HARBOUR 



508 



largest waves are not always so destructive as 



-m.iller ones. Scott Hussell has stated the law 

 that waves break whenever they come to water as 

 dei-p as their own height; so that 10-feet waves 

 should break in in feet water, and 20- feet waves in 

 i \\ater. There seem, however, to bo some 



s which break on reaching water whose depth 

 is ec|ual to twice their own height. Proofs of the 

 depth to which the surface undulations extend 

 base Ix'en given by Sir George Airy, Sir John 

 ('node. Captain Calver, and Mr John Murray, C.E. 

 IJankine has shown that the crest and trough of the 

 ea are not, as was generally believed, equidistant 

 from the level of still water. When / is the 

 length of the wave, H its height from trough to 

 crest, 



H " a 



Crest above still water = -75- H 



H 



Trough below still water = -& - '7854 



It has been held by some engineers that in deep 

 water waves are purely oscillatory, having no 

 power of translation, and therefore incapable of 

 exerting any force against a vertical face of 

 masonry. Tins, however, is incorrect. Were there 

 no wind propelling the 

 waves, no current to in- 

 terfere with their char- 

 acter, and no interference 

 witli one another, such as 

 the reflected wave from a 

 vertical face meeting the 

 next opposing wave, such 

 a theory might be true. 

 True, however, it is not; 

 and all sea- works, in what- 

 ever depth of water they 

 may be placed, will as- 

 suredly have to withstand 

 impulsive action. Be- 

 sides, it must be kept in 

 view that in order to re- 

 duce the expense of con- 

 struction it is essential, 

 where the bottom is soft, 

 to make the foundation 

 a pile of loose rubble or 

 concrete blocks. It fol- 

 lows from what has al- 

 ready been said that the 

 rubble, by shoaling the 

 water in front of the work, 

 will cause the waves to 

 li -come waves of transla- 

 tion before they reach the 

 vertical superstructure, 

 which, assuming the 

 waves to have been simply 

 "iM-illatory, would have 

 reflected them without 



In tidal harlxxirs, or those in shoal water, it in 

 admitted by all that the waves break, and therefore 

 exert an impulsive force. Such works have to 

 withstand ( I ) the direct horizontal force which 

 tends to remove the masonry; (2) the vertical 

 force acting upwards on projecting stones or 

 protuberances, and against the lying beds of the 

 stones; (3) the vertical force acting downwards 

 upon the talus wall, or passing over the parapet 

 and falling iij>on the roadway; (4) the baek- 

 draught, which is apt to remove the soft Lot torn in 

 front of the work; and (">) the blowing action of 

 waves on the air or water which fills the inter- 

 stices of open-work piers. 



In designing the ground -plan of harbours, some 

 rules should i>e kept in view : ( 1 ) the entrance 

 should be always kept seawards of the works of 

 masonry, care being taken that the direction of 

 the piers does 'not throw the sea across the en- 

 trance ; (2) there should be a good ' loose,' or point 

 of departure free of rocks or a lee-shore; (3) 

 spending beaches inside should l>e provided to 

 allow tne waves that pass in to break and spend 

 themselves. A harbour basin surrounded with 

 vertical quay walls becomes a ' boiling pot ; ' this 

 is a point frequently overlooked by engineers; (4) 



Fig. 4. Calais Harbour. 



breaking, and therefore without their having exerted 

 an impulsive force further than statical pressure 

 upon tne masonry. 



There is no fixed rule as to the best profile of 

 any sea-work, which must necessarily depend upon 

 a variety of local peculiarities, such as the nature 

 of the bottom, and the size and quality of the 

 materials obtainable. While a long, sloping break- 

 water does not offer the same amount of resistance 

 to the waves, neither is it in itself so strong, for 

 the weight resting on the face-stones is decreased 

 in proportion to the sine of the angle of the slope. 

 On the other hand, the tendency of the waves 

 to produce horizontal displacement, supposing the 

 direction of the impinging particles to be horizontal, 

 is proportional to the cube of the sine of the angle of 

 elevation of the wall. 



the relation of the width of entrance to the area of 

 a harbour should be a matter of careful study, as 

 upon this depends the tranquillity of the interior, 

 or what has oeen called the reductive power of the 

 harbour. Stevenson's formula for the reductive 

 power is given below : H = height of wave at 

 entrance ; 5 = breadth of entrance ; B = breadth of 

 harbour at place of observation ; D = distance from 

 mouth of harbour to place of observation ; x = 

 reduced height of wave at place of observation. 



HV6 



Fig. 4 represents the harbour of Calais, which 

 was constructed by the French government, and 

 opened on 3d June 1889. Great difficulty was 



