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• Storm water runoff from the railroad and access road embankments will be collected in <br />ditches located at the base of the embankment and conveyed to the nearest arroyo culvert. <br />Erosion of the ditches will be controlled by ditch-check structures (small rock checks or geo- <br />synthetic check dams with heights up to 2-feet). Normal embankment slopes (that is, 3H:1 V <br />slope) will be protected from erosion with vegetation. Steeper slopes will use either geotextile <br />wrap or geogrid reinforcement to protect the slope. <br />Protect the Plant from Storm water Runoff. Protection of the cement manufacturing azea <br />will include the use of diversion ditches and an internal storm drainage system. <br />Diversion ditches are designed to convey the 100-yeaz flood with a maximum depth of 2 <br />feet plus 1 foot of freeboazd (see Figure 4). Disturbed surfaces and embankment slopes will be <br />seeded to protect them from wind and rain erosion. Depending on the slope of the diversion, <br />additional erosion protection may be required, such as a rock lining or reinforced turf mat. <br />Where diversion ditches discharge into existing arroyos, the transition azea is protected by riprap <br />to prevent excessive erosion. <br />Diversion ditches were designed based on the same flow velocity and shear stress <br />limitations as for arroyos. These limitations and design calculations are provided in Part B of the <br />Calculation Appendix. A design schematic for diversion ditches is provided in Figure 4, which <br />includes flow and slope limitations, depending on erosion protection methods used. The figure <br />also shows two options, an excavated and a bermed ditch. <br />• Diversion Ditch #1 (see Figure 3) will temporarily capture and convey storm runoff from <br />relatively small area west of the plant and north east of the mining limit for block year 1 through <br />5. Runoff will be conveyed under the plant access road and railroad and dischazged to Arroyo <br />"F" Sediment and Detention Pond (see below). <br />Diversion Ditch #2 will capture and convey storm runoff from Arroyo "E" and divert to <br />Arroyo "G," where the dischazge will be protected by riprap transition. During mining block <br />year 1 through 5, Diversion Ditch #2 will be sized to convey the entire area (475 acres). <br />However, when the mining progresses to block 6 through I5, the tributary area is reduced to <br />approximately 75 acres. A summary of all diversion ditches recommended for the Project is <br />presented in Table 4. <br />To capture storm runoff within the plant area, a storm sewer system will be constructed <br />with a 25-yeaz flood capacity (see Figure 6). This design frequency exceeds typical storm water <br />standards for industrial land uses, often. in the 5- to 10-year design frequency. The system <br />consists of curb and gutter, storm inlets, and storm sewer to collect local runoff for discharge to <br />Arroyo "F" Sediment and Detention Pond (see below), Calculations to size the storm sewers and <br />storm inlets are provided in Part A of the Calculation Appendix. An oiUwater separator device <br />(or an alternative oil control measure) will be constructed with the storm sewer before <br />dischazging into the Arroyo "F" sediment/detention pond. The discharge into the pond will be <br />protected by riprap transition. <br /> <br />S WMP BC June 28 2002.doq ObhB/02 7 <br />