Laserfiche WebLink
RE. Prowers Aggregate Operators, LLC. - West Farm Pit-Permit No. M-2008-078 <br /> 112c Permit Amendment Application (Revision No. AM-01) <br /> Preliminary Adequacy Review Response 12121118 <br /> -11- <br /> mine, including during final reclamation. <br /> The flood analysis should quantify the velocity and volume of flows expected on site from a <br /> 100-year flood event, as well as the elevation of the 100-year base flood event and its <br /> relation to the elevation of any proposed spillways and reservoir embankments. The flood <br /> control plan should propose mitigation measures such as pit side armoring, inflow and <br /> outflow channels, and/or other appropriate measures. If specific details for these measures <br /> are not known at this time, please commit to submitting them later in a Technical Revision. <br /> Response: A Flood Insurance Study(FIS) was recently(2016) completed by FEMA <br /> for the Arkansas river adjacent to the gravel pit. This was a detailed study that <br /> includes 100-year base flood flows, and regulatory 100-year base flood elevations. <br /> The study shows a 100-year base flood flow of 22,828 cubic feet per second at <br /> Lamar. The 100-year base flood elevations and floodway velocities at various <br /> locations along the pit are shown in the following table. <br /> FIS Geographic Floodway Flow 100-Year Base Flood Depth <br /> Section Location Velocity Flood Elevation at Gravel Pit <br /> (fps) ff ft <br /> P West edge of 2.5 3,602.2 1.4 <br /> ravel pit <br /> O West edge of 1.8 3,600.5 2.0 <br /> Lamar lagoons & <br /> midpoint of pit <br /> N East edge of Pit 1.9 3,599.2 2.0 <br /> Pertinent sections/data are highlighted in the attached FIS study and FEMA Flood <br /> Insurance Rate Maps. <br /> Per Urban Drainage and Flood Control District criteria as shown in their guidance <br /> document "Technical Review Guidelines for Gravel Mining & Water Storage <br /> Activities Within or Adjacent to 100-Year Floodplains". pits adjacent to unstable <br /> alignment rivers that have riverside berms in excess of 400 feet do not require <br /> riverside (slope armoring) or pitside (slope armoring and/or side channel <br /> spillways) bank protection (Section 2.6.1 of the guidance document). The <br /> distance/berm width from the top of the north mining slope to the south river bank <br /> is approximately 800 feet which is much greater than 400 feet. No riverside or pit <br /> side protection is needed for the pit and therefore no Technical Revision to the <br /> permit will be necessary. Any minor erosion to the pit slopes that may occur <br /> during a flood event will be immediately repaired. Any flood flows that enter and <br /> are detained in the pit, or reservoir after the pit is reclaimed, will be pumped back <br /> to the river after the flood event is over, and after any sediment settles from the <br /> water. The pertinent sections from the guidance document are attached for your <br /> reference. <br /> Exhibit H -Wildlife Information (Rule 6.4.8): <br /> 35) The application states that no federal or state threatened or endangered species were <br /> identified in the study area. However, Colorado Parks and Wildlife (CPW) recently <br /> expressed concerns about the state endangered suckermouth minnow for which a section <br /> of the Arkansas River (in the vicinity of the mine site) is considered critical habitat. Please <br /> V M 305 Denver Avenue-Suite D-Fort Lupton CO 80621 •Ph: 303-857-6222•Fax: 303-857-6224 <br />