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1 ' This soil is used mainly for irrigated crops, hay and dry land farming. The soil is usually flood irrigated. The native vegetation is mainly Wheatgrass and sagebrush. ' This soil is in capability subclasses llle, irrigated and Illc, non irrigated. The east-west Interstate 70 corridor including the Last Chance Ditch bisects the north northwest ' trending runoff flow from the airport bench thereby preventing runoff from the airport bench area from crossing the project site. The I-70 right of way includes drainage control measures in the ' form of 24" reinforced concrete pipe that convey drainage from the south side of the highway right of way or the median to the north side of the Interstate. The location of the irrigation and drainage culverts are shown on Exhibits DS-1 and DS-2. The only Interstate drainage culvert ' that crosses under the irsigation ditches is located in the Dry Creek mining area. The drainage study will recommend a diversion ditch be built behind the south pit crest to divert surface runoff to the slough area. The remaining storm water culverts dischazge into the frontage road borsow ' ditch azeas. This storm water runoff is either intercepted by the slough existing irrigation ditches, or is directed towazd Mamm Creek by natural topography. There are no storm water culverts in the west end pit azea. The irrigation pipes entering the respective mining azeas ' discharge into ditch systems that distribute water across the top end of the fields. This study assumes that the head gates controlling the flow to a specific mining azea would be closed during mining. ' The slough area, Dry Creek and Mamm Creek aze protected from disturbance from mining by I either 50 or 100 foot buffers. Therefore, the flows in these features are expected to continue towards the Colorado River as they do now. The overflow channel in the Colorado River, the renewable resource area, is designed to let runoff flow through the azea to deposit sand and ~ gravel. Therefore, no change in flow direction through the renewable resource area is expected. The center portion of the project site and the northwest comer of the west end mining area aze in the I00 year floodplain. Portions of the future mining areas aze also in the 100 year floodplain. The 100 yeaz floodplain delineation is based on a floodplain study for Mesa and Garfield Counties in cooperation with the Colorado Water Conservation Boazd completed by the Army Corps of Engineers, Sacramento District. The topographic mapping is based on aerial photography taken in 1982. The high runoff flows that occurred in 1983 - 1985 have affected the channel of the river. While there are some discrepancies in the floodplain boundary due to the localized channel changes caused by the high flows, it is assumed that the boundary shown is representative of the area that would be affected by the 100 year event. Storm Water Runoff Estimate Methodology J This study assumes that the drainage basin is comprised of the proposed pits and the Interstate 70 corridor including two low spots on the south side of the Interstate. The renewable resource azea is excluded from the study it is north of the proposed pits and it will discharge into the Colorado River. Only two of the storm drainage culverts extending from the south side of the Interstate to the project site have dischazges that are not intercepted by irsigation head ditches or the slough. Discharges from these culverts flow into the Dry Creek mining azea via a culvert under the ~ imgation ditch. The stormwater runoff is diverted azound the balance of the project site by either