Zone IV: Net transport rate on the foreshore 



371. The net transport rate in the swash zone is expected to be a 

 function of local beach slope, sediment characteristics, and properties of the 

 bore propagating upon the beach. No wave or bore information was available 

 for this study, except for some runup measurements from the CE data. Conse- 

 quently, it was not possible to derive a relationship connecting the net 

 transport rate on the foreshore to local wave properties and other factors. 

 However, some qualitative observations were made of the shape of the net 

 transport rate distribution on the foreshore. The region discussed in this 

 section extends approximately from the runup limit seaward to some specific 

 depth corresponding to the point of maximum retreat of the waves in the swash. 

 This depth is a function of the incident waves which cause setup at the 

 shoreline roughly proportional to the breaking wave height. Swash oscillates 

 about the mean shoreline elevation with a range dependent mainly on wave 

 height and surf similarity parameter, even in the field (Guza and Thornton 

 1982, Holman 1986). 



372. For some cases, the net transport rate showed a fairly complex 

 spatial dependence on the foreshore, in particular at the early stages of the 

 experiments. However, the net rate had an almost linear decay with distance 

 for a majority of the cases, both for onshore and offshore transport condi- 

 tions. Figure 50 gives a representative example of the transport rate 

 distribution over the foreshore for CE Case 300, in which different consecu- 

 tive distributions in time are plotted. The slope of the transport rate 

 decreased with time as the profile approached equilibrium, but the shape of 

 the distribution roughly maintained a linear form. In Case 300, the profile 

 retreated shoreward as the foreshore eroded during the run. (The location of 

 the still -water shoreline is indicated by vertical lines for the various 

 distributions in time.) A linear decay in the transport rate implies that an 

 equal amount of material is eroded at all points along the foreshore up to the 

 runup limit (compare field observations of Seymour 1987). 



373. As the foreshore eroded, a step formed extending approximately 

 from the still -water shoreline to the runup limit. The slope of the step may 

 have increased until the angle of initial yield was exceeded (Allen 1970) and 

 avalanching occurred, thereby adjusting the slope to a lower value (residual 



152 



