1983-04-26_PERMIT FILE - M1983067 (7)

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HYDROLOGY REPORT • FLOODING A portion of this site is within the 100-year flood plain of the South Platte River. The bridge structure on Henderson Road is the main man-made obstruction to flood flows in this area . Although a 100-year flood will not overtop this structure, debris will tend to collect around the piers and girders. This may lead to possible scour around the bridge piers and the approach embankments. The floodway lies along the westerly edge of the property. The flood waters entering this site from the south will not be changed historically by this project. The floodplain in this area is 2,550- 3,200 feet in width. The floodwaters of a 100-year flood would reach an elevation of 5,010 to 5,012 feet with velocities between 0. 5 and 5.0 feet Per second. The natural overflow of the lake will be in the same location where the flood would normally be and therefore the pattern of flood flows will not be changed, and the velocity will not be changed. The storage potential of 94 AC-FT in the lake may help during a minor storm, but will have little or no effect on a major or 100-year event. Western Paving is aware that during flood conditions this floodway area may be completely inundated, and must remove all equipment • from the area or risk damage. No equipment, structures, piles, fuel or portable toilets will be allowed in the floodway that cannot be readily removed so as to prevent downstream damage. EFFECT ON GROUND WATER The proposed mining and reclamation will have no adverse impact on the existing ground water. Most of the available gravel to be mined is above the ground water elevation, and will be mined by the dry pit method. Hence, most of the mining will occur above the ground water elevation. The median water table elevation at this site is at an elevation of 5,002 feet. This information was compiled from 52 registered wells listed at the State Engineer ' s office, test hole data , and nearby gravel operations. The second phase of this proposed pit is to be operated in the wet pit phase . This will not require de-watering of the area to be mined. All wells in the vicinity of the proposed pit are at a relatively safe distance and should not be adversely affected by any localized depression of the water table. The ground water flow throuah the site will not be altered after reclamation procedures are completed. The flow pattern near the lake perimeter will be altered slightly, but this will have no impact on regional ground water with a Permanent reservoir. However, ground water monitoring wells will be installed on the south and east • sides of the property before gravel mining takes place. This will insure the vicinity well users that no adverse impact will occur to their wells. Should it appear that some adverse affects may result from the mining operation, measures will be taken by !destern Paving to correct the problem.