Chapter IX 

 TIDES AND CURRENTS IN ESTUARIES AND INLETS 



Paragraphs 



Definition of estuary 436 



Characteristic tides and currents 437-438 



Form of an ideal estuary 438-445 



Prevalence of estuaries of typical form 446-448 



Effect of local contractions and enlaigements 449-450 



Deep channels of estuary form 451-453 



Effect of fresh-water discliarge 454-456 



Differences in tidal range caused by earth's rotation 457-459 



Overtides and overcurrents 460-462 



Tidal bore 463 



Effect of artificial changes in an estuary 464-477 



Tidal inlets, description . 478-479 



Hydraulics of inlet channels 480-481 



Observed relations 482-483 



Overcurrents in inlets 484 



436. Definition of estuary. — The reversing currents produced by the 

 filhng and emptying of the tidal prism of a river that enters a tidal sea, 

 generally dominate the river flow for a considerable distance up the 

 stream. This part of the river usually is funnel shaped, flaring to- 

 wards the entrance. A river mouth of such a shape is called an 

 estuary. The term may be applied as well to any tidal channel of 

 similar shape, even if it does receive any considerable inflow from the 

 uplands. 



437. Characteristic tides and currents in an estuary. — In a typical 

 estuary, the currents often have nearly the same strength and tides 

 nearly the same range at all cross sections; except in the upstream 

 reaches where the tidal flow merges into steady flow. The rate at 

 which high water and low water, and the strength and tmn of the 

 current, advance up an estuary is often so close to -y^gD, the rate of 

 advance of a frictionless progressive wave in a channel of uniform 

 dimensions (par. 339), that this is commonly regarded as the normal 

 rate of progress of the tide. 



438. The ideal estuary. — In the preceding chapter it was shown that 

 the currents in a long closed canal of uniform cross section diminish 

 from the entrance to the head of the canal and the rate at which the 

 tide advances up the canal increases toward the head. The uniformity 

 of the currents in an estuary, and of the rate of advance of the tide, 

 evidently is due to its shape. It is of interest to determine the special 



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