51 



these structures. In the case of the Middle Bay barriers, however, the 

 reduction in storage upstream from the structures represented a large 

 percentage of the total hay tidal prism occupied by the hurricane tides 

 tested, and elimination of this storage area resulted in an appreciable 

 buildup downstream from the structures. In the case of Lower Bay bar- 

 riers, elimination of the entire storage area of Narragansett Bay would 

 have only an infinitesimal effect on tides generated by h-urricanes in 

 the open ocean; therefore, no significant buildup occurred on the seaward 

 side of the Lower Bay structures. Such minor buildup as was indicated 

 by model tests seaward of the Lower Bay barriers is thought to be at- 

 tributable to elimination of local drawdown effects caused by high 

 velocities into the mouth of the bay under existing conditions. 



Effects of Barriers on Tidal Currents 



76. The effects of the barriers on tidal currents throughout that 

 portion of the bay system upstream from the barrier site would be in al- 

 most direct proportion to the effects of the barrier on astronomical 

 tide ranges; a reduction in tidal range of 30 to ko per cent would be 

 accompanied by similar reductions in tidal current velocities. In areas 

 downstream from the barrier site, the mean velocities of tidal currents 

 would be reduced by a factor representing the total reduction in tidal 

 prism upstream from the area in question. The directions of tidal cur- 

 rents would be altered appreciably in the vicinity of barriers, since 

 restriction of an existing wide channel to a single ungated navigation 

 opening would result in f-unneling the entire flow through the ungated 

 opening. The use of a-uxiliary sluice gates, similar to those considered 

 in connection with the Middle Bay barrier plans, would probably prevent 

 undesirable changes in current patterns. Current velocities in the un- 

 gated navigation openings of the plans investigated are affected by the 

 design of the opening and the head differential across the structure. 

 Reductions in total area of navigation openings to effect greater pro- 

 tection to upstream areas from hurricane tides increased head differ- 

 entials across the structures and therefore increased current velocities 



