Ch. 16] CHANNEL HARBORS IN TIDAL ESTUARIES 295 



In some of these harbors the velocities of tidal flow — particularly 

 when reinforced by river flood flows — are competent to move sand, and 

 the problem of sand shoaling is of concern. In general, however, the 

 problem is one of shoaling from silt and clay. Several factors serve to 

 augment the tendency to shoal by silt and clay sedimentation: among 

 these are (1) the slack water periods between successive tides, during 

 which slack water periods the water has no turbulence acting to keep 

 the suspended silt and clay in suspension; (2) the intermixing of the 

 silt-laden fresh water with the salt water brought into the estuary by 

 tidal currents and density currents, this intermixing causing floccula- 

 tion of the suspended material and greatly accelerating its tendency to 

 settle to the bottom. 



Attempts to overcome the problems connected with sand shoaling 

 have been similar to those for river-channel harbors described earlier 

 in this chapter. For shoaling due to silt and clay sedimentation, how- 

 ever, other solutions have been attempted, among them: 



(1) Increasing the tidal prism passing through the harbor area in an 

 attempt to create sufficient velocities to flush out the harbor channel. 

 This increase in tidal prism may be effected in various degrees by im- 

 proving the hydraulic characteristics of the estuary channel above and 

 below the harbor, or by creating a greater tidewater area above the 

 harbor. 



(2) Giving the ebb flow a dominance over the flood flow in the har- 

 bor by a suitable arrangement of training walls or one-way tide gates. 

 The purpose of giving the ebb flow the dominance is to insure the 

 movement of shoal material through the harbor and on downstream. 

 This method is conceivably adaptable to problems of both sand shoal- 

 ing or shoaling from silt and clay. 



(3) Decreasing the tidal prism passing through the harbor to a 

 minimum, thereby keeping the amount of potential shoal material 

 entering the harbor to a minimum. This entails creating what is es- 

 sentially an off-channel harbor, as discussed below. 



Contributions to the following fields would probably serve to assist 

 in the solution of problems dealing with harbors of this type : 



(1) A study of factors affecting the rate of settling of suspended 

 silt and clay in harbor areas, particularly with reference to floccula- 

 tion in the presence of sea water. 



(2) Competence of flowing water to flush sand deposits and silt and 

 clay deposits in various stages of compaction out of harbor areas. 



