2o8 HARBOUR CONSTRUCTION. 



This was exemplified by the apparent ease with which face stones 

 were withdrawn. 



A batter of 9 inches in a foot was at first adopted; this was 

 afterwards reduced to 6 inches, and later still to 4 inches ; but even 

 that was too great. 



(e) The evil effect of parapets in increasing the recoil of 

 waves and the height to which water is thrown; also in 

 unequally loading the foundation, whereby irregular settlement 

 and longitudinal cracks are likely to be brought about. 



The high parapet at Alderney caused the whole force of the 

 waves to be thrown against the superstructure, and immense volumes 

 of water were projected, during storms, to a height of fully 200 feet, 

 by the falling of which the roadway of the breakwater was frequently 

 damaged, and the rubble mound scooped out. 



Sir John Hawkshaw and Sir Andrew Clarke recommended, as 

 a remedial measure, the entire removal of the parapet or promenade 

 wall, as it was called and the formation of a wave-breaker, com- 

 posed of heavy concrete blocks deposited along the seaward face of 

 the breakwater, and extending to slightly above high-water nark. 

 As an alternative, it was suggested that the rubble mound might be 

 raised up to or above the level of high water, as at Holyhead ; but 

 the amount of rubble necessary for this purpose, in view of the 

 exposure of the site and the great depth of water, would have been 

 very large. 



B. Mound Breakwaters of Concrete Bloclcs, thrown 



together pell-mell. 



Breakwaters constructed entirely of pell-mell block-work are 

 few in number, the reason, no doubt, being that a saving in cost 

 may be effected by a judicious use of rubble in combination 

 with the blocks. 



Such combinations are almost endless, but the following may 

 be cited as examples : 



(a) A rubble base, brought up to within, say, 15 or 20 feet of 

 low water, with a pell-mell mound of blocks placed 

 upon it. This system was adopted for the main portion 

 of the breakwater at Algiers, for the outer portion of 

 the breakwater at Port Said, and for the breakwater at 

 Port Erin, 1 Isle of Man. A breakwater of this type is 

 shown in Fig. 47. 



1 In the Port Erin breakwater, the surface of the rubble mound was only from 

 2 to 5 feet below low water of spring tides. 



