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<br />Hydrologic and Hydraulic Analyses <br /> <br /> <br />As an input to BCA, flood hazard areas under existing and various <br /> <br /> <br />future development conditions must be defined. Because the extent of <br /> <br /> <br />flooded land, the magnitude of potential damage and the cost of preventive <br /> <br /> <br />and corrective measures all depend on the magnitude of flood flows <br /> <br /> <br />derived from the hydrologic studies, it cannot be over-emphasized that <br /> <br /> <br />the hydrology is of primary importance to the analysis. The most reliable <br /> <br /> <br />hydrologic techniques consistent with the scope of the project and the <br /> <br /> <br />basic data available should be utilized in the analysis. <br /> <br /> <br />The benefit-cost analysis should include the computation of the <br /> <br /> <br />future flood hydrology for a range of recurrence intervals (at least <br /> <br /> <br />three) including the 100-year event. The recurrence intervals should <br /> <br /> <br />be chosen to give a representative spread in the peak flows, i.e., low, <br /> <br /> <br />medium and high. This will be used to define the base line conditions <br /> <br /> <br />from which the effectiveness "of each flood control alternative will be <br /> <br /> <br />measured. Derivation of runoff hydrographs is necessary. <br /> <br /> <br />Development of the future hydrology will require estimation of the <br /> <br /> <br />type and extent of future development. This information should be <br /> <br /> <br />available at the planning departments of the local jurisdictions within <br /> <br /> <br />the study area. Lacking any usable data, the engineer must make the best <br /> <br /> <br />predictions possible, to the extent of using the services of local <br /> <br /> <br />planners if desirable. Local planners will be familiar with the existing <br /> <br /> <br />development, local subdivision regulations, community preferences and <br />other factors that" will affect the type of development. The limit of <br /> <br /> <br />the development will depend on factors such as topography, soil type <br /> <br /> <br />and political boundaries. Encroachment into the floodplain is an item <br /> <br /> <br />of considerable significance, since flood damages are directly related <br /> <br />54 <br />