1200 



I 



U Jan 78 



2400 



1200 



I 



15 Jan 78 



2400 



1200 



I 



16 Jan 78 



2400 



1^00 

 17 Jan 78 



1* in hours 



Figure 12. A tide gauge record segment from the western side of the 



Atchafalaya delta (Amerada Hess platform, Figure 3) showing the setup 



and setdown of bay water levels associated with cold-front passage 

 (14-17 January 1978). 



Figure 14 summarizes the suggested sedimentological impacts that diversion of 

 fresh water and sediment down the Atchafalaya system will have on the central and 

 western coasts of Louisiana. One of the initial effects of sedimentation in the bay 

 (I950's) was to diminish and finally eliminate a once-productive oyster fishery, Point Au 

 Fer and Marsh Island oyster reefs. With increased sedimentation of highly organic clays 

 and silty clays both in the bay and on the inner continental shelf, the shrimp fishery 

 potential is steadily increasing, however. 



As the deltas from both Lower Atchafalaya River and Wax Lake outlets continue to 

 fill the bay and build onto the shallow continental shelf, delta lobes will merge to form 

 extensive new marsh lands that wilL protrude into the marine environment. At the 

 present rate of nearly 3 km (1.16 mi ) of new marshland added above mean sea level to 

 the Atchafalaya deltas yearly (average 1975-81), by the end of this century, it is 

 estimated, bay filling will be complete and the subaerial delta will be prograding onto the 

 continental shelf. The mean drift system, as well as the wave-induced longshore drift, in 

 this part of the northern Gulf of Mexico favors an east-to-west transport direction. It is 

 safe to assume that the major areas of coastal progradation will be in the immediate 

 vicinity of the delta and along the downdrift coasts. New data concerning the important 

 effects of significant currents generated after the passage of cold fronts suggest that the 

 coarse facies (fine sands) may be skewed somewhat to the southeast after the delta 

 starts supplying coarse sediment to the continental shelf (Adams et al., submitted for 

 publication). However, even assuming that cold-front effects will modify coarse- 

 sediment transport on the shelf, the clays, silty clays, and silts will be spread in front of 

 the prograding subaerial delta and along the chenier coasts to the west (Figure 14). In 

 the short time since the I950's coastal progradation has replaced coastal retreat in many 

 downdrift sites. Sedimentation rates should increase in these areas as Atchafalaya Bay 

 fills and the delta progrades onto the shelf. 



227 



