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I I I I I I I I I I I I I I I I i' I I No USGS gaging stations with adequate periods-of-record were available to verify the peak discharges for approximate floodplains shown in Table 3. Discharges calculated in the AS for determination of detailed floodplains are shown in the Appendix. Boulder County should regulate the approximate floodplains on a case-by-case basis. FEMA requires that a detailed floodplain analysis must be perfonned by a professional engineer and submitted by the developer or development proponent for development projects involving more than 5 acres or more than 50 units (whichever is more restrictive). Hvdraulic Analvsis Hydraulic methods used for the approximate floodplain delineations shown on the Boulder County FIRM panels are not available or have not been documented by FEMA. For future studies, an approximate lOO-year flood elevation at a particular location on the river may be obtained by using the 100-year peak flow value (as provided in this report) in conjunction with an acceptable hydraulic analysis. Hydraulic analyses for determining lOO-year water surface elevations for approximate floodplains may be perfonned using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or other acceptable hydraulic methods, Channel and floodplain cross-sections may be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised. Floodplain Mappine The existing FIRM for Boulder County depicts areas subject to lOO-year flooding based on approximate technical methods, The map does not show flood profiles, base flood elevations, or sufficient detail in order for County officials, engineers, developers, and land owners to make floodplain determinations with any certainty. The Colorado Water Conservation Board can be contacted to request technical assistance for approximate floodplain analyses as needed. 9