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<br />these events were performed uSlng a log-normal probability <br />relationship. <br /> <br />Peak discharge-drainage area relationships for the City of Arvada <br />are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried Qut to provide estimates of the <br />elevations of floods of the selected recurrence intervals. The <br />analyses in this report reflect flooding potential based on <br />existing land use conditions in the community as of November 1988. <br />Any improvements located within the 100-year floodplain which <br />occurred after May 1983 were included based on the "as-built" <br />information provided by the City of Arvada. However, it should <br />again be noted that many of the peak flows used in these hydraulic <br />analyses were based upon future conditions as described in Section <br />3.1. Map and flood elevations will be amended by the developer as <br />development affecting the hydrology or hydraulics of the streams <br />occurs. <br /> <br />The basis of the hydraulic analyses utilized in this study is from <br />the Major Drainageway Planning Report and from the UDFCD plans <br />entitled Ralston Creek Flood Hazard Area Delineation Study, <br />(Reference 3). <br /> <br />For all streams studied, the water-surface elevations for floods of <br />the selected recurrence intervals were computed using the U.S. Army <br />Corps of Engineers HEC-2 step-backwater computer program <br />(Reference 4). Cross sections used in the computations were <br />obtained using aerial photogrammetric procedures based on recent <br />topographic mapping. All bridges, dams and culverts were field <br />surveyed to obtain elevation data and structural geometry. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of + or 0.5 foot for floods of the selected <br />recurrence intervals. The results obtained from the HEC-2 program <br />were adjusted in the vicinity of bridges to more accurately <br />represent actual flooding conditions. In areas where the hydraulic <br />response was more complex, such as areas of split flow, hand <br />calculations were performed. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floodway was computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Insurance Rate Map <br />(Exhibit 2). <br /> <br />Manning's roughness values varied depending on the type of channel <br />and reach of study stream under consideration. However, most <br />values ranged from 0.018 to 0.040 in channel areas and from 0.040 <br />to 0.10 in overbank areas. The Manning's values were chosen by <br /> <br />8 <br />