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this base of the nearby draw. This discharge point will be monitored as part of a CDPHE <br /> Stormwater Discharge permit for the site. The entire subsurface drainage system will also <br /> discharge into this berm and the collection ditches. See Map C-4 for design details. <br /> Typical stormwater controls for disturbed areas of this size are sediment ponds and sumps. <br /> However,there is reason to believe that previous stormwater sumps along this portion of slope <br /> contributed to slides and slope failures in the past. Given that the remediation and reclamation of <br /> this area will be prompt, and the size of a sufficient sediment pond would be a significant portion <br /> of that total disturbance, a more compact, lower impact sediment trapping system is preferred. <br /> The rock filter berm is a system commonly used at coal mines for areas that cannot drain directly <br /> to sediment ponds due to topography. It is a proven system of sediment trapping. Furthermore, <br /> the collection ditches placed downgrade of the filter berm will transport water to an NPDES <br /> discharge point, which will facilitate the monitoring of surface water quality after it passes <br /> through the filter berm. <br /> 2.3.3.2. Subsurface Drainage System <br /> A subsurface drainage system will be installed in the regraded portions of the North Area. This <br /> system will intercept and transfer subsurface water from within the regraded zones to the surface <br /> stormwater system in the North Area. The designs can be seen on Maps C-4 and C-5. Further <br /> detail of their use can be found in the Ground Engineering report in the Geotechnical Stability <br /> Exhibit. <br /> 2.3.3.3. Collection Ditches <br /> A set of armored collection ditches will be used to convey stormwater from both the surface and <br /> subsurface of the North Area to the NPDES discharge point. Map C-4 shows the locations and <br /> designs of these ditches. <br /> To determine the subsurface drainage flows that the collection ditches would have to carry in the <br /> North Area, it was assumed that the volume of water would be less than a third of the total <br /> stormwater runoff(due to the curve number of the regraded ground) and that the water would <br /> flow to the drainage pipes are a lower speed than surface flows. Therefore, Ditches C-3 and C-4, <br /> which have to convey the subsurface drainage system discharge, were designed for 1.5 times the <br /> surface water runoff for their respective drainage basins. <br /> Ralston Quarry Gm%I, icki and Anwmtm PI I A' <br /> January 2019 G-5 <br />