highest still-water level might not produce the highest waves at Roughans 

 Point and, therefore, not the highest overtopping rates. 



115. The. SPN still-water level was estimated to be 13.0 ft NGVD (NED 

 1983) by adding the maximum surge recorded at Boston, about 5 ft, and the 

 maximum probable tide, 7 A ft NGVD and then rounding up to the next foot of 

 elevation. This resulted in a still-water elevation which was almost 3 ft 

 higher than the maximum ever recorded at the Boston gage. Given the unlikely 

 event that a tide with a maximum elevation near the maximum probable tide were 

 to occur sometime during the maximum surge producing northeaster, the proba- 

 bility that the hour of maximum surge (using hour increments) would occur at 

 the hour of maximum tide is only 1/24 (assuming a semidiurnal tide with un- 

 equal highs). Consequently, this combination might fall under the "excluding 

 extremely rare" clause in the definition of the SPN. A better specification 

 of the SPN still-water level might be closer to 12.0 ft NGVD. 



116. This report is mainly concerned with the effect of the SPN on 

 interior flooding at Roughans Point and the propagation of the SPN still-water 

 level throughout the study area which can be easily stated regardless of the 

 exact specification of the SPN still-water level. In considering the interior 

 floods at Roughans Point, the effect of an SPN is straightforward; the inter- 

 ior of Roughans Point would fill to overflowing. The interior water level 

 (approximately 1-2 ft higher than the still-water level in Broad Sound) would 

 be determined by how fast the overtopping volumes would flow over roadways at 

 the west boundary of Roughans Point. The evidence seems clear that given a 

 water level on the order of 12-13 ft NGVD and with the waves appropriate for 

 an SPN, all of the proposed alternatives would be swamped. This can best be 

 seen by considering Figure 36. The only alternative which offers significant 

 protection at the highest return periods is the "Wide Berm + 2-ft Cap." 

 However, even this alternative would not offer protection against the SPN. 



The highest still-water level (in Broad Sound) tested in the simulations was 

 11.2 ft, roughly a 500-year level. Although the SPN would fall well to the 

 right of the edge of Figure 36, the effect of the SPN can be estimated as 

 follows. The extra foot of still water resulting from the SPN would change a 

 2-ft cap down to an effective 1 -ft cap. Furthermore, the larger and longer 

 waves caused by the effect of deeper water in front of the structure and the 

 higher wind speeds of the SPN would further increase the flood levels. 

 Consequently, the interior levels caused by the SPN with the "Wide Berm + 2-ft 



91 



