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commercial acrivity. agricultural purposes. year floodplan within the A portion of this reach is also used for Plate 2 shows the approximate limits of the 100- study area. , B. General Computational Procedures: Each sureyed property is assigned to a category (residential, commercial, public, transportation facilities, utilities, irrigation facilities, agriculture) with as many sub-categories as necessary to represent acurately a property's propensity to sustain flood damage. Each category has an associated depth-damage relationship for structure and contents, expressed as a cumulative percentage of value for each foot of inundation. These relationships were derived from historical data when available, or were obtained from insurance companies, Flood Insurance Administration data, data from other Corps districts, direct interviews, and projects with similar characteristics. The elevation of each property (determined from topographic maps and field investigations) is compared to the elevation of each reference flood (SPF, 100-, 50-year) at the nearest surveyed section to compute the depth of inundation at that location. Each property category is then tabulated in terms of the number of units, average value per unit and aggregate value, within consecutive inundation depth ranges for each reference flood level. For each category, the aggregate value of property at each flood depth is combined with the depth-damage relationship to compute the total single event damages for each level of flooding. Repeating this process gives the discharge-damage relationship for the category in each reach. This is combined with discharge-frequency data to produce the damage-frequency curve. The area under the damagewfrequency curve is computed to derive the probable annual damages for each category, by reach and decade. The above computations are performed for each alternative project, including the without-project condition, with each alternative expresed by a 'different set of discharge-frequency data, the product of this process is the residual equivalent average annual damages for each alternative. The difference between damages in the without-project condition and residual damages for each alternative is the measure of the inundation reduction benefit of that alternative. ~ C. Value of Damageable Property: A sur~'of flood-prone properties was conducted in 1985 to assess the potential magnitude of flood damages with the Alamos a flood plain. Property categories sureyed include residential, commercial, public, transportation facilities, utilities, irrigation facilities, and agriculture. Table 3 displays the value of property within the SPF, 100-, and 50-year flood plains of Alamosa. Table 4 displays, by units and land use, the number of properties located within each of the flood plains studied. $193 million in property is currently located within the SPF flood plain, with the largest single category being residential structures in the south bank flood plain. D. Potential Flood Damages: Historical records show that the potential for severe flooding exists in Alamosa and poses a threat to the citizens and improvements within the flood plain. It is currently estimated that SPF flood would cause damage in excess of $42 million within the study area; damage to residential structures would again be the largest single category from the SPF 8