change in volume per time step is calculated from the new x-contour defined 

 profile between the shoreline and the breaker line. Note that the 

 time -dependent shoreline represents the intersection of the storm surge 

 hydrograph with the face of the schematic berm or dune. This computation 

 yields a total volume of material which is either eroded or deposited in the 

 surf zone as a result of the input storm surge and wave field. 



The key assumption in the model which relates surf zone dynamics to the 

 berm and dune complex is that the total volume of deposition (or erosion) in 

 the surf zone is balanced by erosion (or deposition) from the berm or dune 

 face. If offshore deposition is indicated from Equation 5, then material is 

 uniformly removed from the berm or dune face until the offshore volume of 

 deposition is balanced. If onshore transport is indicated, that volume is 

 supplied uniformly to the face of the berm. Deposition on the dune face is 

 not permitted since dune erosion is considered to be permanent. An offshore 

 volume of erosion or deposition is computed for each time step of the storm 

 surge hydrograph. 



The dune erosion model is capable of accurate predictions of storm induced 

 erosion if the limitations of the model are not severely violated. Limita- 

 tions include the schematic requirements of the dune and berm, and the 

 specification of an equilibrium profile to represent the existing offshore 

 bathymetry which may include bars or troughs. Also, the one -dimensional 

 assumption that alongshore transport is uniform and that erosion is a function 

 of only cross -shore transport can lead to inaccurate erosion predictions if 

 the results are not analyzed with respect to the basic modeling assumptions. 

 Fortunately, in many cases, the restrictions are not too severe and reasonable 

 predictions can be obtained. The fact that many natural beach profiles can be 

 represented by the model was demonstrated by the limited CERC verification of 

 the model to 14 pre- and post-storm profiles (Birkemeier, et. al . 1987). 



STORM SURGE GENERATION 



Results of time -varying erosion of the berm and dune can be computed for 

 any specified storm surge hydrograph. If the frequency of occur-rence of that 

 storm event is known, the erosion volume and berm or dune face recession 

 distance can be directly related to that frequency. In order to develop this 

 relationship, a data base of storm events with the corresponding stage- 



136 



