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FEMA Floodplain <br /> - I <br /> Northern �d: <br /> e� I <br /> mining disturbance <br /> 8459 <br /> - _ Floodplain <br /> elevations _. <br /> Figure 1 —Baseline F000dplain Elevations. North to the top. Cropped from Map C-1. <br /> 1.1.2 Flood Event Velocity <br /> Based on these elevations, and the topography of the creek channel, an approximation of the <br /> hydraulic radius for the 100-YR event can be determined at each elevation location. <br /> FloodStations Floodplain Cross Section wetted Hydraulic <br /> Elevation(ft) (sq.ft.) Perimeter(ft) Radius <br /> 1+70 8459 8979.5 251 36 <br /> 3+20 8448 7687 255 — 30 <br /> 4+19 8433 5054.3 231 22 <br /> 5+26 8430 5257.8 226 11 23 <br /> 6+34 8452 10208.8 278 37 <br /> 7+92 8443 9532.4 279 34 <br /> 8+68 8412 4274.2 207 21 <br /> 9+80 8386 1566.4 172 9 <br /> The average hydraulic radius is 29 feet; this ignores the 100-YR now cross section at station <br /> 9+80 due to its great deviation from the other stations. Also,this station is at the downstream end <br /> of the project's interaction with the floodplain. Using Manning's equation for open channel flow, <br /> a channel slope measured from topography, and a Manning's n of 0.0501, a flow velocity of 60 <br /> feet per second is determined. This is the approximate velocity of the Creek flow during a 100- <br /> YR event. <br /> 2. Mining and Reclamation Plans Impact on Flood Waters <br /> Mining and reclamation plans at the Douglas Mountain Ranch Mine will eventually lower the <br /> topography of the meadow to below current estimated floodplain elevations. This will allow a <br /> ' Mountain streams in clean loose cobbles,rivers with variable cross section and some vegetation growing on banks, <br /> or earth channels with thick aquatic growths((Raymond L.Lowie,2002)OSM, 1982) <br /> DMR Mine Flood Analysis 2 (;rtT 1a icki and Associates.PLLC <br />