bars have associated lows (runnels) on the landward side and occasional 

 drainage gullies across them (King, 1972, p. 339). These systems are 

 characteristic of poststorm beach accretion under a wide range of wave, tide, 

 and sediment conditions (Davis et al., 1972)- Further accretion continues by 

 adding layers of sand to the top of the bar vAiich, by then, is a part of the 

 beach (see Fig. 4-34). 



Berms may form immediately on a poststorm profile without an intervening 

 bar-and-trough, but the mode of berm accretion is quite similar to the mode of 

 bar-and-trough growth. Accretion occurs both by addition of sand laminas to 

 the beach face (analogous to accretion on the seaward-dipping top of the bar 

 in the bar-and-trough) and by addition of sand on the slight landward slope of 

 the berm surface when waves carrying sediment overtop the berm crest 

 (analogous to accretion on the landward-dipping slip face of the bar) . This 

 process of berm accretion is also illustrated in Figure 4-1. 



The rate at which the berm builds up or the bar migrates landward to weld 

 onto the beach varies greatly, apparently in response to wave conditions, 

 beach slope, grain size, and the length of time the waves wDrk on the bars 

 (Hayes, 1971). Compare the slow rate of accretion at Crane Beach in Figure 4- 

 33 (mean tidal range 2.7 meters (9 feet), spring range 4.0 meters (13 feet)), 

 with the rapid accretion on the Lake Michigan shore in Figure 4-34 (tidal 

 range less than 0.08 meter (0.25 foot)). 



Poststorm studies show that the rate of poststorm replenishment by bar 

 migration and berm building is usually rapid immediately after a storm 

 (Birkemeier, 1979; Kana, 1977). This rapid buildup is important in evaluating 

 the effect of severe storms because the true extent of erosion during the 

 storm is likely to be obscured by the potstorm recovery (unless surveys are 

 made within hours after the storm) . 



The ideal result of poststorm 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. Maintenance of coastal 

 protection requires (a) knowledge of the necessary width and elevation of the 

 backshore appropriate to local conditions and (b) adequate surveillance to 

 determine when this natural sand reservoir has diminished to the point where 

 it may not protect the backshore during the next storm. 



e. Prediction of Eroded versus Accreted Beaches. An important aspect of 

 onshore-offshore sediment transport is the distinction between conditions 

 which result in beach erosion and those which produce beach accretion. It is 

 occasionally assumed that a berm characterizes an accreted profile and that a 

 bar characterizes an eroded profile. This is oversimplified in that (1) a 

 berm may be absent on an accreted beach idiere the top of the foreshore may 

 reach the dune or cliff line, (2) nearshore bars do not directly indicate an 

 eroded beach, and (3) a bar and a berm may both be present. Bars are 

 connected in complicated ways with breaker processes (see Battjes, 1974), 

 tidal range, and sediment character and supply (see Krumbein, 1944; Shepard, 

 1950; Saylor and Hands, 1970; Zwamborv., Fromme, and Fitzpatrick, 1970; Davis 

 and Fox, 1972; Carter and Kitcher, 1979; Greenwood and Davidson-Arnott, 

 1979). Berms result from complicated, interrelated processes at the landward 

 edge to the hydrodynamic transport of sediment. 



4-83 



