Astronomical tides and tidelike effects of surge in Stillwater bodies, due to any of the 

 causes previously enumerated, often play an important role in water quality control. The 

 current-producing exchange of water between the harbor basins by tidal action may be 

 essential to the marine ecology and the prevention of a stagnant condition. Consideration 

 should be given in harbor planning to the achievement of the best possible circulation of 

 water through the basins and channels by effective use of the tidal prism. Water level 

 fluctuations on a slower cycle often occur in inland lakes and rivers for various reasons, and 

 although they usually occur too slowly to produce beneficial water-exchange effects, they 

 must be accounted for in design. Typical examples are: (a) seasonal drawdown of water 

 conservation reservoirs, (b) annual changes in the water levels of the Great Lakes, and 

 (c) periodic water level changes in many natural lakes and rivers due to increases and 

 decreases in precipitation over their tributary areas. 

 3. Water Area Shoaling Factors. 



a. Littoral Drift. A principal cause of shoaling at entrances to harbors along the shores 

 of oceans and large lakes is littoral drift (Fig. 13). Because the longshore movement of sand 

 is due mainly to wave action, any structures that change the normal regimen of waves 

 breaking along a coast may influence the littoral movement. If an unprotected channel is 

 dredged through a beach into an inner basin, the wave impinging on either side at the mouth 

 will be refracted in such a way as to cause abnormalities in the wave pattern approaching the 

 Ups of the channel. If the approach of the prevailing waves is normal to the shore, the initial 



Figure 13. Entrance shoaUng by littoral drift in a Massachusetts harbor. 



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