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. . . Jefferson County should regulate the approximate floodplains on a case-by-case basis. FEMA requires that a detailed floodplain analysis must be performed by a professional engineer and submitted by the developer or development proponent for development projects involving more 5 acres or more than 50 units (whichever is more restrictive). Hvdraulic Analvsis Hydraulic methods used for the approximate (Zone A) floodplain delineations shown on the Jefferson County FIS panels are not available or have not been documented by FEMA. For future studies, an approximate lOa-year flood elevation at a particular location on a stream may be obtained by using the lOa-year peak flow value (as provided in this report) in conjunction with an acceptable hydraulic analysis. Hydraulic analyses for determining lOa-year water surface elevations for approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 or HEC-RAS computer program, Flowmaster, hand calculations, 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. FloodDlain MaDDin!! The existing Flood Insurance Study with accompanying Flood Insurance Rate Maps for Jefferson County depict areas subject to lOa-year flooding based on detailed and approximate technical methods. Only for certain stream reaches do the FrS and FIRM panels 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. 5