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<br />The effectiveness of each alternative will depend primarily on how much <br /> <br /> <br />it reduces the damages from this do-nothing case. <br /> <br /> <br />For the do-nothing alternative, compute the flood damages for the <br /> <br /> <br />existing development and for the projected development. For areas of <br /> <br /> <br />uniform flood damage potential (i.e., a residential area of uniformly <br /> <br /> <br />valued homes), per acre factors can be developed for damages for a range <br /> <br /> <br />of flood depths. The floodplain can then be divided into areas of equal <br /> <br /> <br />flood depth, i.e., 0-2 ft., 2-4 ft., and 4+ ft. Flood damages are then <br /> <br /> <br />found by applying the per acre damage factors. The per acre damage fac- <br /> <br /> <br />tors can be computed by estimating typical exposures for each damage <br /> <br /> <br />category and applying the individual damage factors. Table 111-3 summar- <br /> <br /> <br />izes the procedure for obtaining area damage factors for 3 and 4 ft. flood <br /> <br /> <br />depths for a sample low density residential area. <br /> <br /> <br />For existing areas that are not homogeneous with respect to flood <br /> <br /> <br />damage, a more detailed analysis must be performed. For each damage <br /> <br /> <br />category the value exposed to flooding must be known. For example, esti- <br /> <br /> <br />mation of the structural damage to a particular residential unit requires <br /> <br /> <br />knowledge of the value of the structure. The exposure is multiplied by <br /> <br /> <br />the damage factor taken from the appropriate depth versus damage curve. <br /> <br /> <br />Dollar damage is estimated for each damage category, and tabulated as in <br /> <br /> <br />Table 111-4. <br /> <br /> <br />Estimation of future flood damages for presently underdeveloped areas <br /> <br /> <br />will require the development of weighted per acre flood damage factors. <br /> <br /> <br />These factors should be weighted to reflect the probable percentage that <br /> <br /> <br />each land use will be of the total undeveloped area. This information <br /> <br /> <br />was obtained or estimated in the hydrologic analysis, and an example is <br />given in Table 111-5. <br /> <br />, <br />70 <br />