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<br />These detailed computations are performed as follows: The reference <br />flood elevation is substracted from the target protection elevation at <br />the index location. and then this difference is added to the reference <br />flood elevation of the grid cell. This elevation then represents the <br />computed protection level water surface elevation. The amount of pro- <br />tection required by the cell is the protection level elevation less <br />the topography elevation. If the grid cell requires flood protection, <br />the program truncates the elevation-damage curve at the pr~tection level <br />elevation. This process is repeated for each grid cell assigned to the <br />damage reach. Fig. 6 Grid Cell Damage Functions illustrates an example <br />of a grid cell which has a reference flood elevation of 424:5 feet and <br />a topographic elevation of 420.0 feet. The damage reach index location <br />has a reference flood water surface elevation of 427.0 feet and the <br />target protection level is 425.5 feet. The difference in water surface <br />elevations at the index location is a minus 1.5 feet (425.5-427.0). <br />This difference is added to the reference flood elevation of the grid <br />cell to compute the corresponding target protection elevation (424.5-1.5 <br />. 423.0 fee~ for the cell. The resulting truncated elevation damage curve <br />is then aggregated to the index location in the usual manner. The <br />alternative of the flood proofing land use categories within a damage, <br />reach to various frequency flood events may be done for 1) existing con- <br />ditions. 2) alternative land use conditions. and 3) alternative future <br />land use conditions with only the future development flood proofed. <br /> <br />Response to Flood Warning Dissemination.--The temporary evacuation of <br />facilities within a damage reach is a component of the implementation of <br />a flood warning system in conjunction with the people reacting to the <br />flood warning. This type of alternative is difficult.to evaluate. not <br />because of theory. but because it requires the estimation of the effect <br />of the flood warning system on the stage-damage functions for each land <br />use category as temporary protection measures are implemented. To <br />evaluate this alternative. the inputted stage-damage functions are modified <br />for each damageable land use category that will be affected. This modifi- <br />cation in the stage-damage functions should include damage reduction to <br />both the contents and the structure. <br /> <br />Since it is difficult to accurately estimate these damage reductions. <br />several runs could be made for a range of percent damage reduction to <br />calculate the break point necessary for this alternative to be cost <br />effective, and then evaluate whether or not the level of damage reduction <br />can be reasonably achieved. Because the evaluation of this alternative <br />flood loss management measure is done by directly modifying the composite <br />stage-damage functions of the affected land use categories prior to the <br />aggregation of the, damage potential of a grid cell to the appropriate <br />index location, flood warning may be evaluated as an alternative by itself <br />or as an alternative in combination with any of the other nonstructural <br />a lternati ves . <br /> <br />Permanent Evacuation of a Flood Plain.--The evaluation of permanent <br />evacuation of the flood plain requires that the spatial location of the <br />flood plain be defined and all specified land uses be removed from that <br /> <br />13 <br />