202 HARBOUR CONSTRUCTION. 



After the failure of the Madras breakwater, it was found, on 

 examination, that portions of the rubble forming the base had 

 been drawn out from beneath the blocks forming the vertical 

 superstructure at a depth of 23J feet, and that rubble stones 

 weighing from 5 to 50 Ibs. had been displaced at depths of at 

 least 40 feet. 1 



The surface of a rubble mound surmounted by a super- 

 structure should either be carried up well above high water, and 

 be maintained at that level as in the case of the Holyhead 

 breakwater so as to make the waves break and expend them- 

 selves upon it, or it should be kept at such a depth below low 

 water as not to seriously affect the character of the waves, or be 

 exposed to their disturbing action. 



There can be little doubt that failure to recognize this, in 

 the case of the Alderney breakwater, largely contributed to the 

 destruction of that work. 



If it should be intended to make a mound breakwater, with 

 superstructure, answer the purpose of a quay, the rubble on the 

 harbour side must either be kept well down, or jetties must be 

 projected so as to give sufficient depth of water alongside of 

 them to accommodate vessels of the class frequenting the port. 



In determining the direction to be taken by a rubble mound 

 breakwater, special attention should be paid to that of the waves 

 during the heaviest gales, and the work should, if possible, be 

 laid out so as to make the waves break square upon it. When 

 waves break obliquely upon a rubble mound they cause the 

 stone to travel, thereby necessitating an undue amount of 

 maintenance. 



Sometimes it has become necessary to construct groynes to 

 check this travelling of the rubble ; and in other cases, as has 

 already been stated, the more expensive expedient of facing the 

 mound with pitching above the low-water mark has been resorted 

 to. At the outer end of the Holyhead breakwater, old heavy 

 chains were deposited in coils, and shackled together so as to 

 form mats over the surface of the rubble, and they were found 

 to be very effective in checking its movement. 



When mound breakwaters are projected from the shore, they 

 are most expeditiously and economically constructed from a 

 staging, such as was adopted at Holyhead and Portland. 



1 Report by Sir G. Molesworth, consulting engineer to the Government of 

 India. 



