160 



shoaling, and the environmental resources of the area (McAnally et al . , 

 1985). 



Several factors combined to make the Atchafalaya Bay study unusually 

 complex. They included the long period over which predictions had to be 

 made; the migration of the region of delta growth from lacustrine to 

 estuarine to marine environments; a hydrodynamic regime that is variously 

 dominated by river flows, wind- induced currents, tides, waves and storm 

 surges; and the combined deposition of sediments from the sand, silt and 

 clay classes. The investigation included several separate prediction 

 techniques, including: 1) extrapolation of observed bathymetric changes 

 into the future, 2) a "generic" analysis that predicted future delta growth 

 by constructing an analogy between the Atchafalaya delta and other deltas 

 in similar environments, 3) quasi two-dimensional numerical modeling of 

 hydrodynamics and sedimentation, and 4) use of extensive field measurement 

 of water levels, currents and sediments, and laboratory experiments on 

 sediment samples . 



Results have shown a wide range of possible land growth rates for the 

 next 50 years in Atchafalaya Bay. Important results illustrated in 

 Fig. 10.3 indicate sensitivity of delta growth to the subsidence rate 

 (McAnally et al., 1984). These high subsidence rates are caused in part by 

 compaction of thick layers of fine sediments that have been deposited by 

 the Mississippi River and its distributaries over thousands of years. 



The extrapolation results shown in Fig. 10.3 were generated by 

 establishing a relationship between past delta growth and forcing phenomena 

 of river flow and sediment supply, then using that relationship in 

 combination with historically recorded flows to project future delta 

 growth. It did not explicitly include subsidence effects. The generic 

 approach assumed that Atchafalaya delta growth could be considered 

 analogous to other deltas growing under similar conditions and matched 

 observed Atchafalaya delta growth with a generic delta growth and decay 

 time history such as shown in Fig. 10.3. The non-dimensionalized generic 

 curve was then taken to represent a possible future growth and decay cycle 

 for the Atchafalaya. Two results are shown- -generic least squares fit the 

 generic curve to observed Atchafalaya growth in a least squares approach; 

 generic observed forced the generic curve to pass through the observed 1980 



