(3) Build-tip , Hydraijlic model tests have demonstrated 

 that a barrier ccnctructed across ITarragansett Baj"* to "block a storm 

 surge irould create sone "build-up, or increase in the xrater level 

 brelovj the barrier. This hi.iild-up depends :nairJ.y on the relative 

 areas above and beloir the barrier, Ileasurenents Sii.oxred that build- 

 up increases fro:-! a negligible amount at tlie head of the Providence 

 River to a maximuni of 2 to 3 feet in the Middle Bav and falls off 

 to about 0,^ foot near Newport, 



(U) Local wind and wave effects , A barrier that will 

 effectively block ocean waves and hurricane tidal surges may not pro- 

 vide complete protection for all damage areas within the bay. As the 

 fetch above the barrier increases, the reduced waves grow in magnitude, 

 and the wind drives the shallow bay and river waters to higher levels, 

 VJaves generated in the 25-inile length of Narragansett Bay may reach a 

 height of 8 feet from crest to trough. The wind effect in the length 

 of the bay, depressing levels near the mouth and increasing levels at 

 the head of the bay, is 3 to U feet, depending on wind direction and 

 duration. This tipping of' the water surface is greatest in the Upper 

 -■ay where waters are shallow. Appendix B contains a more detailed 

 discussion, 



b. Alternative plans of protection . All proposed barrier 

 sites '.Jere reviewed in the light of these considerations. Four 

 alternative plans for tidal flood barriers were selected as offering 

 the best possibilities for protecting the Narragansett Bay area, A 

 brief description of each follows: 



(1) Lower Bay barrier plan , A plan for rock-fill 

 barriers, \Tith navigational openings, across each of the three en- 

 trances to Narragansett Bay - an East barrier across the East Passage, 

 a V/est barrier across the West Passage, and a barrier across the 

 head of the Salconnet River at Tiverton, This T?lan would provide 

 j;;eneral protection for the Narragansett Bay area, preventing more 

 than 90 percent of the .>120,000,000 damages which the September 1938 

 hurricane tidal flood would cost today. Dimensions of the navi- 

 gational openings through each of the barriers were desir^ed to 

 achJieve a balance beti/een effective protection and the requirements 

 of navigation, and to permit normal tidal flow in and out of the 

 bay without setting up violent currents -round the opeViings, Con- 

 struction xTOuld ranje between :i?69,O0O,0OO and ^109,000,000, depending 

 on the .iFCcsr.ity for additional features for pollution control, fish 

 and x:ildlife and provision for settleijient. 



38 



