72 HARBOUR CONSTRUCTION. 



1858 ; and he went on to say that the tide had the same effect 

 at Wick and " in all places." l 



Sir James Dombrain Inspector of Coastguard Service in 

 Ireland referring to Portrush, says, " The tides are so very rapid 

 outside in the channel, that they keep down the sea whichever 

 way they may be running." 2 



At Peterhead, N.B., where very high seas are at times 

 experienced, the tidal currents run close past the heads of the 

 south bay, and at spring tides they attain a velocity of about 

 3 knots per hour. Their direction corresponds very nearly with 

 that of the troughs and crests of the waves during south-easterly 

 gales. On several occasions, during such gales, I have endeavoured 

 to find out what effect, if any, these currents have upon the 

 height of the waves entering the bay, and, with this object in 

 view, I have carefully observed the waves both at slack water 

 and during the strength of the tide ; in no instance, however, 

 have I been able to detect any change, either in the height or 

 character of the waves, which could be attributed to this cause. 

 I therefore conclude that the shelter afforded by currents is 

 a question of degree, which depends upon their velocity and 

 direction in relation to the height and direction of the waves, 

 the effect of a 3-knot current upon waves of from 15 to 20 feet 

 in height, and running parallel to them, being inappreciable. 



Tidal currents, judiciously directed by means of piers or 

 training banks, constitute the chief force by which the depth 

 at the entrances of most river or estuary harbours is maintained 

 or increased. 



In deciding the width of entrance for any harbour, it should 

 be borne in mind that a strong current out of a harbour is 

 objectionable, inasmuch as it is an obstacle to sailing vessels 

 entering ; and, when opposed by storm waves, a dangerous, short, 

 breaking sea is likely to be set up. 



1 "Report of Royal Commission on Harbours of Refuge, 1859," vol. ii. p. 4. 



8 "Minutes of Evidence, Select Committee on Harbours of Refuge, 1858," p. 125. 



