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Aggregate Industries, Morrison Quarry <br />Amendment No. 7 to Permit M-1973-021 <br />Morrison, CO <br /> <br />October 2021 | 20 <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.