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<br />the flood plain elevation. It is desirable, therefore, to evaluate the <br /> <br /> <br />effect flood plain regulation has on potential flood damage reduction. <br /> <br /> <br />The effectiveness of flood plain regulation in reducing potential damage <br /> <br /> <br />is determined by constructing aggregated elevation-damage curves at each <br /> <br />index location for the future land use pattern "and the future land use pat- <br /> <br /> <br />tern with the regulatory policy in effect. The aggregated elevation-damage <br /> <br />curves are then used as the basis for comparison. <br /> <br />When a land use pattern is subjected to flood plain regulation, the analysis <br /> <br />makes use of the reference flood to determine the flood plain of interest. <br /> <br /> <br />The elevation of the ~egulatory flood (flood plain of interest) at the <br /> <br />index location is determined by a method similar to that for flood proofing <br /> <br />damage reaches to a specified protection level. If the computed regulatory <br /> <br /> <br />flood event water surface elevation is higher than the topographic elevation <br /> <br /> <br />of the grid cell, the elevation-damage function for the grid cell is elevated <br /> <br /> <br />so that the ground elevation (or zero stage) is the same as the flood event <br /> <br /> <br />water surface elevation. The corresponding change in the elevation-damage <br /> <br /> <br />function is shown in Figure 4-10 (based on the grid cell example of Figure <br /> <br /> <br />4-8c). For a regulatory elevation of 423.0 feet, the elevation-damage func- <br /> <br /> <br />tion must be raised 3 feet to reflect the placement of ground elevation or <br /> <br /> <br />zero stage at or above the regulated flood plain. <br /> <br />~ <br />~ <br /> <br />Even though a grid cell has its ground elevation (or zero stage) moved above <br /> <br /> <br />a flood plain, it can still incur damages from the flood plain event. To <br /> <br /> <br />accommodate this, the DAMCAL program has the capability to place the ground <br /> <br />elevation or the zero-damage elevation at the flood plain elevation. Table <br /> <br />4-4 is an example taken from the Trail Creek Watershed (U.S. Army Corps of <br /> <br /> <br />Engineers, 1975) comparing expected annual damages with and without flood <br /> <br /> <br />plain regulation. The comparison is made for a 1990 future land use pattern <br /> <br />with 1) no flood plain regulation, 2) flood plain regulation in which the <br /> <br /> <br />ground elevation of new development must be constructed above the 100-year <br /> <br />flood plain, and 3) flood plain regulation in which new development is re- <br /> <br /> <br />quired to position the zero-damage elevation above the lOO-year flood plain. <br /> <br />4-25 <br />