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12 5 12 5 10 20 30 40 50 60 70 80 90 95 98 99 



PERCENT CHANCE OF OCCURRENCE PER YEAR 



Figure 4. NED stage-frequency curve for Boston 



still-water level would be accurately represented throughout the study area. 

 However, the extra variables involved in simulating waves and wave overtopping 

 volumes would cause added uncertainty in the final frequency curves at 

 Roughans Point. Therefore, it was decided to simulate 150 events in order 

 to increase the confidence that the frequency curves based upon overtopping 

 calculations were accurate. The 150 events were selected and simulated, and 

 then frequencies were calculated in three separate sets, each containing 50 

 events. This was done to establish a measure of confidence in the selection 

 procedure. This confidence calculation will be explained in Part VI. 



19. The selection process involved four steps. First, the stage incre- 

 ments, for which simulations were to be performed, were chosen. As previously 

 mentioned, the highest still-water level on record for the Boston area is 

 10.3 ft NGVD, which occurred during the February 1978 northeaster. As pre- 

 dicted by the NED curve, the 500-year level is 11.2 ft NGVD, and the annual 

 level is 7.9 ft NGVD. Events were selected to duplicate the NED stage- 

 frequency curve below the 500-year level at the Boston gage. Therefore, given 

 the small range in elevation and choosing three sets of 50 events, selections 

 were made every 0.1 ft from 7.9 to 10.4 ft and every 0.2 ft from 10.4 to 

 11.2 ft. Next, the number of events to be selected at each stage increment 



16 



