small compared to the total gross yearly littoral transport, all of the sedi- 

 ment that is transported must be eroded from the nearshore area. 



73. The onshore-offshore transport of sediment is not well understood. 

 Attempts have been made in recent years to establish the equations governing 

 onshore-offshore transport and solve them using numerical models. However, 

 none of these attempts has produced a numerical model that can be used for 

 reliable quantitative predictions of onshore-offshore transport. For example, 

 Wang (1981) evaluated onshore-offshore models developed in recent years by 

 Sunamura (1980), Dally (1980), and Yang (1981). Wang concluded that "Beach 

 profile modeling is a quite recent endeavor. It is a difficult problem be- 

 cause the physical process is complicated and is not well understood. The 

 three models introduced here are not at operational level and are not adequate 

 for quantitative predictions." 



74. Since the equations governing onshore-offshore transport are not 

 completely known, in this study a numerical model is developed that is 

 strongly based upon concepts developed by Swart of the Delft Hydraulics Lab- 

 oratory, the Netherlands (Swart 1974a, 1974b, and 1976). Swart' s concepts 

 were extended to allow the model to consider a variable datum (time-varying 

 tide), a variable wave climate, and onshore transport in addition to offshore 

 transport (Swain and Houston 1983, 1984a, 1984b, and Swain 1984). 

 Governing equations 



75. In his conceptual model, Swart divided a normal beach profile into 

 three zones (Figure 24), each with its own transport mechanism. The first 

 zone is a backshore above the limit of wave runup. If windblown sediment 

 transport is neglected, there is no transport in this zone. The second zone 

 is a developing profile (D-profile) where a combination of bed-load and sus- 

 pended load transport takes place. The dividing point between these two zones 

 is the highest location that waves reach on the beach. Since the tide datum 

 and wave climate vary with time, this dividing point moves with time. The 

 position of maximum runup was determined empirically by Swart and is given by 

 the following equation (all units are metric): 



7650 D 5Q 



0.000143 H°- 488 I ' 93 

 mo 



1 107786 



50 



55 



(98) 



