Creating Synthetic Surge Plus Tide Events 



14. Since the tidal range at Boston (mean range — 9.5 ft and maximum 

 range — 14.6 ft) is much larger than the largest recorded surge (approximately 

 5 ft), the tide is a very important component of the total water level. Rath- 

 er than numerically model the relatively small sample of historical events 

 (surge plus tide), synthetic events were created by combining the historical 

 storm surge time-histories with possible tide time-histories. The basic as- 

 sumption behind this technique is that the surge time-history (edited to re- 

 move the shallow-water effect) of any storm is independent of the tide with 

 which it occurs. In other words, the phenomena which cause tides are not 

 related to the phenomena which cause storms. Therefore, a storm may occur 

 with any tide that is possible during storm season. 



15. Using tidal constituents from NOS analyses of the Boston tide gage, 

 hourly tide heights for the winter season were predicted. The period from 



15 October to 30 April was chosen as winter season, and 19 years of this 

 seasonal record were generated to simulate a tidal epoch. Combining the 

 28 surge time-histories with every possible tide time-history during this tide 

 series would result in more than 2.5 million combinations. Obviously, it 

 would be economically impossible to simulate all these possibilities. Fur- 

 thermore, it is not necessary to simulate a large percentage of the possi- 

 bilities in order to adequately represent the population. In order to form a 

 representative sample of the total ensemble, a random selection process was 

 devised. 



16. The 28 surge time-histories were combined with a large number of 

 tide time-histories. Each of these synthetic surge plus tide time-histories 

 was created from storm and tide time-histories by randomly choosing a starting 

 point in the tide series, matching this point to the start of a storm, and 

 adding the tide and surge levels at each hour for the length of the storm. 

 The resulting large number of possible event time-histories served as the data 

 set from which events were randomly selected for simulation by the numerical 

 models. Each of the 26 storms, in the 20-year partial duration series of 

 surge, was combined with 500 tide time-histories chosen at random from the 

 19-year tide series. Each of these storms was considered to have an equal 

 likelihood of occurrence (each storm did, in fact, occur during the 20 years). 

 The two additional storms (1945 and 1978) were not part of the 20-year partial 



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