the downdrift side. 



these structures 



rate of erosion on 



classic example of 



of sand to beaches on 



The result is that 



actually increase the 



downdrift beaches. A 



this is represented by the jetties built 



in 1898 to stabilize the inlet to 



Charleston Harbor (Neal et al . 1984). 



sand has accreted at 



to the north, while 



the south has been 



In the mid - 1800's, 



Since that time, 

 Sullivans Island 

 Morris Island to 

 severely eroding. 



Morris Island had dunes 10-12 m high and a 

 well developed forest of pines and 

 palmettoes. Presently it is a low, 

 rapidly migrating sand flat. The Morris 

 Island Lighthouse, which was approximately 

 850 m from the shoreline in the late 19th 

 century, now stands 500 m offshore (Figure 

 30) (Neal et al . 1984). 



Recognizing that jetties often cause 

 erosion of "downstream" beaches, newer 

 jetties, such as those at Murrells Inlet, 

 SC, are being built with provisions to 

 move sand from the updrift side of the 



jetties to the downdrift side. At 

 Murrells Inlet the inner section of the 

 north jetty is a subtidal weir jetty, 

 allowing sand to pass over into a 

 deposition basin (Van Dolah et al . 1984). 

 The design was to allow the basin to be 

 periodically dredged, depositing the sand 

 on the downdrift side of the south jetty. 



Seawalls and bulkheads are 

 constructions of last resort (Pilkey et 

 al , 1980). Seawalls reflect wave energy 

 at high tide, increasing the rate of 

 offshore sand transport. This steepens 

 the beach profile, which in turn increases 

 the energy of the waves striking the 

 seawall. Seawalls also increase the 

 intensity of longshore currents, which 

 remove even more sand from in front of the 

 wall. Ultimately the system is self- 

 destructing. 



Seawalls and bulkheads also prevent 

 the exchange of sand between the beach and 

 the dunes during storms. The beach cannot 



Figure 30. The lighthouse at Morris Island, SC (photo taken by O. Pilkey). 



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