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• Introduction <br /> All existing and proposed light use roads will be desi gned, constructed, and maintained to <br /> have adequate drainage control. Generally light-use roads will utilize at grade crossing <br /> of drainage channels, but ditches and culverts will be co nstruc Y.ed in some areas. All <br /> existing light use road culverts are shown in Table 13-5 and depicted on Exhibit 13-2. <br /> The design calculations are contained in Attachment 13-7. A71 proposed light-use road <br />culverts will be sized utilizing the following generic culvert design method. <br />Generic Design <br />Since most all light-use roads are field located, the exact location of culverts are not <br />known until the light-use road is constructed. Peabody proposes to determine the size of <br />a required culvert by using a generic design approach. <br />Fi srt, different culvert diameters (i.e., 24-inch, 30-inch, 36-inch, and 42-inch) were <br />I analyzed to determine the peak flow for each culvert diameter. Tables 13-9-1 to 13-9-4 <br />contain the analysis of these culvert sizes by utilizing Dodson and Associates software <br />(see Attachment 13-1). <br />• + <br />After a peak flow is determined for each culvert diameter, then by utilizing SEDLAD , a <br />drainage area can be determined that will produce that peak runoff for a design storm. A <br />10-year, 24-hour precipitation event was selected as the design storm since it is not <br />known whether the proposed light-use roads would be in operation for less than six months. <br />To determine the peak runoff from a drainage area, four things must be known or assumed. <br />These are: drainage area, time of concentration (tc), curve number, and unit by drogra ph <br />shape. To ensure that the peak runoff is at a maximum for a given area, the time of <br />concentration was assumed to be 0.100 hours, or instantaneous and a fast runoff unit <br />hyd rograph was chosen. The curve number was assumed to be 86. By using these <br />conservative values, a drainage area could be determined that would produce a specific <br />peak runoff flow. Addendum 1 contains the SEDCAD outputs for various drainage areas. <br />Listed below is a summary of the culvert sizes and the corresponding peak flows and <br />drainage areas. <br />• <br />13-9-1 <br />