Residual currents computed from these records are shown in Figure 6. The overall 

 pattern is that of strong flow down the axis of the navigation channel, spreading and 

 reducing in speed on reaching the Gulf of Mexico, then deflection to the west on the 

 inner shelf. Analysis of current data taken on the shelf farther to the west (longitude 

 90° 30') also indicates residual flows to the west (Crout and Hamiter 1981). 



First-order approximations of the sediment mass transported in the Atchafalaya 

 mud stream have been made by taking the product of average suspensate concentration, 

 cross-sectional area of the mud stream, and average drift speed of currents (Figure 6). 

 Conversion to volume transport is made using a density of 375 kg/m (Wells and Roberts 

 1981). When converted to transport per year, the volume of sediment moving in the 

 Atchafalaya mud stream is 53 X 10 m~^, almost half of the volume of sediment that 

 leaves Atchafalaya Bay. Evidence for an intimate connection between Atchafalaya delta 

 development and chenier plain sedimentation can be found in the good time correlation 

 between subaqueous deltaic sedimentation in the bay and the first appearance of 

 mudflats near Chenier au Tigre. Abnormally high river discharge in 1973-75 correlated 

 well with a renewal of mudflat development after a period of erosion in the I960's. 



FUTURE FOR LAND BUILDING ALONG THE CHENIER PLAIN COAST 



We have established that the chenier plain coast is a downdrift recipient of 

 renewed deltaic sedimentation, but that the rate of growth today is insufficient to stop 

 the historic trend of shoreline retreat. There is localized instantaneous progradation in 

 the form of ephemeral and unvegetated mudflats. Because the major effect of subtidal 

 muds is to attenuate incoming wave energy, conditions are being created that are 

 favorable for further sedimentation (Wells and Coleman 1981; Wells and Roberts 1981). 

 Formation of mudflats, then, is the first stage in the feedback loop between coastal 

 energy and shoreline response, which eventually leads to stabilization and progradation. 

 Volume calculations show that more sediment reaches the chenier plain via the 

 Atchafalaya mud stream than appears as new mudflats. For example, if a typical 

 mudflat has a volume of I x 10 m-' to 2 x 10° m-^, then 25 to 50 such mudflats could form 

 each year. Since new mudflats have not been observed to form at this rate, much of the 

 sediment may be spread across the inner shelf as a thin veneer over a longshore distance 

 of perhaps 100 km or more. 



The ephemeral nature of these mudflats suggests that the localized process of 

 shoreline progradation has just begun to accelerate (Wells and Kemp 1981). As a result, 

 we hypothesize that the initial stage of coastal progradation from a new sediment pulse 

 is one of transitory mudflats only. As sedimentation continues, new mudflats will appear 

 and merge with existing mudflats. At its peak of development, the shoreline will become 

 "choked" with fine-grained sediment, mudflats will stabilize and grow seaward, and new 

 marsh vegetation will become estabished. The potential for land building by this method 

 should not be underestimated. The entire chenier plain region itself represents a net 

 coastal progradation of 25 km from the Pleistocene surface contact to the present Gulf 

 of Mexico shoreline. This land building took place in not more than 5,000 years during 

 which the many stranded beach ridges tell us that accretion alternated with erosion. 

 Thus, a conservative estimate of the land-building potential afforded by mudflat 

 accretion is on the order of 5 m/yr or close to the rate at which retreat is now 

 occurring. Accelerated growth of the chenier plain is expected when the subaerial 

 Atchafalaya delta outgrows Atchafalaya Bay, allowing a greater volume of sediments to 



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