CHAPTER XVII. 



SILTING. 



Process described Wave and current action Effect of piers projected from the 

 shore Advance of foreshore Prolongation of jetties Dredging Sluicing 

 Harbours on sandy coasts Outlying isolated breakwaters Direction of drift. 



SILTING is the term commonly used to denote the deposit of 

 matter which is held in suspension by sea-water in proximity 

 to the shore, or the accumulation of that which is made to travel 

 along the sea-bed by the action of waves and currents. 



The process of silting may be described as follows : the 

 waves, together with the tidal or other currents, erode the sea- 

 bed, and also the shores and cliffs along the coast. The material 

 resulting from this erosion is separated and sorted by wave and 

 current action, the larger and heavier particles settling down 

 and being rolled along the bottom, while the smaller and lighter 

 particles are held in suspension by the water. 



These latter depend entirely upon the motion of the water 

 for their support; so when silt-laden water enters a close 

 harbour, or other becalmed area, and the disturbing influences 

 cease or are much reduced, the particles forthwith settle to the 

 bottom by reason of their gravity. 



In the more sheltered parts of a harbour, the agitation of the 

 water is not generally sufficient to stir up this deposit when 

 once it has settled down, so, when the water ebbs away, the 

 material which came in with the flood tide is left behind. 



This process is repeated each tide, and unless means be 

 adopted for removing the material so deposited, the harbour 

 gradually becomes shallower, and is ultimately choked and 

 rendered useless. 



Whilst the waves are keeping the water in a state of agita- 

 tion, and thus assisting the currents in transporting the fine 

 material with which they are charged, they still further help to 



