(2) Geology . All materials with the exception of 5 

 to 10 feet of siorficial orgcmc silts in the viciirity of the 

 ship channel are granular, ranging from fine sand to gravel. 

 Bedrock lies at about UOO feet below m.s.l., but outcrops near 

 the east abutment of the barrier (see Appendix E). 



(3) Design . The major part of the Tiverton barrier 

 is above mean sea level, and therefore an earth-and-rcck-fill 

 structure with the slopes protected with placed rock would give 

 the necessarjf stability at greater economy than rock fill. 



The position of the barrier, about 11 miles above the mouth of 

 the Sakonnet Hiver, would result in a much less severe wave 

 condition than at the East and West barriers, and a top eleva- 

 tion of 20 feet above mean sea level has been selected. 



f . Hydraulic and hydrologlc considerations . The design of 

 the Lower Bay barriers was based on criteria and preliminary studies 

 as follows: 



(1) The design tidal flood of 12.8 feet above mean sea 

 level at the mouth of the bay. This was based on initial studies 

 of maximum winds by the U. S. V/eather Bureau, and of tidal surges 

 by the Beach Erosion Board and Texas A & M Research Foundation. 



(2) Build-up in the flood tide levels below the East 

 and Mest barriers at the mouth of the bay of approximately 0.5 

 foot as determined from hydraulic model tests. The build-up 

 south of the Tiverton barrier is more than 2 feet. 



(3) The design wave height of 2$ and 20 feet at the 

 East and Viest barriers from crest to tro\jgh, derived by the Beach 

 Erosion Board and substantially confirmed by observations of the 

 Narragansett Marine Laboratory. 



(U) Wave rxm-up on the structures above the design tidal 

 flood would result in overtopping of the East and West barriers by 

 6 feet or more. Preliminary stxidies indicated it was iiq^r actio able 

 to design the structures sufficiently high to avoid overtopping; the 

 volvune of water carried into the bay could be absorbed by the 120 

 square-mile drainage area of the bay with a rise of less than 0.3 

 foot. Further study of slopes of structures and stone sizes is 

 reqviired to ass\u*e design of a stable structure. 



1x6 



