and foimdation conditions.. Certain sites otherwise desirable will 

 prove impracticable because of oansatisfactory fovindation conditions j 

 at other sites, excessive lengths or depths may render benefit-cost 

 ratios substantially less favorable, or prohibitive. Several other 

 factors, however, constitvite major considerations in barrier site 

 selection. Among these the most significant are (l) the navigational 

 requirements of vessel traffic within the bay and within the Providence 

 Riverj (2) the necessity of adequate provision for tidal circulation 

 and the discharge of fresh-water drainage j (3) problems created by 

 build-up of the tidal flood level in the area belovr a barrier; and 

 (1;) fetch above the barrier in which water levels and waves may in- 

 crease to damaging elevations, 



(1) Navigational requirements . Except for very 

 important special requirements for Naval craft, which must be pro- 

 vided witn adequate navigational openings in any proposed barrier 

 in the Lower Bay, navigational requirements present no problem at 

 either extreme of the bay, A barrier placed at the head of navi- 

 gation, of course, presents no requirement for navigational openings. 

 In the Lower Bay, vingated navigational openings of wholly adequate 

 width for all present or prospective commercial vessel traffic can 

 be provided while still admitting such a small percentage of the 

 tidal s\irge as would create only a slight rise over the 120-sqviare- 

 mile area of the bay, A barrier placed below the confluence of 



the Blackstone and Providence Rivers, however, would have to be 

 provided with watertight navigation gates which could be closed in 

 time of a hurricane warning, 



(2) Tidal circulation and fresh-water discharge , 



A barrier must be so designed as to provide, either by openings or 

 sluice gates, for adequate tidal circulation to prevent pollution 

 and the passage of interior fresh-water drainage. Barriers placed 

 in the mid-bay or lower bay present no problem in this respect, 

 since tidal circulation would not be adversely affected and the 

 water area above the barrier would be sufficiently large to absorb 

 all fresh-water drainage with only a few inches rise, A completely 

 closed barrier at the head of navigation would present no serious 

 problem, inasmuch as sluice gates would be adequate to provide 

 tidal circulation and fresh-water drainage from the Woonasquatucket 

 and Moshassuck Rivers under normal conditions, and pumping stations 

 could be provided to take over when the sluice gates would be closed 

 during a hurricane alert. Barriers located below the confluence of 

 the Providence and Blackstone Rivers, however, would have to be 

 provided with pumping stations adequate to meet a peak flood flow 

 of 30,000 cubic feet per second from the Blackstone River drainage 

 area, and would therefore require unusual pumping eqviipment and 

 electrical power, 



37 



