park dike is covered with dirt and vegetation, scour can be expected dur- 

 ing peak levels of the design hydrograph. Extensive scouring is not ex- 

 pected, however, due to the dissipation of the turbulence and the short 

 duration of the hydrograph peaks. It is doubtful that the rockfill inside the 

 park dike specified in the plans is necessary. 



A small rubble mound with an impervious core was found to be effec- 

 tive in preventing overtopping when tested with the 1991 profile. The 

 roughness of the stone structure quickly halted the runup, and a much 

 smaller structure than the park dike was found to be sufficient. With a 

 crest elevation of +25.4 ft mlw, there was no overtopping during design 

 storm conditions, with the exception of a minor quantity of splashing. 

 However, runup was observed to approach the mound's crest, and overtop- 

 ping would have occurred at a lower mound crest elevation. Overtopping 

 was observed on tests conducted to simulate beach erosion in front of the 

 seawall, increased swl from sea level rise, or on tests with the 1978 profile. 



Displacement of armor stones occurred with stones averaging 336 lb 

 (prototype) during tests with the 1978 profile. It should be noted that 

 armor stone displacement is common with new construction, and typically 

 decreases as the stones become seated by wave action. Although the num- 

 ber of armor units displaced decreased during each successive set of tests 

 with the 1978 profile and the rubble-mound dike, the amount of wave ac- 

 tion on the dike was less during each successive set of tests due to in- 

 creases in the beach elevation. Because wave action on the dike with the 

 1991 profile was less than with the 1978 profile, and assuming that stones 

 would be seated during storms of less severity than the design event, the 

 336-lb stones are probably sufficient if the 1991 beach profile is maintained. 



If the beach profile returns to a bathymetry similar to the 1978 profile, 

 both park dike and rubble-mound dike will be overtopped during the de- 

 sign storm event and under less extreme conditions. Because sea condi- 

 tions varied for the various tests conducted under this research effort, it is 

 difficult to compare overtopping rates for the different profiles and struc- 

 ture options at Revere Beach. However, the following comparison, based 

 on conditions at the peak of the SPN, may be instructive. 



Table 1 9 lists several tests of Profile 2 tested at the peak of the SPN 

 with an swl of +16.6 ft mlw and a wave period of 15.9 sec. Both 1978 

 and 1991 profiles are included, as are both park dike and rubble-mound 

 dike conditions, as well as overtopping rates without a dike. 



By far the highest overtopping rate was found with the 1978 profile 

 and no dike. With the addition of the beach fill (1991 profile), the over- 

 topping rate was reduced by about 65 percent even with a wave height that 

 was half again as high as that conducted on the 1978 profile. The addition 

 of either the park dike or rubble-mound dike, with the 1991 profile, re- 

 duced the overtopping to nearly zero. The physical model did not include 

 erosion of the 1991 beach profile during the SPN storm. Overtopping 

 rates may be expected to range between those measured with the 1991 and 



Chapter 6 Revere Dike Discussion 



59 



