usually in a stage of rapid decay and are far out to sea on a path that is 
curving away from the coast. The average waiting time between hurricanes 
passing within approximately 100 n.m. of the study area was found to be 
5.7 years (0.175 hurricanes per year) (Prater, Hardy, and Butler in prepara- 
tion). Despite their infrequent occurrence, hurricanes have the potential 
to cause devastating flooding in the study area because of the large storm 
surge caused by high wind speeds and low pressures. The duration of a hurri- 
cane is typically shorter than that of a northeaster. 
186. Stage-frequency curves were developed using a probability model 
in conjunction with a numerical storm surge model. The calculation technique 
can be outlined as follows (Figure 49): 
a. The probability model selected storms and assigned probabil- 
ities to each (no tide) to create separate northeaster and 
hurricane ensembles. 
b. Surge level time-histories at the boundary of the study area 
were obtained either from gage data or numerical simulations 
from the FIMP study. 
ce. Each storm surge time history in the two ensembles was com- 
bined with a large number of tide time-histories to create two 
very large ensembles of synthetic northeaster and hurricane 
surge plus tide events. 
PROBABILITY 
MODEL 
STORM SURGE 
MODEL 
EVENT 
SELECTION 
PROBABILITY 
ASSIGNMENT 
STAGE-FREQUENCY 
CURVES 
MAXIMUM 
FLOOD LEVELS 
Figure 49. Flow chart of project technique 
102 
