310 



PATTERSON-SIMMONS. MODEL STUDIES 



[Ch. 17 



charged with a negative electric potential. This potential being the 

 same in all particles, the latter repel each other, and a state of com- 

 plete dispersion prevails so long as the water is fresh. Contact with 

 salt water, however, causes a base-exchange reaction, whereby the 

 electric potential is instantly neutralized and a process of clotting, 

 technically known as coagulation or fiocculation, results. At first 

 the lumps of coagulated material are quite small; however, as more 

 and more particles are attached, the lumps attain sufficient size and 



Fig. 7. Savannah Harbor model study. 



weight to sink to the bottom, thus effecting deposits in the navigation 

 channel. 



The problem of channel shoaling due to fiocculation of suspended 

 silt by salt-water action is further complicated by the effects of salt 

 water on the velocity of bottom currents. Owing to its greater density 

 as compared with fresh water, salt water tends to occupy the lower 

 layer of the channels as a wedge-shaped mass, with the point of the 

 wedge upstream. The most pronounced effects of salt water on bot- 

 tom currents, therefore, are to increase bottom flood velocities and 

 decrease bottom ebb velocities, which effects reduce the normal pre- 

 ponderance of ebb flow over flood flow and thus decrease the net- 

 seaward movement of material being transported along the bottom. 



The section of the harbor subject to heaviest shoaling is the lower 

 4-mile reach of Front River between the city of Savannah and the 

 intersection of Front and Back rivers. In this reach, for normal con- 



