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• Post Reclamation Analysis. After reclamation, the mining surface will be flatter and have a <br />more uniform slope in the basin. As the result, the time of concentration for the sub-basin <br />hydrology will be longer, resulting in lower flood peaks. In addition, the sediment loads will <br />also be less than existing, because the sub-basin slopes are flatter. However, to be conservative, <br />surface water management facilities aze designed based on peak flows, volumes, and sediment <br />loads from existing topographic conditions, except for in-pit facilities. <br />STORM DRAINAGE PLAN <br /> <br />Surface water will be managed prior to mining, during mining, and after reclamation <br />using several methods and temporary or permanent facilities. Figure 3 provides an overview of <br />the proposed surface water management facilities. A description of each facility is provided <br />below. These methods are divided into ancillary mining, mining, and reclamation activities. <br />Ancillary Mining Activities <br />The construction of the access road, railroad spur, borrow azea, and plant area are defined <br />as ancillary mining activities requiring surface water management facilities. These management <br />facilities have been developed based on the following goals and objectives: <br />• ^ Protect transportation facilities. <br />^ Protect the plant from storm water runoff. <br />^ Control runofffrom disturbed areas. <br />^ Manage process water without surface water dischazge. <br />^ Manage domestic wastewater. <br />Protect Transportation Facilities. The access road and railroad spur are important to the <br />operation of the cement manufacturing facility and will be protected from major damages during <br />the 100-year flood. Culverts are provided where the road and railroad crosses an existing arroyo <br />or creates a sump condition with an embankment. Multiple culverts are specified and designed <br />with low head (i.e. headwater to depth ratio of 1.5 or less) to minimize erosive velocities at the <br />outlet. This approach also minimizes turbulence in the transitions into and out of the culverts, <br />because the broad, shallow flow of the natural arroyos is mimicked. Each culvert will have <br />either flared-end-sections or saddle headwalls to facilitate transitions. Erosion at the outlet will <br />be minimized by constructing rock-lined, pre-formed scour basins. <br />Table 3 summarizes the culverts required for the project. Calculations of pipe size, slope, <br />headwater and erosion requirements are provided in Part C of the Calculation Appendix, <br />Calculations for the drop structure where Arroyo "A" crosses the railroad spur and an example <br />for sizing the preformed, riprap lined scour basin are also provided. An example of the culvert <br />requirements for Arroyo "A" at the railroad spur is shown on Figure 5. Because railroad <br />embankment is limited, the culvert installation includes a drop structure upstream of the <br />• embanlanent to provide adequate headroom. <br />SWMP BC June 28 2002.doc, 06/28/02 <br />