2011-12-16_REVISION - M1977572 (71)

Home Browse Search

• Grading the ground along Berm C to an elevation no higher than 3605. Lowering of Berm C will increase flow conveyance in the overbank area in order to meet the floodplain requirements. • Lowering of the ground along Berm D and Berm E to an elevation no higher than 3607. Grading of Berms D and E will also increase flow conveyance in the overbank area needed to meet floodplain requirements. Berm D should be graded to an elevation of 3605 to minimize the erosion potential of the berm when water flowing overtops Berm D from Pit 3 into Pit 4. • Grading of the active channel's right bank (Berms A -B to D -E) to an elevation 1 -foot below the corrected effective 100 -year water surface elevation profile. Lowering of the right bank ensures that during a 100 -year flood event the floodplain is hydraulically connected between the main channel and the right overbank area in order to meet floodplain requirements. • Lowering of the ground along Berm F to elevations specified and below the corrected effective 100 -year water surface elevations (similar to 1980 profile). The lowering of Berm F will allow 100 -year flows to spread out and provide hydraulic connectivity of the floodplain in order to meet floodplain requirements. • Grading of all pit side slopes at least 5 feet above and 10 feet below the pit's minimum normal water surface to a slope no steeper than a 3:1 per Colorado Division of Reclamation Mining and Safety Regulations. See Appendix D for the determination of the minimum normal water surface elevations in the pits. • Installation of outlet pipes in each pit designed to release water inadvertently captured by the pits during high flow events back to the river. To the largest extent practical, the outlet pipes will prevent the river from frequently backing up through them into the pits. 4.3 Erosion Protection Proposed erosion protection of the Berm A inlet will consist of a turf reinforced mat (TRM) and riprap. The TRM will cover the crest and face of the inlet while the riprap will anchor the top and protect the sides of the inlet. A riprap trench will anchor the toe of the inlet and provide shoreline protection from the normal wave action. 4.3.1 Turf Reinforced Mat The following is documentation supporting the selection of North American Green's (NAG's) P550 TRM (or equivalent). Berm A Spillway Crest Elevation: 3608 feet (NAVD88) Crest Width (Perpendicular to flow): 300 feet Crest Length (Parallel to flow): 20 feet Side Slopes (H:V) 3:1 Spillway Slope (H:V) 10:1 COVALCOO1_Phase 11 Report_082211.docx 4.2 ANdERSON CONSUETING ENGINEERS, INC.