154 HARBOUR CONSTRUCTION. 



hose was never allowed to be more than 2 feet below the surface of 

 the river. If that precaution was neglected, and the hose was inserted 

 3 feet below the surface, salt water was immediately pumped up." 1 



The late Mr. David Stevenson, referring to observations on 

 the River Dee at Aberdeen, says 



"It was then clearly ascertained that on certain occasions there 

 was a perceptible outward current at the surface during the whole 

 period of the flood tide, and that simultaneously there was a current 

 of sea water running inwards. The salt water flowed in underneath, 

 raising up the fresh water, the fresh water in reality running out 

 over the top of the salt water." 2 



This tends to show that the tidal water should be chiefly 

 relied upon for the maintenance of channels at river-mouths, 

 and that every effort should be made to increase the volume and 

 efficiency of the tidal flow. 



The value of upland water must not, however, be underrated, 

 because during the ebb tide it combines with the tidal water 

 and increases the scouring effect. 



Before deciding upon the site of an entrance or navigable 

 channel, the nature of the bottom ought to be ascertained by a 

 series of borings or probings. These should be carried to the 

 full depth of channel contemplated; 3 and in the event of rock 

 being found at such a level as to interfere with the forming or 

 deepening of the channel on the site first selected, other borings 

 should be taken with the view of finding a more desirable one. 



Such an investigation, although it may prove somewhat 

 costly, can scarcely fail to be of great value in revealing the 

 facts of the case, and enabling recommendations to be made with 

 greater confidence. 



Rock may be removed by dredging, after it has been broken 

 up either by blasting or by some of the other methods now 

 adopted. They are, however, all costly ; and should such work 

 have to be performed in a breaking sea upon a bar, its difficulty 

 and cost would be greatly increased. 



1 Min. Proc. Imt. CM, vol. xxi. 2 Ibid., vol. xxi. 



3 In considering the depth requisite for vessels of any given draught, due 

 allowance should be made for waves, and also for send, or the plunging of vessels 

 in a rough sea, which may amount to several feet below their normal line of 

 flotation. For vessels of moderate size, two-thirds of the height of the waves, 

 trough to crest, is sometimes taken as the greatest send, one-half the height of the 

 waves being allowed for vessels of larger size. 



