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STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />131 }Sherman SL, Room '_ I S <br />Denver, Colorado 80203 <br />Phone. 13031 866-3561 <br />FAS: 1303) 83'8106 <br />DATE: March 13, 2001 <br />~~ <br />DIVISION OF <br />MINERALS <br />GEOLOGY <br />R E C L A M A T I O N <br />MINING•SAf ET7 <br />TO: Gregg Squire RIB Owem <br /> Grnernor <br /> <br />FROM: <br />Allen Sorenson ~ GrrR E. lValrhrr <br />E.ecun.r D~rrao. <br /> Michael B. EnnF <br /> <br />RE: <br />Reclamation Permit Application to Construct a Lined Gravel Pit, Drvmm~ D~recior <br /> Coulson Excavating Company, Inc., Bonser Pit, File No. M-2000-156 <br />I have reviewed the above referenced gravel pit application and have identified the following issues to <br />be addressed prior to recommending approval of the proposed plan for reclamation. The scope of this <br />review is limited to consideration of hydrologic and pit liner installation issues. <br />1. Possible Flooding_Imnacts <br />The proposed operation would mine gravel within approximately 100 feet of the Big Thompson River <br />and at approximately 1300 feet upstream of the I-25 Bridge. The Division is concerned with the <br />potential for off-site impacts if the river down cuts the pit banks during a flood and temporarily or <br />permanently alters the course of the river's mainstem. Down cutting that may result if the proposed <br />pit captures the river could result in both upstream and downstream impacts, possibly including <br />impacts to the 1-25 Bridge abutments. The Division suggests that a review of the Flood Insurance <br />Rate Maps (FIRM) for the pit location may provide useful information on the potential for flooding <br />impacts. <br />• Where are the floodplain and floodway boundaries in the area of the proposed pit? <br />• What is the anticipated depth, velocity, and flow direction of flood waters relative to the pit? <br />It maybe that the pit area becomes a placid backwater during a flood due to channel constrictions at <br />the I-25 crossing. This condition would be much less of a concern than if the pit area is subject to <br />deep, high velocity flooding, in which case the Applicant may consider installation of spillways to <br />pass floodwaters safely through the pit, or may consider increasing the mining setback distance from <br />the river. These types of mitigation measures are described in Urban Drainage and Flood Control <br />District publication "Technical Review Guidelines for Gravel Mining Activities," (December 1987) <br />and in the Division of Minerals and Geology Publication "In-Stream Aggregate Extraction and <br />Reclamation Guidance Document;' (August 1998). Flooding mitigation measures, if needed, may <br />also be designed based on the results of site specific floodplain modeling studies that consider the <br />location and shape of the proposed pit. The documents cited above are available for purchase from <br />