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c. Such other measures to minimize erosion resultant siltation and disturbance to the <br />prevailing hydrological balance. • <br />As stated earlier, some haul road segments exceed a 10% grade for more than 300 feet. Culverts will <br />be installed to reduce watershed areas for these particular road segments, thereby reducing runoff and <br />flow velocities. However, erosive velocities may occur during the design storm event. SCC will <br />construct porous rock dams and/or sumps in the roadside ditches, at various points between culverts, <br />to aid in dissipating the erosive velocities and assist in minimizing the amount of sediment reaching <br />some of the culverts along these roads. They have proven to be an effective sediment control <br />measure along several haul roads in Seneca II-W. <br />Runoff from all roads located within the mine disturbance area will be routed to sediment ponds Irefer <br />to Exhibit 12-1, Operations Planl. Therefore, no adverse environment damage will occur outside the <br />proposed disturbance area. The roads and ditches will be maintained daily as part of routine mine <br />operations. The outlets of culverts will 6e armored, as necessary, to control sediment and downslope <br />erosion. Refer to the haul road designs (Exhibit 13-11.3 and 13-11.4j for culvert locations. Refer also <br />to Exhibit 13-11.5, proposed Haul Roads R, C and D Profiles, for road and ditch segment which <br />exceed 10% grades. <br />Tab 13, Attachment 13-12, Drainage and Sediment Control Plan, contains haul road ditch and culvert • <br />designs. <br />Surfacing. See Exhibit 13-9, Typical Haul Road Details, for typical detail. <br />Culverts. Each haul road will be 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 before being released from the permitted area (see Exhibit 13-2, Surface Water Control Plan, for <br />general locations and Exhibit 13-9, Typical Haul Road Details, for typical details of drainage structures) <br />and Figures 13-3, Typical In-Wash Culvert, and 13-4, Typical Cross•Drain Culvert, for culvert <br />installation. <br />Culverts will be installed to avoid plugging or collapse and to avoid erosion at the inlets and outlets. <br />Riprap will be installed as necessary to reduce the exit velocity. The minimum width will be the width <br />of the natural downstream channel. The riprap will be sized in the field by Seneca's project engineer <br />based on the "as-built" slope of the culvert and final configuration of the exit channel slope area. The <br />sizing shall be based on the Federal Highway Administration's HEC No. 11 "Use of Riprap for Bank <br />Protection" or other standard methods. All pipes will have a minimum cover of 12 inches. Culverts • <br />and drainage ditches will 6e maintained periodically to prevent accumulation of debris on the culverts <br />Attachment 13-1, General Hydrology Design Methods. <br />18 Revised 7/99 <br />