162 



Atchafalaya delta land area. The quasi-two dimensional approach employed a 

 one -dimensional numerical model with two subsidence rates to establish a 

 range of possible results and to define sensitivity to subsidence. 



Prediction of bed movement due to sediment erosion-deposition was 

 carried out by the application of the two-dimensional, depth- averaged 

 numerical model in Savannah Harbor, Georgia (Ariathurai et al., 1977). The 

 reach of the estuary under investigation was 6,400 m in length, between 

 stations 1 and 3 (Fig. 10.4). The turning basin, near station 2, was the 

 region of heaviest shoaling. Flow and sedimentary data were collected at 

 the three stations previously. Given the flow field (from measurements) , 

 the model was verified against measured suspension concentrations. 

 Fig. 10.4 shows predicted bottom evolution with tide for the period 2300 hr 

 on September 24, 1968, to 0500 hr on the following day. The semi-diurnal 

 spring range of tide during this period was 2.6 m. The initial condition 

 corresponds to the situation at 2300 hr when the ebb current was decreasing 

 but there was no measurable deposition. The bottom topography shown in the 

 figure at that time may therefore be considered to represent the bed which 

 was not scoured by the prevailing current. As observed, heavy deposition 

 occurred by 0500 hr, at slack water following flood, particularly in the 

 region of the turning basin. Measured deposit thickness on the order of 

 1 m there agreed with the model result. Modeling effort such as this one 

 can be used to generate predictive scenarios for sedimentation patterns 

 provided the expected hydrodynamic and sedimentary boundary conditions are 

 well known. 



10.4 WETLAND RESPONSE 



Shallow bays surrounded by extensive wetlands will expand rapidly in 

 response to a rise both because of the gentle slope and the deterioration 

 of the marshes in response to salinity increases. For example, Barataria 

 Bay, Louisiana, has increased its surface area about 10 to 15 percent over 

 the last century in response to about 1 m of local relative sea level rise 

 (National Research Council, 1987b). In general, however, although wetlands 

 are critically important as a buffer against shoreline erosion, their 

 response to sea level change is complex and not yet fully understood in the 

 quantitative sense. 



