Ch. 17] SAVANNAH HARBOR, SAVANNAH RIVER 311 



ditions of tide and fresh-water discharge, the initial contact between 

 salt water from the ocean and silt-laden fresh water from the upper 

 watershed occurs. Furthermore, the effects of salt-water currents on 

 bottom velocities in this reach of the harbor are very pronounced, there 

 being a condition of almost complete stagnation on the bottom during 

 the ebb period. The resultant of the above conditions is that very 

 rapid shoaling occurs in the navigation channel, and extensive dredg- 

 ing, at an average annual cost of approximately $250,000, is required 

 to maintain project depths in this relatively short length of channel. 



The model used for a study of the above problem reproduced an 

 area of the Atlantic Ocean from Wassaw Sound on the south to Hilton 

 Head Island on the north and offshore to about the 40-foot contour of 

 depth; the Savannah River from its mouth to the head of tide; and 

 the maze of interconnecting tidal channels tributary to the Savannah 

 River. The model was of the fixed-bed type, all channel and over- 

 bank areas being molded in concrete to scales of 1:1,000 horizontally 

 and 1 : 150 vertically. Automatic tide controls reproduced the tides 

 in the ocean portion of the model, and the scale discharge of fresh 

 water, representing the fresh-water flow of the river, was introduced 

 at the upstream end. The salinity of the model ocean was main- 

 tained at the measured salinity of the prototype, so that salinity cur- 

 rents and their effects were reproduced accurately throughout the 

 model. No attempt was made to reproduce the prototype process of 

 shoaling in the model; instead a comprehensive study was made of 

 the effects of each plan on hydraulic conditions throughout the har- 

 bor, and on this basis the probable effects of the plan on prototype 

 shoaling were predicted. 



Plans designed and tested to reduce shoaling in lower Front River 

 consisted in deepening, widening, and extending upstream the project 

 channel, constructing submerged sills across the mouth of Back River, 

 constructing a dam and tide gate in Back River and a dredge cut 

 across Hutchinson Island above the city of Savannah, and replacing 

 the tide gate of the latter-mentioned plan with a solid dam. Tests of 

 widening, deepening, and otherwise improving the Front River project 

 channel indicated that bottom ebb velocities in lower Front River 

 could be increased appreciably by this method. As was expected, im- 

 provements in the hydraulic efficiency of the channel allowed the tide 

 wave to run more freely and thus effected increases in velocities 

 throughout Front River. Tests of submerged sills across the mouth of 

 Back River indicated that this method would have little, if any, effect 

 on velocities in Front River. By far the greatest increase in ebb 



