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• c. Such other measures to minimize erosion resultant siltation and disturbance to the <br />prevailing hydrological balance. <br />$ulf3Gldg. See Exhibit 13-20 for typical detail. <br />Culverts. Each haul road was designed, constructed, and maintained to have adequate drainage <br />using ditches and culverts to safely convey the peak runoff from a 10-year, 24-hour precipitation <br />event to sediment ponds before being released from the permitted area (see Exhibits 13-4 and <br />13-5 for general locations and Exhibit 13-20 for typical details of drainage structures). <br />Culverts were installed to avoid plugging or collapse and to avoid erosion at the inlets and <br />outlets. Riprap was installed as necessary to reduce the exit velocity. The minimum width is <br />the width of the natural downstream channel. The riprap was sized in the field by Peabody's <br />project engineer based on the 'as-built slope of the culvert and final configuration of the exit <br />channel slope area. The sizing was based on the Federal Highway Administration's HEC No. 11 <br />'Use of Riprap for Bank Protection° or other standard methods. All pipes have a minimum cover <br />of 12 inches. Culverts and drainage ditches are maintained periodically to prevent accumulation <br />of debris on the culverts inlets and outlets. <br />The following is an outline of the general design procedures used in the design of culverts: <br />• <br />1. Identify the need for a structure from topographic maps, and a visit to the 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 higher <br />headwater is the value that the culvert design is based. Attachment J contains a <br />complete description of both the PIPE and TRAP programs. <br />• <br />13-12 <br />