200 HARBOUR CONSTRUCTION. 



These slopes were pitched with the view of putting a stop 

 to the disturbance of the rubble; but the smooth face of the 

 pitching on the seaward side so increased the effect of the 

 drawback of the waves that the rubble at the toe of the pitching, 

 or just about the low- water level, was disturbed by it more than 

 ever. This caused a great deal of trouble, and necessitated the 

 formation of a masonry toe composed of large carefully worked 

 blocks of granite, dovetailed into each other so as to increase 

 their stability. 1 



The return water down the slope tripped up the oncoming 

 waves, and caused them to break more heavily upon the slope 

 than they would otherwise have done. A section of this break- 

 water will be found in Fig. 40. 



The seaward slopes of Table Bay breakwater are approxi- 

 mately 1 in 2J from the top, which is 15 feet above high water 

 of spring tides, to the level of high water. From high water to 

 low water, 1 in 5; from low water to 15 feet below that level, 

 1 in 7 ; and thence to the bottom, 1 in 1 J. The slopes upon the 

 harbour side are 1 in 2 from the top to 5 feet below low water ; 

 and thence to the bottom they are 1 in 1J. The width at the 

 level of high water is 110 feet. 



It is worthy of note that the slopes of this breakwater, as 

 formed by the severe gale which occurred in the year 1865, 

 remained practically undisturbed for 24 years, or until the gale 

 which occurred on July 30, 1889. This gale, however, further 

 pulled down the rubble and flattened the slopes to the inclina- 

 tions above stated. 



In order to obviate the inconvenience of stone being thrown 

 over the crest of this breakwater into the harbour during storms, 

 a wall or " backbone " of concrete has recently been constructed 

 along it. 



An objection often urged against rubble mound breakwaters 

 is, that the stones of which they are composed are subject to the 

 same wear from wave-action as those which form boulder and 

 shingle beaches. No doubt the surface stones are rolled about 

 and rounded by the ceaseless action of the waves, and by their 

 grinding one against the other, just as in the case of natural 

 beaches ; but the annual waste from this cause, even in such a 

 large breakwater as that at Holyhead, is very small. 



It is unreasonable to expect that works, especially sea-works, 



1 The pitching has now been extended over the benching so as to form an apron. 



