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<br />. <br />. <br />. <br />. <br />,. <br />. <br />. <br />'. <br />. <br />. <br />. <br />I. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />I: <br /> <br />using the lOO-year peak flow value (as provided in this report) in conjunction with an acceptable <br />hydraulic analysis. Hydraulic analyses for detennining lOO-year water surface elevations for <br />approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps <br />of Engineers' HEC-2 computer program, Flowmaster, or other acceptable hydraulic methods. <br />Channel and floodplain cross-sections may be obtained from detailed topographic mapping or <br />from field surveys. Cross,sections obtained from USGS quadrangle maps are not advised. <br /> <br />To verify the floodplain shown on the FHBM, the Town's topographic mapping and Flowmaster <br />hydraulic software was used to calculate normal depth for Calamity Draw immediately south of <br />Nucla. Normal depth represents the depth at which the channel would flow absent any <br />downstream channel constrictions that may cause the depth to increase. If downstream <br />constrictions exist, the normal depth calculation will result in a floodplain width that is too <br />narrow. The analysis shows a lOO-year floodplain width of approximately 160 feet for the cross- <br />section for normal depth, while the FHBM shows a lOO-year floodplain width of approximately <br />240 feet. It is not clear for Calamity Draw whether downstream constrictions exist which may <br />cause a wider floodplain. Therefore, it is advised that for planning purposes, the floodplain <br />shown on the FHBM be used with no modifications. <br /> <br />F100dDlain MaDDine <br />The existing FHBM for Nucla depicts areas subject to lOO-year flooding based on approximate <br />technical methods. The map does not show flood profiles, base flood elevations, or sufficient <br />detail in order for County officials, engineers, developers, and land owners to make floodplain <br />determinations with any certainty. The Colorado Water Conservation Board can be contacted to <br />request technical assistance for approximate floodplain analyses as needed. <br /> <br />5 <br />