35 



effects of the plan on maxim-um elevations of the 1938 hurricane tide and 

 the 20-knot design hurricane tide. The effect of the plan on times of 

 high tide are shown in table 3- Astronomical tide ranges were reduced 

 at all gages upstream from the barriers^ the average reduction being of 

 the order of 25 to 30 per cent, while the times of high water at upstream 

 gages were delayed by as much as 1.8 hours. The peak of the hurricane 

 tide at Providence was lowered from +15.1 ft to +7.8 ft mlw for condi- 

 tions of the 1938 hurricane tide and from +I7.2 ft to +9.1 ft for condi- 

 tions of the 20-knot design hurricane tide. The maximum elevations of 

 hurricane tides at all other gages upstream from the barriers were ap- 

 preciably reduced, while the times of high water at these gages were de- 

 layed by as much as 2.0 hours. Ro measurements of current velocities 

 were made in the navigation openings of the East and West Passage struc- 

 tures during model tests of plan 29. 



50. It is emphasized that data contained in this report showing 

 the effects of barriers on hurricane tide elevations throughout the bay 

 system apply only to the gravitational component of the hurricane - 

 generated ocean tide; the effects of local wind setup must be added to 

 elevations presented herein to arrive at maximum elevations that would 

 obtain during a hurricane. The reductions in hurricane tide elevations 

 effected by plan 29 at Providence and other points throughout the upper 

 bay appear quite large, but it must be remembered that some damage by 

 hurricane tides begins when the water-surface elevation at Providence 

 exceeds about +G.G mlw, and this elevation would be exceeded appreci- 

 ably by adding the wind component to the model test data presented 

 herein. 



51. The width of the East Passage navigation opening for plan 29 

 was considered at that time to be about the minimum that could be toler- 

 ated by the Wavy. A further reduction in widths or depths of the naviga- 

 tion openings of the plan, which obviously would have been required to 

 reduce the absolute elevation of hurricane tides at Providence below that 

 at which damage begins, was considered untenable at the time. It was 

 therefore concluded that a Lower Bay barrier plan alone, having ungated 

 openings for navigation, could not simultaneously meet the requirements 



