2021-02-03_REVISION - M2020008 (4)

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Rifle Gravel Pit #1 January 2021 (Amendment to File No. M-2020-008) 5.2.1.2 Water Handling and Dewatering Stormwater Management SCI will apply for coverage under Permit No. COR400000, CDPS General Permit (for) Stormwater Discharges Associated with Construction Activity at least 10 days prior to the commencement of construction activities disturbing greater than or equal to 1 acre. This permit has several requirements that address water quality and stormwater management, including monitoring and reporting, and the development and implementation of a Stormwater Management Plan (SWMP). Note this permit allows for the construction of a clean water diversion for mitigating off-site water from comingling with the mining activities. SCI will use a clean water diversion to intercept site run-on water and reroute it around the construction activities to its historical point of discharge from the property. No stormwater nor dewatered waters will be discharged into the clean water diversion and the appropriate stormwater management controls will be installed to prevent contaminants from flowing into the channel. See the clean water diversion channel detail in the revised drawings. The clean water diversion will be a v-notch shape approximately 2.5 feet deep with 2:1 side slope, 1% channel slope with a Manning's n of 0.030 (lined channel bottom with some gravel) -67 cfs. This configuration was selected based upon two rough calculations 1) the existing tailwater area is approximately 25 feet wide; 3.5 feet deep and -0.06% slope from east to west, with an estimated capacity of 55 cfs; 2) there is an existing 24" diameter corrugated metal pipe at a 5% slope (n-0.025) on the east portion of the Rifle Pit property conveying tailwater from the adjacent property with an estimated capacity of 28 cfs. Hence, the clean water diversion is sized to accommodate a potential range of run-on (28-56 cfs). The purpose of this clean water diversion channel is to eliminate the off-site water from comingling with the construction activities. Dewatering SCI will also apply for coverage under Permit No. COG603000 (Discharges Associated with Subterranean Dewatering or Well Development) or Permit No. COG608000 (Discharges to Surface Water from Well Development and Pumping Test Activities) to manage dewatered water. The specific permit will be determined once the water is sampled and analyzed. SCI negotiated a temporary license agreement with the Shideler's (Parcel 217908100527) to the east that will allow SCI to bury a 12" dewatering pipeline that will collect and convey pumped dewatered water to the Colorado River (Attachment A drawings). This pipeline will have an energy dissipation structure upstream of the discharge point to the Colorado River and an armored outfall to prevent erosion and sedimentation discharges into the Colorado River. The trenching and installation of the proposed dewatering pipeline will create temporary impacts to federally jurisdictional wetlands located on the Shideler's (Parcel 217908100527), and will also require a permanent outfall structure on the bank of the Colorado River, which will constitute a permanent impact to that jurisdictional water. A range of alternatives has been considered for this project and every effort has been made to avoid the discharge of dredged or fill material into waters of the United States. However, there are no practicable 16