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Adequacy Clarification Grand River Park Project, M-2006-046 January 8, 2007 Page 2 2. Clarification of Comment 24. As noted above, certain aspects of Comment 24 are revised and superceded by the most recently developed water management strategy. Basin Configuration. The dimensions of the basin at the north end of Phase 1 B are modified as noted in Comment 10, as explained above. The applicant examined various options for locating the storage capacity in the clarification/regulating basin, placing the volume both above groundwater and within the alluvial flow (using special impervious construction). Given the revised estimate of volumetric requirements, the - basin-will be broad and shallow, with all storage capacity located above the existing groundwater table. Outlet to Colorado River. Previous plans assumed a gravity outlet from the basin, such as a lined trapezoidal channel, but no rational scenario results in a grading plan conducive to a gravity outlet to the Colorado River. In addition, scenarios for a gravity drain to the Colorado River involve introducing a break in the floodway levee; as a flood risk avoidance strategy, the applicant will avoid any activity that would threaten the integrity of the floodway levee, including avoiding the construction of a spillway. Therefore, discharge to the Colorado River is by pumps and piping, which will draw water from a sump within the regulating pond and return water to the Colorado River at an outlet safely located to prevent erosion of the levee. Spillways. Flood waters in the main channel of the Colorado River have no route to surcharge into the subject property by virtue of the floodway levee constructed along the northeastern property boundary. The limits of the flood fringe do include the proposed mining cells, and flood waters would migrate into the site by a circuitous route well removed from the main channel. The effectiveness of spillways is highly situational, and, as a result of recent analysis, only one spillway is called for (between Phase 16 and Phase 3A) as shown on the current Revised Exhibits C-2 and F. To avoid the unnecessary acceleration of flood waters, a spillway is not desirable at the fringe location where the 100-year flood would first encroach upon the site. Furthermore, based on additional data regarding the groundwater table across the site, showing groundwater in mining cell areas barely below the land surface, the strategy of turning off pumps at early flood stage will result in almost no void on site that would be subject to large differential head pressures from an adjacent flooded area. Only the interface between Phase 1 B and Phase 3A, because of the presence of underground gas infrastructure, is deemed appropriate for a spillway.