Cycle of Profile Erosion and Recovery 



Erosion 



19. Starting from a depositional profile characterized by a high, wide 

 berm, steep foreshore slope, and a relatively flat offshore, if the incident 

 wave steepness exceeds the critical steepness, the beach begins to erode. If 

 the surge is high enough, storm waves move sand from the berm and dune off- 

 shore in the region shoreward of the main wave breaking point. As the 

 material moves offshore, the beach slope in the surf zone becomes milder. As 

 a result, one or more offshore bars may form, and existing bars may move 

 farther seaward (Birkemeier 1985a). Figure 4 illustrates the offshore move- 

 ment of a bar resulting from the passage of a series of three storms at Duck, 

 North Carolina. The last and largest storm, occurring 13-15 November 1981, 

 caused the most beach erosion and greatest seaward shift of material. 



PROFILE LINE 188 

 5 OCT 81 



17 OCT 81 



■ 3 NOV 81 

 16 NOV 81 



200 400 600 81 



DISTANCE FROM BASELINE, m 



Figure 4. Offshore bar movement resulting from 



three storms at Duck, North Carolina 



(Birkemeier 1985a) 



20. The mechanisms controlling offshore bar formation and movement are 

 not well understood. Two main mechanisms are known to exist. The classical 

 mechanism is that the bars are formed at the break point of storm waves; the 



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