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HT. SUBAERIAL 

 IVOR'S ISLAND 



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AVERAGE WATER LEVEL 

 DURING STUDY PERIOD 

 DEER ISLAND 



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AVERAGE LEVEL 

 EUGENE ISLAND 



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4 8 12 16 20 



HOURS AFTER FRONT PASSAGE (WIND SHIFT) 



Figure 13. Mean water levels in Atchafalaya Bay following cold fronts. 

 These data are summarized for 35 cold fronts between 1 January 1979 and 

 30 April 1980. 



Additional effects associated with water-level elevation near the coast will tend to 

 offset marsh deterioration caused primarily by the numerous processes collectively 

 described as subsidence. These "backwater effects" are caused by deltas at the mouths 

 of major flood-water outlets at the coast. This process, plus similar effects produced by 

 water-level elevation during the passage of cold fronts, provides a new supply of 

 sediment to the marshes, causing aggradation of the surface. 



In summary, diversion of Mississippi River water and sediment to the coast through 

 the Atchafalaya system has led to the following conclusions concerning impacts on 

 central and western Louisiana coasts: 



(1) New marsh lands are being added in the vicinity of the active Lower 

 Atchafakiya River and Wax Lake Deltas at an average rate of about 3 km /yr 

 (1.16 m^lyr) (average 1973-81). This trend will continue as long as present flow 

 levels are maintained. 



(2) Downdrift coastlines are starting to accrete as a product of advected clays and 

 silty clays from the Atchafalaya River source. The rate of coastline 

 progradation should increase as the delta builds onto the continental shelf and 

 makes sediments more available to the downdrift areas. 



(3) "Back-water effects" result from water-level elevation during cold-front 

 passages and inefficient dispersal of sediment-rich flood waters at the coast 



228 



