2019-03-18_PERMIT FILE - M2018016 (15)

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I00-YR event to spread out across the meadow, increasing the wetted perimeter of the flow cross section and reducing the hydraulic radius. Any decrease in hydraulic radius will lead to a decrease in the velocity of flow; therefore, it can be assumed that the final flood condition of the reclamation plan will feature lower velocities and therefore less erosive forces. Figure 2 shows an estimate of the lower flood elevation proposed by mining. This is based on a typical cross section(based on station 3+20, which has a hydraulic radius closest to the average)of the West Fork of Clear Creek and the proposed mining plan. The hydraulic radius of this estimated post-_ mining flood is 8 feet versus the hydraulic radius of the baseline data at 30 feet. This large decrease in hydraulic radius is caused by an increase in wetted perimeter as the flow is spread out across the mined-out property. a>,o Baseline topo. Est. Final ... . . Flood Elevation for Zone M4P — M>P Post-mine topo eam• enov — e3ea 200' ,9D' 18V ,10' I. I.. 14U 130' 12V 110• tar N' W ]V W W 40 W W 10 0 10' 2V X 40' W' W 1V 80' N' IW 110' 120' 13P IQ' IW IW 1 1W' 1- 200 21V Figure 2-Estimated Change in Flood Elevation. All elevations and distances in feet. This change in flood patterns is beneficial to the surrounding community as it slows flood water flows. The mined-out area will act as a hydrologic sink,pulling flood water and energy onto the site and absorbing it. Since no mining is proposed below the elevation of the West Fork of Clear Creek's typical flows, and the site is graded towards the creek,there is no risk of pit capture. 2_1 During Mining - Stormwater Berm During mining, in order to prevent the unwanted discharge of stormwater from the site, a riverside berm will be maintained along the West Fork of Clear Creek. Urban Drainage guidelines for a riverside berm require a top width of 100'. Mining activity proposed at the DMR Mine does not require the berm be steeply sloped on the pit side, which is usually the situation for riverside berms at gravel pits. The Urban Drainage guidelines acknowledge that site conditions deferring from those that the guidelines are designed around(the South Platte River) and may requiring modification on a case-by-case basis. Thus, the designs associated with the DMR Mine along the river have a basis in Urban Drainage design guidance with modification to best suit the site's unique condition. The stormwater berm along the creek will be maintained with a total width of 110' with the natural 2.5H:I V slope on the riverside of the berm, a 10' wide flat top, and a 100' pit side slope at 16.7H:IV. The berm will be six feet tall when measured from the final anticipated pit floor. The berm will be vegetated using the approved topsoil stockpile seed mix for the pit on all portions that are disturbed. Its location and extents can be seen on Map C-213. DMR Mine Flood Analysis 3 crem lxwkW and ns.«ian.PLIA'