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<br />I <br />I <br /> <br />I <br />I <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />relatively narrow in width and steep in slope (approximately 66 <br />feet of drop per mile or 1.3%). The channel is irregular in <br />cross section, with bank widths of between 90 and 150 feet. The <br />channel bed through the North St. Vrain reach consists primarily <br />of cobbles and small boulders, with sandbars appearing near the <br />mouth of the North St. Vrain. <br /> <br />There are several gravel mining operations in the floodplain <br />within the study reach. Due to the transient nature of the <br />gravel mining, and to the radical changes which mining of this <br />type makes to the topography of the floodplain, the floodplain <br />analysis contained in this report applies only to the current <br />conditions of the floodplain as reflected by the topographic maps <br />used in the floodplain delineation. Another floodplain analysis <br />should be performed if gravel mining operations continue to <br />significantly alter the topography within the floodplain. <br /> <br />Obstructions to flood flows exist along the entire study reach. <br />Trees, brush, and other vegetation growing along the streambank <br />tend to impede floodwaters, causing backwater conditions that <br />increase flood depths upstream. Manmade obstructions such as <br />buildings, bridges and culverts also impede floodwaters, causing <br />backwater conditions that increase the flood flow depths upstream <br />of the obstruction and high velocities downstream of the <br />obstruction. Brush and other debris washed downstream during <br />flooding may collect at channel constrictions at bridges and <br />culverts, thus creating a damming effect. This damming effect <br />may cause overbank flow, which in turn may erode bridge abutments <br />and approaches. Where debris has collected at channel <br />constrictions, increasing flood flows may cause a mass of debris <br />to break loose, causing a wall of water and debris to surge <br />downstream until another obstruction is encountered. In some <br />instances, debris may collect to the point where structural <br />capacity of a bridge is exceeded, causing the bridge to fail. <br />Figure 1 on the next page identifies the study area. <br /> <br />-4- <br />