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. " . ~ ~ 8 . . . i o o ht.urn treqlW'l'lCY . " . ~ ~ 8 . . . r o Return , tr-.quency (AVl!:RAGl!: AMIftI'AL ) (DAMAGB EXISTIlIG) (CONDITIONS ) r I ! Area ben.ath curve equal. averaga annual d"'9' under exiating conditions 10 20 30 40 50 as 'Probability GRAPH A ~ Damag' Potential Bxiatlnq Condition. Ar.. ben.ath curve aquals Iverl'll annual damag, undar i~prov.d condition. 10 20 30 40 50 aa' Probability GRAPH B - D..... potent.ial .ttar i.prove..nta conatruotad (A~GB ANNUAL ) ~ (OAHAG!: IMPROVED) - (CONDITIONS ) "WAAGE ANNUAL BQJ:PIT FROM IMPROVEMENTS Flqura IV-I -60- The next step is to determine the damage derived from each storm inundation. To accomplish this, a graph of depth of inundation versus damage as a percentage of residence value was developed. The curve is based upon data supplied by the U. S. Army Corps of Engineers that they utilize for establishing damage potential for the flood insurance program. Damage to the channel and street crossing must be calculated. TO do this, curves relating velocity of flow and depth of inundation to dollar damage to the crossing and channel were developed. Figure IV-2 is a sample tabulation sheet for damage to public facilities including crossings and channel sections. These damages represent the cost of replacing grass, filling eroded areas, replacing curb and gutter and other costs associated with returning the drainageway to useable condition. The final step in determining the average annual damage poten- tial is plotting the values on a graph of damage potential versus return frequency as a percentage. A curve is then drawn connecting the plotted points and the area below the curve determined by use of a planimeter. The area is con- verted into an average annual damage potential. The average annual damage potential reduction (average annual benefit) is converted to a present worth of benefits by use of a present worth factor obtained from compound interest tables. For purposes of this report a 6% interest rate and a 40 year life expectancy WaS utili~ed. The present worth factor at 6% for 40 years equals 15.046. As a comparison, if a 100 year life expectancy had been assumed, the present worth factor would only have increased to 16.618. Forty years was used, since it dppears to be a rAa~on~ble length of life for the average improvement before m~jor repair~ or repl~~emPnt would ~ necess~ry. It is interesting to see the effect that using a lower interest rate has On the present worth factor. The 40 year, 5% present worth factor equals 17.159 and the lOO year 5% present worth factor equals 19.848. The Corps of Engineers often uses the latter figure. Use of the 100 year, 5% value would increase the villue of the benefits for this project by 32%. The higher value is felt to be unrealistic with the present monetary situation. -61-