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ENGINEERING METHODS Hydrolol!ic Analysis Hydrology information for the streams with approximate floodplain delineations on Lake County's FHBM has been estimated in accordance with the CWCB' s "Guidelines for Determining 1 00- Year Flood Flows for Approximate Floodplains" (Guidelines), The IOO-year flow for each of the streams with approximate floodplains (in all of Lake County) can be estimated by utilizing the regression equation for Arkansas River Basin, Subregion Ark-6, The equation is QIOO = 45,98(A)o.717 ,where QUHI is the IOO-year peak flow in cubic feet per second, and A is the upstream (contributing) drainage area in square miles, Figure 1, presented at the end of this report, displays the Arkansas River Basin with Subregions Ark-l through Ark-6. The regression equation is only valid for tributary streams having drainage areas between 4 square miles and 330 square miles. This equation is valid for the Arkansas River mainstem upstream of the Lake Creek confluence only, The remainder of the Arkansas River mainstem (downstream of the Lake Creek confluence) is exempt from the regression equation, 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, A procedure for determining hydrology information for approximate floodplains of lakes, ponds, and local drainages has not been established, 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 barns, and individual single residential structures, The 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 transportation facilities, or construction of flood control facilities, Lake County should regulate the approximate floodplains on a case-by-case basis subject to a detailed hydrologic and hydraulic analysis performed by a professional engineer, in accordance with the determinations of the Colorado Water Conservation Board regarding the applicability of the Guidelines, Hydraulic Analysis 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 town officials. The telephone discussions addressed any historic flood problems which may have occurred in Lake County. FEMA 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 lOO-year water surface elevations for approximate floodplains may be computed usingFEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, or other acceptable hydraulic methods.