are insde un of the difference bet^-jeen annual losses under pre- 

 project and post-project constriiction conditions. An alternative 

 method used for compprstive purposes vms an evaluation of savings 

 by the recoinnended protective works in a recurrence of hurricanes 

 and storms that have occurred in the p?st SO years. The 50-year 

 period vias selected to conform with the economic life of the pro- 

 ject used for cost amortization. The alternstive method resulted 

 in benefits of a com.parable magnitude to those derived by the 

 damage-frequency method. The benefits derived by the damage-fre- 

 quency method were used in determining benefit-cost ratios. 



b. Average axmtial losses . The Narragansett Bay area was 

 divided into 8 damage areas and 13 sub-areas for the purpose of eco- 

 nomic analysis. A description of these areas is shoi-m on Table D-U. 



Recurring stage-damage d.ata for individual properties, refer- 

 enced to the peaV- elevation of the 195U hurricane flood, was sum- 

 marized for each of the damege areas which are i-n.thin the planned 

 protection. The stage-damap'e curve i-ra-s combined id-th stage-fre- 

 quency dats to develop a damage-freqiiency curve. The development 

 of the stage-frequency curve is described in Appendix B, The 

 damage-frequency cur'/e vras plotted with damage as the ordinate ajid 

 percent-chance-of-occurrence (reciprocal of frequency) as the ab- 

 scissa. The area under the damage-frequency cu.rve is a measure of 

 the average annual loss. By this method, the average annual losses 

 in the areas of Narragansett Bay ■'.^;hich would be protected by the 

 plan amounts to $5,922,000, at 1956 price levels, of which 

 ■r^l,697,000 occurs in the area above the Fox Point barrier, and 

 :r?U,225,000 betvieen Fox Point barrier and the Lower Bay barriers, 



A recurrence of the 3 major hurricanes and 22 other storms 

 causing tidal flooding which occurred in the past 50 years would 

 occasion losses evaluated at #227,200,000, or $U,5UU,0C0 anntially, 

 under present economic conditions. Table D-5 shows a tabulation 

 of losses which would result in the areas to be protected by the 

 plan under present economic conditions in the event of a recurrence 

 of the tidal-flood stages produced by destructive hurricanes and 

 storms which have occiirred -vrithin the last ^0 years. The curves on 

 Plate D-1 are representative of the stage-damage and damage -frequency 

 relationships found in the Narragansett Bay areas, 



c. Average annual flood-prevention benefits . Average annual 

 flood-prevention benefits were derived by determdning the differ- 

 ence betifTeen annual losses under present conditions and those re- 

 maining after construction of the projects in the plan. A stage- 

 frequency curve reflecting post-project conditions was used to de- 

 velop post-project annual losses. Annual benefits in the protected 

 areas of the Narragansett Bay area amount to :/5,902,000, of which 

 fpl,697,C00 would accrue to the area above the Fox Point barrier and 



D-13 



