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. . . A detailed study or other hydrologic analysis must be performed for projects involving most of the Arkansas River mainstem or streams with drainage areas that fall outside the applicable range of the two methodologies discussed above. The Guidelines methodology is not appropriate for development involving residential subdivisions greater than 50 lots or occurring on sites greater than 5 acres, commercial construction, industrial construction, construction of public facilities, or construction of flood control facilities. The hydrologic methodology developed in the Guidelines is appropriate for designation and approval of approximate floodplain information and for use in simple floodplain analyses for development such as open space, agriculture, farm bridges, pole bams, and individual single residential structures. A procedure for determining hydrology information for approximate floodplains of lakes, ponds, local drainages, and overflows from irrigation canals and ditches has not been established, Prowers County should regulate its approximate floodplains on a case-by-case basis. A detailed floodplain analysis may be required in certain cases and should be performed by a professional engineer and submitted by the developer or development proponent. An approximate floodplain analysis can be submitted by a developer or development proponent, for the cases listed above, in accordance with the determinations of the Colorado Water Conservation Board regarding the applicability of the Guidelines. Hvdraulic Analvsis The hydraulic analysis for the preparation of the FHBM likely consisted of interpretation of USGS quadrangle mapping by HUD's engineering consultant, along with telephone discussions with local officials. The telephone discussions addressed any historic flood problems which may have occurred in Prowers County. The FIA did perform and did not have the benefit of any detailed hydrologic and hydraulic analyses for the preparation of the FHBM, Future hydraulic analyses for determining 100-year water surface elevations for approximate floodplains may be computed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, or other acceptable hydraulic methods. Floodnlain Mannin!!: The official floodplain information for Prowers County consists of the Flood Hazard Boundary Map (FHBM), dated July 12, 1977, The FHBM was originally prepared by the Federal Insurance Administration (FIA), but was later converted to a Flood Insurance Rate Map by letter from FEMA. The FHBM is the only available floodplain mapping for the unincorporated portions of the County at this time. Based on CWCB field observations and conversations with the Prowers County Land Use Administrator, the Zone A floodplain shown on the FHBM for the most downstream reach of Willow Creek (east of the Lamar city limits) is not accurate. The floodplain area depicted on the 5