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-2- Plate I (100-scale map) shows the Phase 1 Drainage Plan proposed by • Colorado Westmoreland, Inc. The plan proposes to route storm runoff from most disturbed areas through 5 retention ponds located downstream from the TLS bench. As seen in Lhis plan, vegetative buffer strips are to be extensively used to control sediment. htoreover, the vigorous revegetation program currently underwzy also will greatly reduce the amount of erosion. EPd has taken a portion that the proposed drainage plan (Plate I) is not acceptable since the regulations require that stormflows generated from all disturbed areas be routed through settling ponds. Accordingly, a modified plan has been developed which will route stormflow from the Air Shaft bench to the ponds, as well as waters generated from the west side of Westmoreland Canyon. This modified plan would be in compliance with the regulations, but would also increase the flood hazard in lower Westmoreland Canyon. Because "credits" are allowed for on-site erosion control, the analysis presented below was made to determine the amount of dead storage capacity that should reasonably be designed into the system. PREDICTED EROSIDr; FROM BENCHES The modified Universal Soil Equation (USLE) recently developed by Utah State Univeristy (1976) was used to estimate po*-ential erosion and to also numerically show to what extent the proposed combinations of cross-ditches, benches, buffer areas, and revegetation will reduce erosion upstream from the proposed settling ponds. The procedure is discussed in some detail in Appendix I. However, the serious reader is also encouraged to consult the Utah State (1976) reference fora comprehensive treatment of the subject. Cross-Section of Orchard 4alley Mine The modified USLE requires a representative Cross-section for determination of the LS factor. It is believed that the cross-section shown in Plate A is a conservative basis for analysis since it depicts most of the disturbed areas at the mine. The slope was resolved into 9 major segments whose length and slope are summarized in Table 1. (It should be noted that the slope above the Portal Bench was not included in the LS factor analysis since this is an area of massive rock outcrops.) The parameters used in the headings of