VARIOUS TYPES OF BREAKWATERS. 201 



can be made to last for ever without maintenance ; and such 

 splendid examples of rubble mound breakwaters as at present 

 exist show conclusively that works of this class may be main- 

 tained at a moderate cost, and that in this respect they do not 

 compare unfavourably with many breakwaters of the vertical 

 type (see pp. 216, et seq.). 



At depths varying from about 12 feet to 15 feet below low 

 water of spring tides, a mound composed of rubble of compara- 

 tively small size say stones of from 10 cwt. upwards will 

 remain undisturbed in heavy seas, if no superstructure be placed 

 upon it. 



During the construction of the Alderney breakwater, the 

 rubble which was deposited to form the mound was allowed 

 three years to consolidate before putting the superstructure upon 

 it, and it was found that the heavy winter storms did not 

 disturb it below the level of 15 feet below low water of spring 

 tides. 



The placing of a vertical structure upon a rubble mound, 

 however, at once alters the pre-existing conditions, and, where 

 no disturbance previously occurred, an excavating action will be 

 found to come into play. This is caused by the downward 

 action of the waves playing against the face of the wall, and 

 by the force of their recoil, which was so great at Alderney 

 that it ploughed out the mound to a depth of 20 feet below low 

 water for a distance of from 80 to 90 feet from the face of 

 the wall. 



In the mound of the Colombo breakwater, stones weighing 

 three tons and upwards were disturbed at depths varying from 

 10 to 14 feet below low water, and that by waves which never 

 exceeded 15 feet in height. 



At Port Elizabeth, some rubble was deposited at the head of 

 the north jetty in order to protect it from scour. The depth at 

 low water was 22 feet, 1 and the layer of stone was 3 feet thick. 

 Its surface was thus 19 feet below low water. This rubble was 

 all thrown in by hand, and no stone weighed more than from 1 

 cwt. to 1J cwt. Although waves of 15 feet to 20 feet in height 

 passed over it, it was never disturbed in the smallest degree. 

 There was, however, no superstructure upon it other than one of 

 comparatively open pile-work, which only broke, but did not 

 reflect, the waves. 



1 The range of ordinary spring tides was 7 feet. 



