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.~ O~~ ~j DRAINAGE STUDY FOR THE 32 ROAD GRAVEL PIT The purpose of this study is to determine the effect of a proposed gravel pit expansion on historic drainage conditions. This study is supplementary to the Conditional Use Application which has been transmitted to Mesa County under separate cover. The forementioned application contains pertinent information regarding site location, surrounding land use, and topographic features in the area of the gravel pit. The reader of this report should carefully review the application prior to studying the information contained herein. The accompanying Drainage Map depicts two separate historic drainage basins. One basin located east of the proposed gravel pit will not have any substantial influences as a result of the mining operation. Therefore, this drainage study will not evaluate the easterly basin. When gravel extraction is complete, no change will result in the historic drainage patterns. The rational method was selected to estimate peak run-off flows. Flow estimates included herein will not be used to size any new drainage structures. The studied drainage basin is limited to the area above an existing 18 inch diameter pipeline which crosses part of the existing gravel pit area. O = CIA C = 0.2 (Undeveloped Areas) Storm Drainage Manual, Table 402 Tc = 1700 feet B 0.8 fps = 23 min. 2100 feet 0 1.0 fps = 35 min. 58 min. INTENSITY Iio = 0.95 Iioo = 1.45 AREA = 89.9 acres Oio = 0.20 x 0.95 x 89.9 = 17.1 cfs O~ao = 0.20 x 1.45 x 89.9 = 28.1 cfs An overflow ditch will be away from the existing 18 culvert is exceeded. The ground, as indicated on t Capacity for the proposed sheet. provided to direct excess runoff inch culvert when capacity of the ditch will carry overflow above ~e grading plan to the river. ditch is shown on the accompanying