Transport Rate Equations 



402. The distribution of the transport rate was calculated using 

 relationships developed for the four different zones of the profile described 

 in Part V. In the surf zone, i.e., the region of breaking and broken waves, 

 the distribution of the transport rate is mainly a function of the energy 

 dissipation per unit volume. Seaward and shoreward of the surf zone, semi- 

 empirical relationships derived from the LWT experiments are applied to 

 calculate the transport rate distribution. The magnitude of the transport 

 rate in all transport zones is governed by that calculated for the part of the 

 surf zone where broken waves prevail (Figure 43, Zone III). 



403. The direction of net cross-shore transport is determined in the 

 model by the criterion described in Part IV, which is based on the deepwater 

 wave steepness and the dimensionless fall velocity (Equation 2). Although the 

 criterion was developed to predict formation of bar and berm profiles, 

 examination of associated cross -shore transport rate distributions showed that 

 this relation was applicable to predict the direction of net transport as 

 well. Onshore transport is predominant if a berm profile is formed, whereas 

 offshore transport is predominant if a bar profile is formed. According to 

 the criterion, material is transported offshore or onshore along the full 

 length of the active profile at a specific instant in time. This is a good 

 approximation if the profile is not too close to the equilibrium shape for the 

 existing incident waves , in which case the transport rate would tend to be 

 mixed, i.e., both onshore and offshore transport might occur along different 

 regions of the profile at the same time. If the model is applied to field 

 conditions, the mean wave height should be used to determine the direction of 

 transport by Equation 2. (As discussed below, significant wave height should 

 be used to calculate the breaking waves and transport rate.) 



404. Both Moore (1982) and Kriebel (1982) used transport rate formulas 

 for the surf zone in which the rate was proportional to the excess energy 

 dissipation per unit volume over an equilibrium energy dissipation which the 

 beach profile could withstand without changing shape significantly. Dean 

 (1977) showed that an equilibrium profile derived from the concept of a 

 constant energy dissipation per unit water volume from the break point and 



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