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. ENGINEERING METHODS Hvdrolol!ic Analvsis Hydrologic information used for the FEMA floodplain delineations in Yuma is not available or was not documented by FEMA. However, acceptable hydrologic information was developed by Hydro- Triad, Ltd. using the SCS rainfalI-runoff methodology. A complete description of the procedure can be found in chapters 1II and N of the Phase A Report. The City of Yuma 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 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 Town of Yuma FHBM panel are not available or have not been documented by FEMA. For future studies, an approximate lOO-year flood elevation at a particular location in the floodplain may be obtained by using the 100-year peak flow value (as provided in the Phase A Report) in conjunction with an acceptable hydraulic analysis. Hydraulic analyses for determining I DO-year water surface elevations for approximate floodplains may be performed 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. Floodvlain MavviDl! The old Flood Hazard Boundary Map for Yuma 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 necessary for City officials, engineers, developers, and land owners to make floodplain determinations with any certainty. The FHBM was determined by FEMA and CWCB staff to be inaccurate. Better floodplain mapping is depicted in the Phase A Report dated November 1976. FLOODPLAIN MANAGEMENT Local Ordinance Requirements Communities that manage their floodplains in Colorado do so most often through the local regulatory process. This requires a community to adopt an ordinance or regulation, which sets forth the performance standards by which development wilI occur in identified flood hazard areas. Nationwide and in Colorado, regulations are enforced within the area of the lOO-year floodplain. . The basic requirement of these regulations is that new development and significant improvement of existing development cannot occur in the I DO-year floodplain without a floodplain development permit being obtained. Obtaining such a permit requires compliance with engineering standards. In communities like Yuma that have approximate floodplain information, the engineering standards have been more general. Structures must be built with materials and techniques that "minimize flood damages." For communities that have detailed floodplain information, more specific 3