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'The technique developed for automatically generating elevation-damage functions adapts this traditional method to the grid cell data bank concept. The methodology consists of constructing a unique elevation-damage relationship for each grid cell within the flood plain (based on topographic ground ele- vation, land use. and composite damage function assigned to the grid cell) and aggregating all the grid cells assigned to a particular damage reach to the appropriate index location. using a reference flood as the mechanism-for adjusting for a sloping water surface profile. Damage Reaches.--Damage reach boundaries are selected based on the traditional procedure that includes determining reaches with,consistent parallel water surface profiles for a range of discharges while maintaining the economic detail desired for analysis. The boundaries extend to a reason- able lateral distance from the stream to the flooded area of the largest flood event determined necessary for economic-damage evaluations. plus an arbitrary vertical distance (say 5 feet). Fig. 1 Damage Reach Delineation illustrates a typical damage reach delineation. The damage reaches are encoded and pro- cessed into the grid cell data bank with each cell within a reach assigned the reach identification value. The damage reach identification is used to aggregate grid cells to the appropriate damage reach index location. Reference Flood.--Since flood profiles result in different water surface elevations throughout a damage reach. a reference flood is required to properly adjust the elevation for aggregation purposes of each cell within the reach with respect to the index location. Each cell is assigned a reference flood water surface elevation which is used with the reference flood elevation at the index location to adjust the composite damage function for proper aggregation of damages at the index location. The reference flood should be an event within the range which is critical for flood damage computatio~a mid-range flood (say 25 to 50 year) is a better choice than a rare flood such as a 500-year exceedance interval event. If the flow profiles are consistently parallel throughout the potential damage range, the selection of the reference flood is less critical. The reference flood elevations should be determined from detailed water 'surface profile analysis. If water surface profiles are not available. the slope of the flood profile through a damage reach may be assumed to correspond to the slope of the thalweg of the main stream or the slope of the adjacent flood plain itself. Fig. 2 Reference Flood Concepts illustrates the adjustments performed using the reference flood. Composite Damage Functions.--The general objective of the analytical methods developed are to provide a consistent and expedient methodology of evaluating a range of nonstructural alternatives for existing and selected alternative future land use patterns. The concept of using gen- eralized composite'stage-damage relationships for the land use category assigned to each grid cell was selected as the mechanism to perform the analysis rather than the conventional individual structure approach. The use of these generalized functions provides the capability of expediently evaluating alternative, land use 'patterns that are consistent with the existing (base) condition land use pattern. 3 .