SILTING. 285 



fill up harbours by rolling sand and shingle along the sea-bed, 

 and driving them into the harbour entrances. They also heap 

 up drift materials against obstacles, such as piers, etc., which 

 run across the shore along which the drift travels; but when 

 such piers extend into deep water, a considerable time often 

 elapses before the drift material as distinguished from that 

 held in suspension finds its way into the harbour, because the 

 corners which the piers form with the coast, constitute traps 

 for it. As, however, these fill up, an advance of foreshore takes 

 place, and the material finds its way out to and round the pier- 

 heads. Should these occupy a salient position, and extend well 

 into the tide-way, the current past them will scour much of this 

 away, and enable the depth of water at the entrance to be more 

 easily maintained than under other conditions. Nevertheless, 

 a current in passing an entrance often expands into the harbour 

 and creates eddies, by means of which some of the drift material 

 is almost sure to be carried in, and deposited under the lee of 

 the piers. Total immunity from dredging can therefore scarcely 

 be expected. 



In order to combat the advance of foreshores, jetties are 

 often extended seaward; but such prolongations, unaided by 

 dredging, are generally found to be of little permanent benefit, 

 inasmuch as each extension of the pier causes a corresponding 

 advance of the shore. The rate of such advance depends upon 

 the physical conditions peculiar to each site, and especially upon 

 the quantity of sand in motion. This varies greatly in different 

 localities hence, while the extensions of the jetties at such 

 ports as Calais and Dunkirk have failed to produce the desired 

 results, the prolongation of those at Port Said has been more 

 successful ; so much so, indeed, that it has been estimated it will 

 take three centuries for the foreshore at Port Said to advance to 

 the end of the piers, assuming its present rate of accumulation 

 to be maintained. 1 



In order to keep open the Ymuiden harbour, it has been 

 found necessary to dredge, on an average, about 650,000 cubic 

 yards of sand per annum, at an average cost of about 22,000. 



Such an expenditure as this can only be met by a very large 

 traffic, and there can be little doubt that but for this Ymuiden 

 harbour would ere now have ranked amongst the failures of 

 harbours on sandy coasts. In view, however, of the very large 



1 Min. Proc. Inst. C.E., vol. Iv. 



