PART VI: STAGE-FREQUENCY CURVES 



85. In this section, the method for establishing stage-frequency curves 

 will be described for both the still-water locations and for the interior of 

 Roughans Point. 



86. The goal of this project was to produce stage-frequency curves for 

 two distinct processes. The first process involved the interaction of storm 

 surge and tide to produce still-water levels at coastal (and river) locations, 

 and the second process combined waves with the surge and tide to produce flood 

 levels behind seawalls due to wave overtopping. Although the simulation of 

 these two processes involved some different steps, development of frequency 

 curves for the two processes once the water levels are determined is essen- 

 tially the same. 



87. Probability was assigned to each of the events selected for simula- 

 tion, as described in Part II. By assigning the probability to the maximum 

 still-water level caused by the event at each numerical gage location, stage- 

 frequency curves can be constructed by the following method. First, an array 

 of possible stages at each gage location is established with a discretization 

 interval (0.1 ft for this project). Next, for all 150 events, the probability 

 masses assigned to each event are accumulated in the stage interval which 

 brackets the maximum water level that occurred for that event. Exceedances 

 can be determined for any interval by adding the probability of that interval 

 to the sum of the probabilities of the intervals above it. After this was 

 accomplished for the total set of 150 events, the process was repeated for 

 each of the three sets of 50 events. This produced three additional sets of 

 stage versus exceedance relationships which were used for confidence 

 calculations. 



88. The range of stages modeled in the still-water level portion of 

 this study was just over 3 ft (from 7.9 to 11.2 ft NGVD). All of the re- 

 sulting 33 discretization intervals did not receive probability. Some 

 intervals received probabilities from several events causing in places (in the 

 array of stages) a series of heights where no event deposited its probability. 

 This occurrence results in a jagged line when the stage-frequency is plotted. 



89. There is no physical reason why adjacent height intervals should 

 have greatly different probabilities. The jagged nature of the raw curves is 

 caused by trying to represent a continuous process (all possible storm events) 



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