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on the Federal Highway Administration's HEC 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 following fs an outline of the general design procedures used in the design of <br />culverts: <br />1. Identify the need for a structure from topographic maps, and a visit to the <br />site; <br />2. Determine the size and hydrology of the watershed in question Exhibits 13-4 and <br />13-5). <br />3. Perform SEDIMOT II computer run to determine the peak runoff from a 10-year, <br />24-hour precipitation event. Attachment I contains the typical inputs used for <br />these SEDIMOT II calculations. <br />4. Using the peak flow rate from SEDIMOT II, the tailwater depth downstream is <br />calculated using the Manning's Equation. The Manning's Equation calculations <br />are generally performed on a personal computer utilizing a program written by <br />Dodson and Associates entitled TRAP. Another program written by Dodson and <br />• Associates entitled PIPE is then used to perform the culvert design analysis. <br />Basically this program determines the capacity of the culvert using two <br />procedures, inlet control and outlet control. The procedure resulting in the <br />higher headwater is the value that the culvert design is based. Attachment J <br />contains a complete description of both the PIPE and TRAP programs. <br />Table 13-3 contains an inventory of existing and proposed culverts at the Nucla Mine. All <br />existing culverts are adequate. Figure 13-4 and Figure 13-5 show typical installation of <br />culverts. Other applicable methods include the use of charts developed by the Federal <br />Highway Administration, published in Hydrologic Engineering Circular HEC-5 (FHA, 1980), <br />and Hydrologic Design Series HDS-3 (FHA, 1980). <br />Charts published in HEC-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 nomography. To be conservative and to allow for adequate freeboard, <br />Peabody usually uses "projecting" conditions. <br />As virtually all culverts have free outf alts, inlet control assumptions can be verified by <br />13-28 Revised 04/11/88 <br />