148 



approximate, does simulate the overall trends suggested by the measurement. 

 Dredging plans called for a deepening of the channel between the entrance 

 (end of jetties) and King's Bay ranging from to 4 m. A 4% rise in the 

 salinity was predicted 20 km up the entrance as a result of dredging 

 (Environmental Science and Engineering, Inc., 1980). 



The aforementioned examples are merely illustrative of the basic 

 phenomenon of interest, and are not meant to demonstrate the power of 

 available technology with respect to physical or numerical models and their 

 application. The subject matter has been covered extensively in literature 

 on fluid mechanics and hydraulic engineering. For a fuller treatment, the 

 work of Fischer et al . (1979) may be cited. Here, it will suffice to make 

 reference to a recent study conducted to examine the potential effects of 

 deepening the lower Mississippi River on salinity intrusion (Johnson et 

 al., 1987). In Fig. 9.5, the duration of the saline wedge intrusion (days 

 per year) is plotted against distance above the head of a number of passes 

 (distributaries) in the vicinity of New Orleans. Two curves are shown, one 

 for a 12 m deep channel (present depth is 9 m) and another for a 17 m deep 

 channel. These curves were generated numerically. The model was 

 calibrated against data obtained during the 1953-54 low- flow period. The 

 results essentially are illustrative of the effect of channel deepening 

 from 12 m to 17 m. Thus, for example, this effect would increase the 

 duration of the wedge at 180 km from about 40 days to about 60 days per 

 year. 



9.5 RESEARCH NEEDS 



Scientific work in understanding the basic processes of mixing between 

 salt and fresh waters is likely to continue well into the future, in order 

 to steadily improve predictive methodologies, which have at present 

 attained a reasonable degree of sophistication. However, further 

 improvements are highly desirable. 



An area which deserves further consideration is mixing under wave 

 action, or combined wave and tide action. At present the effect of waves 

 in this respect is only weakly understood. Much of analysis carried out so 

 far appears to have been directed towards situations dominated by tides 



