The ideal result of post-storm beach recovery is a wide backshore 

 that will protect the shore from the next storm. Beach recovery may be 

 prevented when the period between successive storms is too short. Main- 

 tenance of coastal protection requires knowledge of the necessary width 

 and elevation of the backshore appropriate to local conditions, and 

 adequate surveillance to determine when this natural sand reservoir is 

 diminished to a point where it may not protect the backshore during the 

 next storm. 



4.525 Bar-Berm Prediction . High, steep waves scour the beach, eroding the 

 foreshore into a simple con cave -upwards profile. The material eroded from 

 the beach is deposited offshore as a longshore bar. Waves of low steepness 

 tend to push sand onto the beach, usually as migrating longshore-bar sys- 

 tems which eventually become part of the beach. In contrast to the concave- 

 up eroded profile discussed previously, the accreted profile is concave- 

 downward. Idealized eroded and accreted profiles (measured in a prototype- 

 scale wave tank) across the beach and nearshore zone are shown in Figure 

 4-28. 



To design a beach that contains a reservoir of sand in the backshore 

 sufficient to survive a design storm, a minimum requirement is the ability 

 to distinguish wave conditions that cause eroded profiles from those that 

 cause accretion. Usually, it is assiomed that a berm characterizes an ac- : 

 creted profile and that a bar characterizes an eroded profile. (See Figure 

 4-28.) Tliis picture is somewhat idealized. A sharp berm crest between 

 backshore and foreshore is often lacking, and on some beaches the berm is 

 absent, so that the top of the foreshore reaches the dune or cliff line. 

 Berms are illustrated in Figures 4-1 and 4-27. 



Similarly, the idealized longshore bar seaward of an eroded beach, 

 (middle profile of Figure 4-18) is often absent, and in its place there 

 may be several subdued bars or a platform extending to the breaker line 

 at nearly constant depth. 



a. Longshore Bars . The term bar has been applied to a number of 

 quite different coastal features, including barrier islands (the "off- 

 shore bar" of Johnson, 1919), ridge-and-runnel systems, and linear shoals. 

 (Duane, et al., 1972.) Longshore bars are unrelated to any of these fea- 

 tures. They appear to most nearly resemble a ridge-and-runnel system, 

 but differ in that longshore bars are located at the breaker position and, 

 at least in part, are eroded out of the bottom by the falling breaker, 

 whereas the ridge-and-runnel system is an accretionary feature migrating 

 landward across the surf zone. 



The typical longshore bar, as described from the observations of 

 Shepard (1950) and the experiments of Keulegan (1948) , is a ridge of sand 

 parallel to the shore and formed at the breaking position of large plung- 

 ing breakers. Longshore bars seem most directly related to the height of 

 larger breakers (not necessarily of maximum height). The depth to the sea- 

 ward bar increases with the height of the larger breakers along the Pacific 

 coast. (Shepard, 1950.) Bars form readily in tidal seas, but seem better 



4-78 



