2008-07-25_GENERAL DOCUMENTS - M1999025

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Page 4 July 22, 2008 See response to concerns 2 and 3 above. 6. Any diversion of flood waters would reduce the river stage at flood peaks and result in above normal deposition of sediment in the downstream "island" area, thus altering the existing channel/slough/backwater system. Mr. Miller emphasized that his main concern is the reduction of flows during low flow periods such that the northern channel that defines the large island northwest of the Soaring Eagle Pit, experiences minimal or no flow conditions. WWL noted that flow measurements collected during low-flow periods showed that the north channel receives approximately 60 percent of the total river flow compared to 40 percent in the south channel. In addition, the culvert design inlet would limit flow diversions to less than 0:3 percent of total river flow for the full range of anticipated events up to the 100-year return interval. 7. The project itself would increase the risk of diverting the main river, regardless of the final state of the pit because pit operations are removing the more resistant coarse materials in the floodplain area. WWL and WE concur that the mining of the gravel from the floodplain area will remove the shallow, more resistant materials. As discussed in subsequent paragraphs, the final proposal presented herein includes construction of a levee dike from the upper end of the project (approximately river station 2.7 - see Figure 1 attached) to just downstream from the river split at the upstream end of the island (roughly river station 1.95) This dike is intended to reduce the potential for the required 150',Lft wide unmined barrier between the river and the pit to be breached and for the pit to, consequently, become the main river channel. However, without a hardened dike completely around the pit, the possibility for such an occurrence will remain and, at some conceivable flow quantity, is likely. The key permit requirement stated by the ACE is requirement number 2 above: "The permiee will maintain the gravel pit as deemed necessary by the Corps of Engineers to prevent the pit from becoming the main channel of the Colorado River." This requirement was discussed during the meeting. Mr. Ken Jacobson stated that it was not the intention of the ACE to require GJ Pipe to prevent capture of the Colorado River by the pit under all possible flow events, nor to implement engineered mitigation of the section of bank between the inlet and outlet. The main intent of the requirement was to design an inlet facility such that the inlet did not create a weak point in the barrier between the pit and the river that would increase the potential for capture of the river at the inlet itself. WWL and WE acknowledged this statement and put forth the question of possibly installing a levee or dike along a portion of the bank between the inlet and outlet to reduce pit capture in this area. Mr. Jacobson responded that the ACE would not require such a dike, but would work with the project proponents to facilitate its permitting if a dike was deemed necessary or desirable. Mr. Jacobson agreed to make these permit clarifications in writing Post-Meeting BEC-RAS Simulations Following the July 15 meeting, WWL conducted several HEC-RAS simulations to investigate the influence of a levee on stage elevations in the area adjacent to and immediately downstream of Soaring Eagle Pit. The proposed dike would have a top elevation above the 100-year flood stage and would start at the upstream end of the project (approximately river station 2.7) and end at a point just downstream from the upstream-most point of the island. The results of the simulations showed that during up to and including the 100-year event, (a river discharge of approximately 75,000 cfs),