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Aggregate Industries, Morrison Quarry <br /> Amendment No.7 to Permit M-1973-021 FYZ <br /> Morrison,CO <br /> outlet. All drainage diversion structures were sized based on calculated 100-year peak flow <br /> rates and runoff volumes to limit impacts to downstream structures in UD#2. Both detention <br /> basins will include emergency spillways, designed to accommodate 500-year peak flow with <br /> 50% grate blockage. Detention Basin 2, the inlet basin, will be installed at approximate El. 7,040 <br /> on the northern side of the South Quarry western face to intercept UD#1 along its natural flow <br /> path. Water will pass through the outlet control structure to a culvert to be installed on the <br /> existing upper western bench at El. 7,040, with a slope of 0.2% carrying water south along the <br /> bench towards UD#2. Once the culvert exits the western face bench, the slope will increase to <br /> 1.1% and pass through a control structure to enter Detention Basin #2. From there, water will <br /> discharge to UD#2. The drainage diversion will remain after mining activities are complete. The <br /> draft final drainage design can be found in Appendix 3, and Exhibit F.3 presents its final <br /> configuration. <br /> It is anticipated that stormwater may collect in the lower elevations of the South Quarry after <br /> significant weather events. Per the Storm Water Management Plan (Rev. 6, June 2021), visual <br /> observation will be conducted during and after storm events, and stormwater that has not <br /> infiltrated into the subsurface within 72 hours will be pumped to Outfall 003 for discharge. Once <br /> Phase III mining is completed, a long-term drainage structure will be designed and installed. <br /> The final design and cost estimate for this structure or a permit modification to remove this <br /> drainage requirement will be submitted within 24 months of this amendment signature. <br /> Schedule for Phase III Buildout <br /> Phase III development is anticipated to have a duration of approximately 12 years. The West <br /> Quarry will be mined concurrently with the South Quarry, and approximately 11 acres of the <br /> West Quarry and 86 acres of the South Quarry will be impacted during Phase III. <br /> October 2021 1 20 <br />