CHAPTER VII. 

 IMPROVEMENT OF RIVER OUTLETS. 



THE improvement of the mouths of rivers is a complicated problem, and its proper 

 consideration would require a volume. It is only intended here to refer briefly to 

 the general aspects of such work. 



Formation of Bars. On reaching the sea the current of a river is suddenly 

 checked in its flow and the sediment held in suspension cannot be carried further. 

 The result is the formation of bars, and these bars generally grow in length out into 

 the sea and rise in height to such an extent that the water, in trying to escape from 

 the river, forces its way through new channels or side outlets of shallow depth. The 

 splitting up of the river into a number of divisions reduces the velocity of the current 

 and in consequence its power to keep open a channel. 



There are few rivers entering the sea that are not obstructed by a bar across the 

 mouth, and the permanent removal of this is the object in view when improvements 

 are made. This bar is caused (i) by the deposit of suspended matter carried by the 

 current of the river into the quiet water, and (2) by the action of sea waves. When 

 more material is brought down by the river than can be carried away by the current or 

 tide, improvement by jetties can only be temporary, and recourse must be had to canals 

 or dredging. Where a littoral current exists it is necessary to lead the river out to it 

 by jetties, which contract the waterway and increase the velocity of the current. There 

 is a constant tendency of the sea to throw up bars along the shore, and were it not for the 

 ebb and flow of the tide and the action of the natural currents of rivers the mouths of 

 the latter would become impassable by reason of these barriers. Bars formed by the 

 sea are generally in tidal streams, while those formed by the river are most common in 

 rivers flowing into tideless seas. 



Principles Governing Tidal and Non-tidal Outlets. Vernon-Harcourt has laid down 

 the following principles for improving tidal rivers:* 



" (i) The tidal flow should be admitted as far up a river as possible, and all barriers 

 to its progress removed, so that the period of slack water may be reduced to a mini- 

 mum. By this means also the area of inevitable deposits is enlarged, and thus the 

 deposit does not unduly shoal the channel when the fresh-water discharge is small, and 

 the volume of tidal water flowing through the outlet is thereby increased. 



" (2) The fresh-water discharge should not be abstracted, if possible, for supplying 

 canals or for other purposes, but should be directed into the upper end of the main 



* Rivers and Canals, p. 235; and Improvement of the Maritime Portion of Rivers. 



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