waves can decrease when the bottom velocities are insufficient to transport 

 the sand particles up the steeper slope produced by the foreshore 

 accumulation. 



RULE 4: Water pushed by waves into a groin compartment will sometimes 

 return offshore in the form of rip currents along the sides of 

 groins . 



In this way, groins may actually increase the amount of sediment which 

 moves offshore as well as the distance seaward that it travels. Dean (1978) 

 explains three mechanisms for creating rip currents between groins. The first 

 is the simple channeling of the longshore current which can push up against 

 the groin and then jet seaward (see Fig. 5-lla). 



The second mechanism is the differential wave setup at the shoreline 

 created when waves approach at an angle to the beach (see Fig. 5-llb). The 

 wave-sheltering effect of the updrift groin produces a smaller setup at the 

 updrift side of the groin compartment. This causes a circulation cell to be 

 established in which water flows (a) toward the updrift groin along the 

 shoreline, (b) seaward along the updrift groin, (c) downdrift along a line 

 seaward of the groins, and (d) back to the beach along the downdrift groin. 



The third mechanism is the differential wave setup at the shoreline 

 created when the waves approach normal to the beach (see Fig. 5-llc). The 

 setup is smaller adjacent to each groin due to the energy dissipation caused 

 by the interaction of water motion with the groin structures. This produces 

 two circulation cells within each groin compartment in which water flows (a) 

 along the shoreline from the center of the groin compartment toward each 

 groin, (b) seaward along each groin, (c) toward the center of the groin 

 compartment along a line seaward of the groins, and (d) back to the beach in 

 the center of the groin compartment. The circulation cells pick up sand at 

 the beach and deposit it in the deeper water seaward of the groin. The effect 

 is a sand loss at the beach even through the water recirculates. 



The appearance of rip currents, with their detrimental effects on the 

 beach, is difficult to predict. They are another uncertainty in groin 

 design. Dean (1978) suggests that the rip current problem can be compounded 

 if the groin spacing is the same as the rip current spacing under natural 

 conditions of the study area. This further emphasizes the importance of 

 understanding the physical environment of the study area. 



RULE 5: The percentage of the longshore transport which bypasses a groin 

 will depend on groin dimensions^ fillet dimensions, water level, 

 and wave climate. 



Sand can bypass a groin by traveling over its top (overpassing) or around 

 its seaward end (endpassing) . Overpassing will depend on the level of the 

 sand immediately adjacent to the groin. If the sand level is too low, the 

 longshore drift will not be carried over the groin; it will accumulate next to 

 the groin. As the sand accumulates and the fillet is formed, the level may 

 eventually rise enough to allow overpassing. However, the extent to which a 

 fillet can grow vertically is controlled by the wave and tide climate, not the 

 height of the groin. 



5-37 



