were assigned to the 27-member storm ensemble according to the portion of the 
stage-frequency curve that they represented. 
191. The interaction of surge and tide is significantly nonlinear in 
shallow water. Also, the contribution to flooding in back-bay areas from the 
overtopping and breaching of barrier islands is a highly nonlinear process. 
For these two reasons, in shallow water, surge and tide cannot be separately 
modeled and then added together to produce a combined water level. However, 
in FIMP, the boundary of the nearshore grid was in relatively deep water and 
far enough away from the barrier islands so that it was unaffected by the 
above-mentioned nonlinearities. Therefore, it was possible to simulate surge 
without tide on the global grid, and then linearly add tidal time histories to 
the surge time histories. By combining multiple tides with each storm, a 
large number of synthetic surge plus tide events were created. This process 
created more than 600,000 hurricane and 18,000 northeaster surge plus tide 
time-histories. 
192. With the assumption that a storm has an equal probability of 
starting at any point during a tidal cycle, stage-frequency curves were con- 
structed for surge plus tide water levels. This was done on the boundary of 
the nearshore model as well as at several open-coast locations such as Sandy 
Hook. 
193. It was not feasible to model on the FIMP nearshore grid any but a 
small portion of the large ensemble of events created by the convolution of 
global results with tide. A procedure was therefore devised to select events 
to be modeled. A total of 51 hurricane plus tide and 40 northeaster plus tide 
events were simulated. The stage-frequency curve at the nearshore boundary 
was discretized by height with a single event selected at each height interval 
to represent all events having maximum water levels falling in that interval. 
At these discrete locations on the global stage-frequency curve, the maximum 
water level occurring during each selected event was assigned a probability 
equal to the probability mass of its interval of the stage-frequency curve. 
After the event was simulated on the nearshore grid, this probability was 
assigned to the maximum water level caused by the event at various locations 
throughout the study area. Thus, stage-frequency curves were created for 
multiple locations throughout the FIMP nearshore grid. 
105 
