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STATE OF COLOKADO <br /> DIVISION OF MINERALS AND GEOLOGY <br /> Department of Natural Resources <br /> 1313 Sherman BI., Room 215 <br /> Denver,Colorado 80203 <br /> Phone:(303)866-3567 <br /> FAX:(303)832-B10b <br /> DEPARTMENT OF <br /> NATURAL <br /> RESOURCES <br /> Romer <br /> DATE: May 26, 1997 Go,erno, <br /> James S.Lochhead <br /> TO: File No. M-81-302 Executive Director <br /> Michael B.Long <br /> FROM: Allen Sorenson i Division Dnecior <br /> RE: Guidelines for Floodplain Evaluation <br /> TECHNICAL GUIDELINES <br /> The stream channel and floodplain are both integral parts of the <br /> natural conveyance of a stream. The floodplain carries flow in <br /> excess of the channel capacity and the greater the discharge, the <br /> further the extent of flow over the floodplain. <br /> The first step in any floodplain analysis is to collect data, <br /> including topographic maps, flood flow data from nearby gauging <br /> stations, and surveyed cross sections and channel roughness <br /> estimates at a number of points along the stream. <br /> It is my understanding that there is an accepted flood discharge <br /> for the 100-year flood on So. Boulder Creek, so the next step is to <br /> determine the profile of water surface elevation along the channel . <br /> This analysis can be carried out assuming steady, gradually-varied, <br /> nonuniform flow using a one-dimensional model such as HEC-2 (U. S . <br /> Army Corps of Engineers, 1982) . <br /> After the water surface elevations have been determined, the area <br /> covered by the floodplain is delineated. The lateral extent of the <br /> floodplain is determined by finding ground points on both sides of <br /> the stream that correspond to the flood profile (water surface) <br /> elevations . Ground elevations in the floodplain are determined <br /> from topographic maps; the floodplain is determined by following <br /> the contour line that corresponds to the flood profile elevation <br /> for a particular area. The floodplain delineation is only as <br /> accurate as the topographic maps used, so some verification through <br /> field survey is advisable . <br /> REGULATORY ASPECTS <br /> In order to provide a standard national procedure, the 100-year <br /> flood has been adopted by FEMA as the base flood for purposes of <br />