used to predict the 92.5 percentile of erosion, the following equation is 

 used: 



5^(* 1/2 t, S»)*" (.4, 



Once the erosion quantity is determined from Equation 13 (or Equation 14), 

 the poststorm shape is moved along the surge level until the eroded area 

 equals the estimate. 



90. Because the MSBWT model is designed to predict the average (or 

 maximum) erosion for a particular area, it was verified (Balsillie 1985b) by 

 comparing actual and predicted poststorm profile shapes to 32 cases where 

 measured changes were within 1.5 m^/m of the actual Q e av „ . In addition, 

 damage to three piers (two in Florida, one in California) was correlated to 

 the maximum wave crest elevation predicted by the model. 



Properties of the MSBWT 



91. This section relates the properties of the MSBWT model to the list 

 of requirements for dune erosion models stated in Part III. 



a_. The deepwater wave height and period entered are required by 

 the wave transformation model in order to determine the depth 

 of the innermost trough. Wave steepness is included through 

 use of a surf parameter defined as the wave steepness divided 

 by the square root of the beach slope (Balsillie 1984c). 



b. The model requires input of the maximum surge level above the 

 still-water line, including the wave setup. 



c_. The effect of grain size is included only as it affects the 

 offshore slope and the predicted equilibrium poststorm shape. 



d. The actual beach profile shape is fitted to an equilibrium 

 offshore shape. The true shape of the offshore is not 

 required . 



e_. The onshore boundary is dependent on the type of profile 

 (nonflooded, flooded, or breached). Each is treated 

 differently. On nonflooded profiles, a 1:1 slope is used for 

 the eroded dune above the surge level. 



f_. Cross-shore transport is modeled through the volumetric 



requirements of bar formation. The model has provisions for 

 accounting both dune erosion and overwash. Volume losses are 



45 



