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RETURN PERIOD, VRS 

 LEGEND 

 -set a STAGE FREQUENCY 

 s" c STILL-UATER LEUEL 



THREE SET COOPARISON 

 OAK ISLAND 



Figure 57. Stage-frequency curves, three-set comparison, Oak Island 



the still-water level of the event but also upon the magnitude, direction, and 

 duration of the event's waves. Since the selection process used still-water 

 level as its only criterion, two events which were selected with the same 

 still-water level could have very different wave characteristics and, there- 

 fore, could cause very different flood levels at Roughans Point. Stage- 

 frequency curves for the three sets of 50 events are shown in Figures 58 and 

 59 for the "Wide Berm" and the "Wide Berm + 1-ft Cap" alternatives, respec- 

 tively, which had the most and the least variation among the three sets, of 

 any of the six Roughans Point structure combinations. 



112. Assuming that for any given return period the calculated stage is 

 a normally distributed random variable, an estimate of the probable error PE 

 can be calculated using the three stage-frequency curves generated indepen- 

 dently from the three sets of selected events. PE estimates the 50 percent 

 error bounds about the mean value in a series of measurements. Equation 18 

 states the relationship between PE and standard deviation a as 



PE = 0.6745 a 



(18) 



87 



