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1 <br />1 <br />1 <br /> <br />1 <br />1 <br />DRAINAGE FACILITY DESIGN <br />General Concept <br />The site will be approached as there is no disturbance to the site. The existing gravel pit will be <br />assumed to be non-existent in historical calculations. This historical flow will then be <br />conservative for the actual existing conditions. The proposed runoff will assume that '/~ of the <br />proposed mining site is disturbed. It is also assumed that most of the existing gravel pit will be <br />recovered before the second half of the additional mining disturbance occurs. The proposed <br />runoff calculations were based on Alternative C of the Environmental Analysis dated February <br />2000. <br />The proposed runoffwill be detained and released at or below historical rates using the detention <br />' pond. The pond will also include a WQCV for sediment control. All flow from the site will be <br />directed to the detention pond and released into the existing ditch along Colorado State Highway <br />No. 67. <br />' Existing Drainage Characteristics <br />Currently, the western portion of the site is disturbed from an earlier gravel pit. The runoff the <br />site is released without detention into the existing ditch along Colorado State Highway No. 67. <br />' For historical runoff calculations, the site will be assumed to have no disturbance. The historical <br />runoff was 11.16 cfs for the 5-year storm and 26.70 cfs for the 100-year storm. Please see <br />Appendix ] ;Rational Calculations. <br />' Proposed Design Drainage Characteristics <br />' Using the assumption of only % of the proposed mining site disturbed at one time (5.5 acres). <br />The proposed runoff is 48.2] cfs for the 5-year storm and 98.79 cfs for the 100-year storm. This <br />runoff will be directed to the detention pond using channels as needed. <br />' The WQCV for the site is 0.5 acres fora 40-hour release time. The depth of this volume is 3', <br />which gives the riser pipe a 0.75 in2 per row. A 8" PVC riser pipe with 16 -'/4" holes every 4" up <br />' will be used. The bottom of the pond will be at 9182.00 and the top of the WQCV will be at <br />9185.00. <br />' The additional storage will be used to detain the 5-year and 100-year design storms to release at <br />or below historical rates. A 36" CMP riser pipe with a trash rack will be used to release the flow. <br />2 - 3" holes at 9185.125 will be used to release the 5-year storm at 0.50 cfs. This shows a major <br />' reduction in flow compared to the historical flow of 11.16 cfs. The maximum elevation of the <br />pond during the S-yeaz storm will be at 9186.25. <br />' The top of the 36" CMP riser pipe will be at 9186.50. The release rate for the 100-year storm <br />will be 16.88 cfs at a maximum elevation of 9187.13. This also shows a major reduction in flow <br />' compared to the historical flow of 26.70 cfs. A 30" CMP pipe at 1% minimum will be used for <br />' \\Urw~16Tb742321\Mule Credo Gravel PitVteporb\Drun~ge Report 032100.d°c <br />