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drainage using ditches and culverts to safely convey the peak runoff from a 10-yea r, <br />24-hour precipitation event to sediment ponds before being released from the permitted <br />area (see Exhibit 13-9 for typical details of drainage structures). <br />Culverts will be installed to avoid plugging or collapse and to avoid erosion at the <br />inlets and outlets. Riprep will be installed as necessary to reduce the exit velocity. <br />The minimum width will be the width of [he natural downstream channel. The riprap will be <br />sized in the field by Peabody's project engineer based on the "as-built" slope of the <br />culvert and final configuration of the exit channel slope area. The sizing shall be based <br />on the Federal Highway Administration's HEL No. 11 "Use of Riprap for Bank Protection" or <br />other standard methods. All pipes will have a minimum cover of 12 inches. Culverts and <br />drainage ditches will be maintained periodically to prevent accumulation of debris on the <br />culverts inlets and outlets. <br />The general design procedures used in the design of culverts is contained in Attachment <br />13.1. <br />Table 13-3 contains an inventory of proposed culverts at the yoast Mine. Figure 13-3 and <br />Figure 13-4 show typical installation of culverts. Other applicable methods include the • <br />use of charts developed by the Federal Highway Administration, published in Hydrologic <br />Engineering Circular HEC-5 (FHA, 1980), and Hydrologic Design Series HDS-3 (FHA, 1980). <br />Charts published in NEC•10 (FHA, 1978) are also used; however, exit velocities must then <br />be determined by other methods. Headwater conditions are typically examined by using <br />HEC-5 inlet control Homographs. To be conservative end to allow for adequate freeboard, <br />Peabody usually uses "projecting" conditions. <br />As virtually all culverts have free outfal ls, inlet control assumptions can be verified by <br />the "Pipe Flow Charta" in XDS-3. If flow in the culvert has a free surface, entrance <br />control exists, and exit velocity can be approximated by using the greater of normal or <br />critical velocity determined by the "Pipe Fl or Charts" in HOS-3. <br />Naintenence. Access roads will be inspected on a deify work basis at the Toast Mine. If, <br />upon inspection, it is determined that repairs are needed, appropriate repairs such as, <br />but not limited to, blading, filling of potholes, replacement of the road surfaces, <br />• <br />28 <br />