possible to derive an empirical transport relationship for the surf zone 

 specifically applicable to onshore -directed transport. 



517. The net direction of cross-shore transport may be predicted using 

 Equation 2 in terms of the deepwater wave steepness and dimensionless fall 

 speed. As the grain size increases for given waves, the tendency for onshore- 

 directed transport also increases. By consideration of the dimensionless fall 

 speed, Dean (1973) explained the tendency for onshore transport to occur in 

 terms of the relation between the elevation to which a particle is suspended 

 and the distance it falls during wave passage. A hydraulically heavy particle 

 falls to the bottom during the onshore portion of the wave motion because of 

 the greater settling speed, resulting in net movement onshore. 



518. The criterion for distinguishing bar and berm formation is closely 

 related to the transport direction and used in the model to determine trans- 

 port direction, as discussed previously. The same basic transport relation- 

 ship (Equation 33) is used whether onshore or offshore transport occurs. A 

 beach that is not in equilibrium with the waves and unable to dissipate 

 incident wave energy uniformly over its length will experience transport until 

 equilibriiam is attained if exposed to the same wave climate for a sufficiently 

 long time. For onshore transport, the net transport rate in the model is 

 assumed to be proportional to the energy dissipation per unit volume, similar 

 to the situation for offshore transport. Also, the term which modifies the 

 net transport rate due to the local bottom slope is incorporated. Seaward of 

 the break point, exponential decay (Equation 21) of the transport rate is 

 imposed with a spatial decay coefficient as given from the LWT experiments. 

 The same value of the spatial decay coefficient, 0.11 m"^' is applied indepen- 

 dently of wave and sand parameters. 



519. Both the location of the plunge point and the value of the spatial 

 decay coefficient between the break point and plunge point are determined in 

 the same manner for both accretionary and erosional profiles. Since the 

 magnitude and direction of the transport rate seaward of the plunge point 

 depend on the transport rate in the surf zone, the transport will be onshore 

 if the transport is directed onshore in the surf zone. On the foreshore, a 

 linearly decreasing transport rate is applied to the runup limit with the 

 decay starting from the shoreward end of the surf zone. This shape is 



220 



