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• 35 <br />VI, DESIGN OF CULVERTS, CHANNELS, AND SEDIMENT PONDS <br />The proposed drainage system consists of a series of culverts, channels, <br />and sediment ponds throughout the permit boundary. Many of the drainage struc- <br />tures are already present and the rest have been proposed in the G.E.C Minerals' <br />permit application. Hydrology Map 2, Appendix A, illustrates the drainage <br />network. The existing and proposed structures are analyzed with several ob- <br />jectives in mind. <br />' 1. Impound all runoff from areas which would produce large quantities <br />• of sediment or contaminated runoff. <br />• 2. Provide ditches and culverts along roads to prevent erosion from un- <br />controlled runoff. <br />3. Prevent disturbance of the natural sediment balance of the study area, <br />by not greatly increasing or decreasing the concentration of sediment <br />r leaving the lease area. <br />4. Utilize existing structures as such as possible and recommend modi- <br />fications if necessary. <br />5. Design channels and diversions which are stable. <br />Utilizing the results of the water and sediment runoff analysis, the size, <br />slope, shape, and other necessary design parameters for proposed structures <br />were determined. For the existing structures, the adequacy of their present <br />configuration was determined. The design parameters for the post-mining <br />condition are based on channel restoration, therefore, the channels must be <br />considered permanent. <br />6.1 Internal and Noncontributing Drainage <br />The internally draining and noncontributing areas were defined in the Pre- <br />liminary Data Base Report and are delineated on Hydrology Map 2, Appendix A. <br />The necessary requirement for the internally draining areas is that the natural <br />or man-made depressions be large enough to collect all the runoff occurring in <br />the area. In order to determine this, the storage volume of the depressions <br />and the runoff volume must be known. The storage volumes for each area were <br />presented in the Preliminary Data Base Report. The volume of runoff is esti- <br />mated using the .Green-Ampt infiltration eouation. This is the same method used <br />. in N,ULTSED. The parameters necessary for the determination are the rainfall <br />hyetograph and the soil infiltration properties. The 10-year rainfall event <br />for the Magpie Creek drainage was used. The soil infiltration properties used <br />