74 HARBOUR CONSTRUCTION. 



being formed. Storms at Kurrachee are, however, unknown; 

 and the largest waves, which are brought up by the south-west 

 monsoon, do not exceed 15 feet in height, trough to crest. 1 



Similarly, at Colombo, notwithstanding that the site of the 

 harbour is fully exposed, Mr. Kyle states that the greatest height 

 of waves observed by him during the construction of the break- 

 water there was 12 feet in the offing, and 15 feet just outside 

 the breakwater. 2 



It is thus evident that no general rule can be laid down for 

 calculating the height of waves from the length of fetch and 

 depth of water alone. 



Points to be considered in judging of the height of waves 

 likely to assail a sea-work have already been treated of in 

 Chapter II. 



In observing the direction taken by heavy waves a very 

 important point in laying out sea-works it should be borne 

 in mind that the shape of the contours of the sea-bed often 

 varies very much at different levels, and that waves have a 

 tendency to accommodate themselves to, or be guided by, the 

 direction of such contours. 



Thus, unless it is known that the formation of the sea-bed 

 is such as not to influence the direction of the general run of 

 the waves, it cannot, with safety, be assumed that waves of, say, 

 20 feet in height will advance in the same direction as waves 

 whose height is only 10 feet, notwithstanding that the direction 

 of the gale generating them may be precisely the same in 

 each case. 



Submerged gorges or converging sunken rocks, even at 

 considerable depths, cause waves to heap up and assail, with 

 increased force, any portion of a work up to which they may 

 lead. Such a form of bottom is not uncommon, and it may 

 always be regarded as an adverse feature. It is, therefore, of 

 importance that a careful study be made of the contours of the 

 sea-bed immediately in front of, and to some distance seaward, 

 of a proposed work, more especially if such be situated on a 

 rocky coast. 



It often happens that a change of wind into a quarter from 

 which the exposure is small will increase the sea in a bay or 

 harbour, by causing the waves to wheel in the manner before 

 referred to. 



1 Min. Proc. Imt. C.E., vol. xliii. 2 Ibid., vol. Ixxxvii. p. 90. 



