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Grand River Park Project, M-2006-046 <br />Response to Adequacy Letter of August 1, 2006 <br />6 September 2006 <br />Page 17 <br />because the floods in this watershed are governed by snowmelt) and turning off <br />pumps if there is a risk that flows will overtop the channel banks. <br />To mitigate against any residual chance the pit will be captured, another key <br />factor can be controlled. Each pit will have an armored spillway, to be lined with <br />8-inch or larger riprap, such spillway to be located at the leading corner of the <br />excavation with the lowest rim elevation relative to the flood surge. Initially, since <br />the low point on the river relative to the site is immediately adjacent to the bridge <br />at the interstate, floodwater will surcharge into the site through the outlet <br />structure of the clarification basin. A spillway will be built between the <br />clarification basin and the production area of Cell 1. <br />Should flood flows exceed the capacity of the main Colorado River channel, flow <br />in the flood fringes could enter the Phase 16 Mining Area from the east side. <br />Flow in the flood fringe is generally lower velocity and shallower than main <br />channel flows and the potential point of entry is a significant distance from the <br />main channel. It is unlikely that the magnitude and duration of potentially erosive <br />velocities would be sufficient to create a headcut back to the River and allow flow <br />in the main channel to be captured. Moreover, after a pit is filled with water - in <br />this case before or shortly after reaching flood stage -the effective elevation of <br />the pit for migrating waters is the same as the surrounding hood fringe and there <br />is no erosive effect below the rim elevation. <br />25. The applicant states that aCottonwood-dominated riparian area exists at <br />the northeastern fringe of the site and wetlands/wet meadow areas are <br />located at various locations across the site. The mining plan indicates that <br />dewatering will occur in all phases of the site. It is the Division's <br />experience that a drop in ground water levels of less than two feet can be <br />fatal to cottonwoods. Please indicate how the cottonwoods and wetlands <br />will be monitored and protected from such impacts. <br />Recovery of sand and gravel is a temporary process, which may significantly <br />disturb the immediate vicinity of construction materials extraction. This is the <br />justification for reclamation planning rather than a fact to be denied. It is possible <br />that all vegetative communities on the site will be affected to some degree by <br />mining or dewatering. There is no Construction Materials Rule prohibiting such <br />impacts and it is standard practice to account for appropriate replacement of <br />vegetation in the reclamation plan. In this case, the reclamation plan will emulate <br />and enhance current ecological functions of the site. <br />Regarding plant communities referenced in the Division's comment, the <br />reclamation plan does not specifically depend on the maintenance or <br />reestablishment of those communities in any particular form. Nonetheless, to the <br />greatest possible degree, the applicant intends to avoid impacts to both <br />