The Multiple Shore-Breaking Wave Transformation Model 



84. The MSBWT model (Balsillie 1984c) resulted from a series of 

 investigations of breaking wave properties (Balsillie 1984a, b) in order to 

 estimate the height of storm waves crossing low-lying inland areas. 

 Recently, the erosion process has been added to the model, and it is now used 

 to estimate storm effects on nonflooded, flooded, and breached profile types 

 (Balsillie 1985b). 



Governing equations 



85. The MSBWT model is based on a combination of the physics of surf 

 zone wave dynamics and statistical estimates of unknowns such as erosion 

 quantities and bar/trough shapes. The basic assumption of the wave 

 transformation model is that energy in the surf zone is not uniformly 

 dissipated but is dissipated through the breaking and reformation of incident 

 waves (a more realistic assumption). Breaking waves are required to be 

 plunging breakers. This is more appropriate than the spilling wave 

 assumption, since waves typically break by plunging on most coasts of the 

 United States. 



86. The model is based on the assumption of an initial equilibrium 

 profile shape for the offshore and uses the actual beach shape. Beginning 

 with the surge height (including wave setup), a design wave period, and a 

 20-ft (6-m) wave, waves in 1-ft (0.3 m) high increments are propagated shore- 

 ward. At computed plunge points, an offshore bar and trough are formed, the 

 shapes of which are determined by the statistical analysis of bar/trough data 

 for the depth of the particular breaker. 



87. Since the bars form from the convergence of sediment both from 

 offshore and onshore, a net deficit of material exists at the trough of the 

 innermost bar. The depth of this trough, plus an additional depth based on 

 the depth of bed liquefaction, is used to generate a poststorm profile shape 

 extending up to the surge level. Similar to the Vellinga model, a 1:1 slope 

 is assumed for the face of the poststorm dune (above the surge level). 



88. The eroded volume is separately computed based on the following 

 dimensional (metric) equation (Balsillie, 1985a, 1986): 



43 



