6 Revere Dike Discussion 



58 



The proposed park dike with the crest lowered to provide only a 1.5-ft 

 rise from toe wall to crest was found to be sufficient to prevent nearly all 

 overtopping during the design storm event using the post-storm 1991 

 beach profile. Revere Boulevard, of course, would be completely flooded. 

 In the model, waves overtopping the seawall and crossing Revere Boule- 

 vard would flow part way up the park dike in a solid sheet of water across 

 the width of the flume. As the runup decreased, the sheet of water would 

 be reduced to a few "fingers" or thin streams of water that flowed much 

 further up the slope of the dike. Under the most severe conditions of the 

 SPN, most sheet flow did not extend more than one-third to one-half the 

 distance to the crest of the dike before separating into a few "fingers." 

 All overtopping observed with the park dike under design storm condi- 

 tions with the 1991 bathymetry occurred when one of the "fingers" 

 reached the crest of the dike. At no time did the solid sheet of runup 

 reach the crest. It is anticipated that prototype runup on a park dike cov- 

 ered with vegetation and paths would be less than observed in the wave 

 flume. If the profile in front of the seawall is maintained at a bathymetry 

 similar to the 1991 survey, the park dike with a crest elevation of +24.0 ft 

 mlw should be adequate to prevent nearly all overtopping during the de- 

 sign storm event. It should be recognized, however, that in any random 

 sea event there is a possibility of an event occurring that exceeds the con- 

 ditions tested in the physical model. 



Decreasing the elevation of the seawall decreased the rate of overtop- 

 ping over the park dike. With the toe wall maintained at a constant eleva- 

 tion, decreasing the freeboard of the seawall increased the freeboard of 

 the toe wall over the seawall by the same amount, increasing the effective- 

 ness of the toe wall. Although there was more overtopping of the lower 

 seawall, the increased effectiveness of the toe wall resulted in less water 

 overtopping the dike. Tests conducted to determine effects of a failure of 

 the seawall to the elevation of the roadway found that overtopping rates 

 were lower than with the seawall in place. 



Wave breaking occurred either on the slope in front of the seawall or 

 across the seawall onto Revere Boulevard. By the time wave action 

 crossed the toe wall onto the park dike, most of the turbulence had dissi- 

 pated, and flow on the dike appeared to be predominantly laminar. If the 



Chapter 6 Revere Dike Discussion 



