study, since hurricanes do not significantly contribute to stage-frequencies 

 in the project area, and since northeasters are difficult to parameterize, a 

 modification of the historical approach was used. Historical storm surge 

 time-histories were combined randomly with tide time-histories to produce syn- 

 thetic event water level time-histories. Probabilities were assigned using 

 data from a nearby tide gage. This process is explained in detail in the 

 following paragraphs. 



Choosing Storm Surge Time-Histories 



11. Regardless of the approach selected, data in the vicinity of the 

 study area are essential for identifying inputs to the numerical modeling and 

 for assigning probabilities. This project was fortunate in having convenient 

 sources for the necessary data. The National Ocean Service's (NOS's) Boston 

 tide gage has been in continuous service since 1922. This gage is located at 

 Commonwealth Pier in Boston Harbor which is less than 5 miles from the study 

 area. Wave hindcast information was available for deep water adjacent to the 

 study area from the WES Wave Information Study (WIS). Hourly wind data were 

 available from Logan International Airport which is less than 5 miles from the 

 study area. The 20-year period from 1956-1975 was chosen from which to gather 

 data for use in the numerical modeling. This period was selected because 

 information was available from the above mentioned sources in all the 

 necessary data categories: water level, wind, and wave. 



12. By defining storm surge as the difference between measured water 

 level and predicted tide, a partial duration series of storm surge time- 

 histories (26 storms) was extracted from the Boston tide gage data. A minimum 

 value of the maximum surge, 2.5 ft, was used to define those storms which had 

 a reasonable probability of causing significant flooding. If surges much 

 below the 2.5-ft level were combined with possible tides, it would be unlikely 

 that any of the resulting events would be selected as one of the relatively 

 small number of events to be modeled (only 150 events with water levels from 

 7.9 to 11.2-ft NGVD were selected). The value of 2.5 ft was chosen using the 

 following guidance: with this surge level, only 5 percent (Harris 1981) of 

 the hourly tide heights are high enough so that the combined surge plus tide 

 would be greater than 7.9 ft NGVD. The combination of surge and tide and the 

 selection of the events to be modeled are explained later in this report. 



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