2023-11-20_PERMIT FILE - M2023040

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Exhibit G-1 RIVERCITY C O N S U L T A N T S Date: August 7, 2023 Project: MA Concrete Rolling Hills Gravel Pit Subject: Drainage letter The purpose of this letter is to address the pre-mining and post-mining drainage conditions for the proposed Rolling Hills Gravel Pit. This is a 231.E-acre project consisting of two parcels to be mined for sand and gravel aggregate and will be restored to rangeland as an interim condition prior to eventual residential development.The project is located at 841 Coffman Rd, Whitewater, CO 81527. The existing site is open rangeland.The proposed permitted area is 231.6± acres and the proposed mining area (affected area) is 83.4 acres. The property is owned by M&D Enterprises, dba MA Concrete. Approved site access for the property is along Coffman Road.The approved access point on Coffman Road will be the only access point. No permanent structures are being proposed. The parcels are not located in a regulatory Flood Zone and no hazard areas have been identified for the parcels. Existing drainage patterns will not be altered by mining activities. Existing drainage generally flows west to east across the project area. The gravel deposits to be mined are situated primarily at the tops of the knolls on the project property. The terrain will be flattened on the tops of the knolls. Runoff will not be concentrated within the mining areas, and existing drainage patterns will be maintained. The 2-year peak flow is 1.0 cfs, the 10-year is 29.1 cfs, and the 100-year is 162.3 cfs. Drainage calculations are included with this letter. All runoff from the site is tributary to the Gunnison River to the south of the project area. The drainage and erosion control plan for the gravel pit mining area will be to contain all stormwater in the mining and processing area on site through a combination of grading and berms. As mining progresses, vegetated stormwater berms will be installed on the downhills sides of the mining areas to prevent water from exiting the site. In addition, standard size hay bales (50-601b size) will be installed in a tight row along the bottom toe of the berm, and will be staked into the ground using rebar that extends at least 18" below ground level. The hay bales will be left in place until revegetation is complete. As the project area is developed in the future, the site development should be designed to follow existing drainage patterns and contain any increase in runoff from roofs, irrigated landscaping, and impervious surface areas on site with no off-site impact. Concentrated discharge should be avoided. Enclosures: Exhibit G-2- Drainage Map-MA Concrete Rolling Hills Gravel Pit Drainage Calculations MA Concrete Rolling Hills—Drainage Letter 1 of 1