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Grand River Park Project, M-2006-046 Response to Adequacy Letter of August 1, 2006 6 September 2006 Page 16 applicant/operator to acquire a CDPS permit prior to discharging any water from a point source on affected land at the mine site, i.e., at the time it is necessary for such a permit to be obtained, not at the time the minerals reclamation permit application is submitted."). The applicant will provide written demonstration of an approved CDPS discharge permit to the Division prior to any offsite discharge and prior to the initiation of any mining activities. 24. Based on a review of the Pre-Mining and Mining Plan Maps, most of the proposed mining operation will take place within the 100-year floodplain of the Colorado River. Please describe the mitigation measures that will be implemented to reduce the potential of the 100-year flood event from capturing the pit and causing off site damage. Pit capture occurs when the channel of a stream shifts such that the majority of the flow is captured in a nearby excavated area. Model floodplain codes, including those adopted by Garfield County local jurisdictions, include direct authority to regulate this hazard. To our knowledge, the Construction Materials Rules contain no reference to pit capture or this type of alteration of natural watercourses, and none is cited. Assuming the Construction Materials Rules present some authority and rational basis to regulate flood hazards, the first step of hazard planning is to assess the risk. In this case, the Division seeks to judge the risk of the Colorado River shifting its main course to inundate excavated areas on the mining site and cause off-site damage. The cross-section of the Colorado River floodway adjacent to the site conveys the majority of the flow of the river; if an equivalent cross-sectional area were to erode away and back-cut toward the floodway, a transfer of stream energy could accelerate the erosion and form a channel between the river and the pit. However, with appropriate planning as described below, even in a worst-case scenario at the subject site, it is not necessary to plan for this contingency. In a worst-case scenario, flood waters would begin to enter the flood fringe at the eastern edge of the site and begin downcutting as they enter the pit. If de- watering pumps were turned off in anticipation of the flood stage, the storage capacity within the pit will be quickly filled to an elevation near the ground surface, thereby preventing downcutting below the natural groundwater elevation. This in turn prevents the back-cutting of the cross-sectional area necessary to carry any substantial amount of the stream capacity. A "split flow" would require more cross-sectional area than is available above the natural groundwater elevation. Any significant risk of pit capture is therefore mitigated by monitoring the forecast flood stage (flood stages are known well in advance