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• ~' ,~~ ~ i III IIIIIIIIIIIIIIII <br />STATE OF COLORADO <br />Roy Romer, Governor <br />DEPARTMENT OFNATURALRESOURCES <br />MINED LAND RECLAMATION DIVISION <br />FRED R. BANTA, Director <br />DATE: October 27, 1989 <br />T0: Dan Hernandez <br />FROM: John T. Doerfer <br />RE: Hydrologic Adequacy Review, Corrective Action Plan for <br />Snyder Quarry Access Road, Castle Concrete Company, <br />File No. M-77-210 <br />I have reviewed the hydrologic aspects of the corrective action plan proposed <br />by Castle Concrete Company for the Snyder quarry access road received <br />October 6, 1989. I discussed these comments with you previously nn <br />October 12, 1989. In addition, I read over the comments submitted by other <br />parties you forwarded yesterday. Following the review of these materials, I <br />am providing you with the following comments on hydrologic aspect, of the plan. <br />Hydrol ogi c P1 an El ements <br />Six (6) culverts and a silt retention facility are proposed for construction. <br />The culverts will improve drainage from upland hill slope areas intro the <br />drainageway referred to as the "Snyder fork" of the Black Canyon drainage <br />basin. These hillslope areas are undisturbed and do not require sediment <br />control. A siltation facility is proposed in the previously-disturbed "Old <br />Scale Site". The actual dimensions and sizing of this facility are not <br />specified in the plan although an estimated size is given. The discussion <br />acknowledges that more explicit engineering designs must be provided. The <br />silt retention facility is included as a part of the plan to protect water <br />quality. The regrading and stabilization efforts on Slide Area No. 1 will <br />remove the area next to the rockslide where sediment presently is being <br />deposited. Building a pond in the area proposed will provide for sediment <br />control and act to replace the structure which is to be removed. <br />Drainage <br />The runoff amounts and peak flows used in the design and sizing of culverts <br />were based upon previous studies. Small subbasins were defined based upon <br />likely culvert locations and peak flows resulting from 10-year, 25-year and <br />100-year return interval precipitation events were calculated for each. The <br />25-year event was chosen as the design storm. A map showing subbasins and a <br />summary of hydrologic computations was provided in the appendix. iKy specific <br />comments on culvert designs are as follows: <br />215 Centennial Building, 1313 Sherman Street Denver, Colorado 80203-2273 Tel. (30:1) 866-3567 <br />