B-25. WIND SET-UP 



Calculations made by the New England Division were based 

 on a step method formula derived by the Beach Erosion Board: 



AS = d^ 



^ ^ 2KU^ AF _ 3_ 



4 



AS = incremental wind set-up in feet 

 d_" average depth of water in feet 

 U = velocity of wind in miles per hour 



A F = incremental fetch length in statute miles 



K = coefficient (1.165 x 10"^) 



The above formula was used in deriving the wind set-up above 

 the proposed site of the East barrier for a design hurricane with 

 and without the barrier with U = 76 mph. A step method was used 

 progressing up the 25.5 miles of bay with the depth of water vary- 

 ing according to the fetch length assumed. The depth ranged 

 between 20 feet and 130 feet. Without the barrier in place, the 

 initial elevation was the design hurricane elevation, 13 feet above 

 msl, in which the set-up was computed to be 3.0 feet. With the 

 barrier in place, the initial elevation north of the barrier was 

 elevation Ii..5 feet above msl, in wh?.ch the total set-up computed 

 was 3*5 feet. In the latter case, allowance was made for the 

 tilting effect the wind creates on the water surface, depressing 

 the lower end of the pool and raising the upper end. Furthermore, 

 at Newport, due to the narrow opening which limits the flow of 

 water through the proposed opening, the effect of a hurricane wind 

 of 76 mph is also to depress the water surface. It was assumed, 

 therefore, that the Newport end of the pool would be 0,9 foot 

 lower than the starting elevation and the upper end would be 2.6 

 feet higher than an elevation without wind at Providence. The 

 latter modification was applied to model tests without wind 

 effect . 



The resulting high-water profile from model tests, reprod- 

 ucing the 1938 hurricane has a flat slope. At Newport, the 

 model shows that Stillwater levels (without wind set-up) are 

 about ii.5 feet and at Providence about 5.1 feet above mean sea 

 level. Wind set-up, calculated from the above formula and mod- 

 ified as described, was superimposed upon the model test profile 

 for the 1938 hurricane tide resulting in an additional rise of 

 about 2.6 feet at Providence, or a total rise of 7.7 feet above 

 mean sea level as shown on Plate 8 of the main report. 



B-25 



