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<br />then aggregating the damage potential of all grid cells assigned to a par- <br /> <br /> <br />ticular damage reach to an appropriate index location. <br /> <br />DamaRe Reaches. Damage reach boundaries are selected based on a pro- <br /> <br /> <br />cedure of determining reaches with consistent parallel water surface profiles <br /> <br />for a range of discharges while maintaining the economic detail desired <br /> <br /> <br />for analysis. The lateral boundaries extend a reasonable distance from the <br /> <br />stream to the edge of the flooded area of the largest flood event deemed <br /> <br /> <br />necessary for economic-damage evaluations, plus an additional horizontal <br /> <br /> <br />increment to insure coverage of proposed and future conditions (Figure 4-1 <br /> <br /> <br />illustrates typical damage reach delineations). The damage reaches are <br /> <br />encoded into the grid cell data bank and each cell within a particular reach <br /> <br /> <br />is assigned a damage reach identification value or code. The damage reach <br /> <br /> <br />identification is then used to insure that the grid cells are aggregated <br /> <br /> <br />with respect to the appropriate damage reach index location. <br /> <br />I <br />I <br />I <br />, <br />I <br />I <br /> <br />Reference Flood. Since a flood profile yields different water surface <br /> <br /> <br />elevations throughout a damage reach, a representative (or reference) flood <br /> <br /> <br />event is required to properly adjust the topographic (or first floor) ele- <br /> <br /> <br />vation of each grid cell for aggregation purposes within a particular reach <br /> <br />(with respect to the index location). Each cell is assigned a reference <br /> <br />flood water surface elevation which is used with the reference flood eleva- <br /> <br /> <br />tion at the index location to adjust the individual damage functions (for <br /> <br /> <br />proper aggregation of damages at the index location). <br /> <br />The reference flood should be an event within the range which is critical <br /> <br /> <br />for flood damage computation, a mid-range flood event (a 25 to 50 year event) <br /> <br />is often more appropriate than a rare flood with a 500-year exceedance <br /> <br /> <br />interval. Historic flood events are often used as reference floods. If <br /> <br /> <br />the flow profiles for different discharge rates are consistently parallel <br /> <br /> <br />throughout the potential damage range, the selection of the reference flood <br /> <br />is not as critical. The reference flood elevations should be determined <br /> <br />4-6 <br />