Table 19 



Comparison of Overtopping Rates at Peak of SPN 



Table in 

 which 

 data are 

 listed 



Year of 

 Survey 



Dike 



SWL 



Wave 

 Height 



Wave 

 Period 



Over- 

 topping 

 Rate 



12 



1978 



None 



16.6 



8.86 



15.9 



1.1980 



8 



1991 



None 



16.6 



12.65 



15.9 



0.4156 



15 



1991 



Park 



16.6 



12.70 



15.9 



1 



16 



1991 



Rubble-Mound 



16.6 



12.70 



15.9 



0.0000 



17 



1978 



Park 



16.6 



11.00 



15.9 



0.0960 



18 



1978 



Rubble-Mound 



16.6 



11.00 



15.9 



0.06180 



^ Overtopping rate too small to be measured. 



1978 profiles because erosion of the 1991 beach profile during the SPN 

 storm can be expected. 



If the beach erodes back to the 1978 profile, overtopping of either dike 

 will occur. The last two entries in Table 19 give overtopping rates for the 

 park dike and rubble-mound dike with the 1978 profile. The dikes greatly 

 reduced the overtopping rate compared to conditions without any dike, al- 

 though the overtopping rates with either dike may still be unacceptable if 

 the beach is eroded to the 1978 profile. 



As a qualitative reference, Fukuda, Uno, and Irie (1974) measured and 

 filmed waves overtopping a seawall fronted by a concrete revetment dur- 

 ing severe storms. The films were then viewed by a panel of coastal ex- 

 perts who estimated the degree of danger posed by the overtopping. 

 Averaging the results of the panel, it was determined that at a location 

 10 ft behind the structure, overtopping rates greater than 0.0002 cfs/ft 

 would prohibit a vehicle from driving past at high speed, damage to a 

 house could be expected at an overtopping rate of 0.0007 cfs/ft, and over- 

 topping rates greater than 0.002 cfs/ft would be dangerous for a walking 

 person. These overtopping rates assumed an average over several hundred 

 waves and could be increased by a factor of 10 for a location 30 ft behind 

 the structure. For protection of a relatively densely populated coastal 

 area, Goda (1985) reports an overtopping rate of 0.1 cfs/ft as an adopted 

 guideline in port areas in Japan. These overtopping rates assume an aver- 

 age over several hundred waves (Goda 1985). 



Based on the overtopping rates given above from Fukuda, Uno, and 

 Irie (1974), overtopping rates with either dike may be hazardous if the 

 beach erodes to the 1978 condition. However, conditions listed in Table 19 

 occurred only at the peak of the SPN hydrograph and the dikes eliminated 

 95 to 98 percent of the overtopping compared to the 1978 condition. 

 Lower swl's caused substantially less overtopping (see Tables 17 and 18). 



60 



Chapter 6 Revere Dike Discussion 



