Inequality of Offshore/Onshore Transport 



On most beach and barrier systems, a well-established cycle of sediment exchange 

 exists between the beach and shoreface in response to storm and fair weather 

 conditions. In general, sediment is eroded from the beach and nearshore bars under 

 storm conditions and stored in bars located farther offshore or deeper on the lower 

 shoreface. Under fair weather conditions, a variable proportion of this material may 

 move onshore and return to the beach, resulting in accretion. This pattern of change has 

 been well documented for southeastern Australian beaches (Short 1978). The proportion 

 of sediment returned to the beach is dependent, among other factors, on the maximum 

 depth from which waves can transport sediment landward under constructive fair 

 weather conditions, compared to the maximum depth which sediments are transported 

 seaward under erosive storm conditions (Figure 7). 



To estimate the effectiveness of sediment return from offshore, the threshold 

 depth for the initiation of sediment motion was calculated for wave conditions of 

 A-second period, 30 cm high; 5-second period, 50 cm high; 7-second period, 200 cm high; 

 lO-second period, 300 cm; and 1 5-second period, 400 cm high. The first two of these 

 conditions represent typical constructive conditions on the Louisiana coast. The last 

 three conditions are typical of winter frontal storms and force l-to-3 hurricanes. For the 

 first two cases, 4-second and 5-second waves, the critical threshold depth offshore from 

 the Chandeleur Islands was 5 m and 6 m, respectively. For the 7-second, lO-second and 

 1 5-second waves, critical depths offshore from the Chandeleur Islands were around 40, 

 60, and 150 m, respectively. Murray (1970, 1972) measured near-bottom currents off the 

 Florida Panhandle in 3.6 m of water during the passage of Hurricane Camille and off the 

 southern Chandeleur Islands in depths of 20 m during winter frontal storm passage. In 

 both cases, the near-bottom current field velocity vectors were directed shore-parallel 

 and offshore during a frontal passage associated with strong, onshore winds and high 

 wave energy. 



These data indicate the presence of a strong inequality in offshore-onshore 

 transport related to the storm-dominated characteristics of the wide, shallow Louisiana 

 continental shelf. Sediments transported offshore during these storm events under wave- 

 and wind-induced nearshore circulation encounter offshore- and longshore-directed 

 near-bottom currents. Only that sediment deposited above the 5 to 6 m depths on the 

 shoreface will be available for subsequent return to the barrier system. This mechanism 

 represents another means of permanent sediment loss from the Louisiana barrier system, 

 and may explain the extensive offshore sand sheets seaward from the Chandeleur Islands 

 reported by Frazier (1974). 



THE LATE LAFOURCHE COASTAL BARRIER SYSTEM 



Barrier Development 



The Late Lafourche delta barrier system consists of the Bayou Lafourche erosional 

 headland, the Caminada-Moreau coast, and two nearly symmetrical sets of flanking 

 barriers, Caminada Pass spit and Grand Isle to the east, and the Timbalier Islands to the 

 west (Figure 10). The barriers have developed as the shoreface retreated, actively 

 reworking distributary sand bodies of Bayou Lafourche and the beach ridges of Cheniere 

 Caminada (Harper 1977). The sediment dispersal pattern consists of the longshore 



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