analyze the stability of the terraces. The <br />difference between the two terrace designs 1s sketched below: <br />OSPI <br /> <br />3~ <br />3~ <br />The two terraces would have almost equal stability with a slight advantage of <br />OSPf's design since the overall face slope would be less. OSM's terrace <br />arrangment also provides greater erosional stability (stability to water <br />erosion) since the terraces at 50 foot (vertical) intervals would reduce the <br />length of the slope. The reduction in soil loss is in proportion to <br />• reductions in the slope length according to the Universal Soil Loss Equation <br />A = RKLSCP.* The soil loss reduction achieved by halving [he length of the <br />slepe is about 15 percent (assumes a 400 foot slope of 30 percent reduced to <br />200 feet). Cololdyo has recognized the need to reduce slope length by <br />installation of contour furrows along the face of the fill. <br />Enclosures <br />ce: Bill Penny/Office of the Regional Solicitor <br />David Maneval/OSM Headquarters (2 copies) <br />Keith Kirk/OSM, Region V <br />0 <br />ColoWyo <br />*"Preliminary Guidance for Estimating Erosion on Areas Disturbed by Surface <br />Activities in the Interior Western United States," Interim Final Report, USDA <br />• and USE PA, July, 1977, EPA, 908/4-77-005. <br />