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• The minimum width will be the width of the natural downstream channel. The riprap will be sized in the field by Peabody~s project engineer based on the "es-built" slope of the culvert and final configuration of the exit channel slope area. The sizing shall be based on the Federal Nighway Administre[ion~s HEC No. 11 "Use of Riprep for Bank Protection" or other standard methods. ALl pipes will have a minimum cover of 12 inches. Culverts and drainage ditches will be maintained periodically to prevent accumulation of debris on the culverts inlets and outlets. The general design procedures used in the design of culverts is contained in Attachment 13-1. Tables 13-5 and 13.6 contain an inventory of existing end proposed culverts at the Seneca II-N Mine. All existing culverts are adequate. Figure 13-3 and Figure 13-4 show typical installation of culverts. Other applicable methods include the use of charts developed by the Federal Highway Administration, published in Hydrologic Engineering Circular HEC-5 (FHA, 1980), end Hydrologic Design Series HDS-3 (FHA, 1980). Charts published in HEC-10 (FHA, 1978) are also used; however, exit velocities must then be determined by other methods. Headwater conditions ere typically examined by using • HEC•5 inlet control nomograph s. To be conservative and to allow for adequate freeboard, Peabody usually uses "projecting" conditions. As virtually ell culverts have free outf alts, inlet control assumptions can be verified by the "Pipe Flow Charts" in HDS-3. If flaw in the culvert has a free surface, entrance control exists, and exit velocity can be approximated 6y using the greater of normal or critical velocity determined by [he "Pipe Flow Charts" in HDS-3. Monitoring. A section of the Naul Road G alignment traverses the sideslope of a hill. Although the stability analyses indicate that the road cut will be stable, as an added safety measure, Peabody will monitor the slope above and below the road alignment. Rebar will be driven into the ground at locations indicated on Drawing No. 9111-5, Attachment 13.3A (Ivey Report). These monuments will initially be surveyed after installation prior to road construction. End-of-construction conditions are the most critical periods for cuts in clays. Therefore, [he monuments will be surveyed for movement immediately after the cuts have been completed. They will be surveyed monthly for the firs[ quarter and quarterly on the same schedule as Pond 005. If no movement is detected after one year, monitoring will be suspended, upon approval by the Division. 27 Revised 01/06/94