70 HARBOUR CONSTRUCTION. 



One very noticeable instance of such a " race " with which I 

 am familiar, is that at Kyle Rhea, between the Isle of Skye and 

 the mainland. This channel (Sleat Sound) is trumpet-shaped, 

 having a width at its southern entrance of about 5 miles, which, 

 in a distance of 20 miles, contracts to less than half a mile at the 

 Kyle Rhea ferry. Through this narrow channel the tidal 

 waters rush with great velocity, in consequence of the Isle of 

 Skye blocking up the channel between the mainland and the 

 Western Hebrides to the extent of about 70 per cent. 



The Sound of Jura contracts in a somewhat similar manner 

 causing a rapid race through its narrow northern channels. 



In some rivers, or tidal estuaries, the configuration of the 

 coast, coupled with a shoaling bottom, occasions a great range 

 of tide, and the rush of water into the tidal compartment 

 is often so great as to cause what is known as a " bore " 

 (see p. 61). 



Another cause of acceleration of the tidal currents may be 

 here referred to, namely, that resulting from capes or headlands 

 ponding back the water. 



At such points the main body of water to seaward is, by its 

 inertia, slow to give way or accommodate itself to these irregu- 

 larities of coast-line, the result being that the passage or transfer 

 of the inshore water from one side of the promontory to the other 

 is obstructed. A " head," or difference in level, is thus produced 

 on the up-stream side, resulting in a current which often attains 

 a considerable velocity past the salient points. 



In designing harbours this fact should not be lost sight of, 

 since, in projecting long breakwaters into the sea to form a 

 harbour, the tidal current running past the entrance may be so 

 increased in velocity as to render it difficult for ships to take the 

 harbour at certain times of the tide. An example of this may 

 be found at the harbour of Ymuiden, on the west coast of 

 Holland. The piers forming this harbour were projected into 

 the North Sea for a distance of nearly a mile. Before they 

 were built, the velocity of the flood current at spring tides 

 along that part of the coast was from 2 to 2 knots per 

 hour, according to the direction and strength of the wind. 

 As the piers advanced, the velocity of the current past their 

 seaward ends steadily increased; and by the time they were 

 completed the current past the entrance, during flood tide of 

 springs, attained the inconvenient velocity of from 3 to 4 knots 



