REVERSAL OF COASTAL EROSION BY RAPID SEDIMENTATION: 

 THE ATCHAFALAYA DELTA (SOUTH-CENTRAL LOUISIANA) 



Harry H. Roberts 

 Ivor LI. van Heerden 



Coastal Studies Institute and Departnnent of Marine Sciences 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



In early I950's Atchafalaya Bay began experiencing sedimentation, which marked 

 the initiation of a new major delta lobe In the Mississippi River Delta complex. This new 

 era will be characterized by rapid progradation and marshland growth In parts of coastal 

 Louisiana that have been typified by coastal retreat for hundreds of years. Although the 

 Atchafalaya River has long been a distributary of the Mississippi, it was not until the 

 early I950's that the Atchafalaya Basin had filled sufficiently to allow significant 

 quantities of sediment to be transported to the bay. The I950's and I960's marked the 

 period of subaqueous growth when the bay bottom accreted with prodelta clays and silty 

 clays. As a product of the abnormally severe 1973 flood, the Atchafalaya Delta became 

 a subaerial feature characterized by sand-rich lobes which are prograding at a rapid 

 rate. During 1972-77 approximately 32.5 km^ (12.6 mi ) (above low tide level) of new 

 marshland was added to Atchafalaya Bay as a product of sedimentation from Lower 

 Atchafalaya River Outlet. Similar orocesses are occurring at the mouth of Wax Lake 

 outlet, where, by early 1976, 2.20 km (0.85 mi ) of new land existed. 



Systematic monitoring of changes within the delta system over the last 4 years has 

 shown that delta growth responds directly to flood volume and duration. The years 1976 

 through 1978 can be characterized as average in terms of discharge. Analysis of 

 LANDSAT imagery reveals that Wax Lake suffered a net loss of subaerial expression 

 during this period owing to the combined effects of subsidence, compaction, and winter 

 erosion. Comparison of aerial photographs for a section of eastern Atchafalaya Delta 

 reveals a similar trend. Land loss was reversed during the major flood in 1979. 



The delta has evolved by channel bifurcation and bar fusion, processes by which 

 coarse distributary-mouth bars fuse into larger sand bodies through selective elimination 

 of the delivery network. These processes are accomplished by rapid growth of 

 mid-channel bars and sealing of feeder channels by subaqueous levee growth. The 

 presence of deltas at Lower Atchafalaya River and Wax Lake outlets has elevated water 

 levels near the coast during floods (backwater effect), causing sediment-rich water to be 

 transported into surrounding marshes. A similar response results from setup prior to 

 cold-front passage. The net effect is marsh aggradation and restoration in flood areas. 

 Rapid sedimentation since the I950's has reversed the traditional trend of coastal erosion 

 in the vicinity of Atchafalaya Bay and is now initiating a new growth phase of the 

 downdrift chenier plain. 



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