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Table 13-3, Culvert Summary, contains an inventory of existing culverts at the Yoast Mine. All existing <br />• culverts are adequate. Proposed culvert designs are shown on Exhibits 13-11.3, Proposed Haul Road <br />B Extension, and 13-11.4, Proposed Haul Roads C and D, and in Attachment 13-12, Drainage and <br />Sediment Control Plan. Applicable methods include the use of charts developed by the Federal <br />Highway Administration, published in Hydrologic Engineering Circular HEC-5 (FHA, 19801, and <br />Hydrologic Design Series HDS-3 (FHA, 19801. <br />Charts published in HEC-10 (FHA, 19781 are also used; however, exit velocities must then be <br />determined by other methods. Headwater conditions are typically examined 6y using HEC-5 inlet <br />control Homographs. To he conservative and to allow for adequate freeboard, Seneca usually uses <br />"projecting" conditions. <br />As virtually all culverts have free outfalls, inlet control assumptions can be verified by the "Pipe Flow <br />Charts" in HDS-3. If flow in the culvert has a free surface, entrance control exists, and exit velocity <br />can be approximated by using the greater of normal or critical velocity determined by the "Pipe Flow <br />Charts" in HDS-3. <br />• <br />• <br />18a Revised 2199 <br />