30 HARBOUR CONSTRUCTION. 



composite breakwater, in relation to the levels of high and low 

 water and to the magnitude of the waves, is of great importance. 



The breakwater at Alderney was l of the composite type, and 

 the level to which the rubble mound was carried varied in 

 different sections of the work. In the most exposed places, it 

 was only carried up to, or perhaps it would be more correct to 

 say maintained at, the level of 12 feet below low water of 

 spring tides, and it sloped away to seaward at the rate of about 

 1 in 5J. 



The range of spring tides at Alderney is 17 feet, and the 

 seas are exceptionally heavy. 



There can, I think, be no doubt that the violence with which 

 they assailed the work was greatly increased by the slope of the 

 rubble mound having been formed at the level it was. 



In general, waves break on entering water the depth of 

 which but little exceeds their height, trough to crest ; therefore, 

 at some period during each tide storm waves at Alderney would 

 be made to break just as they reached the superstructure, and 

 consequently just at the time when their greatest destructive 

 pow"er had been attained. 



From observations which are recorded oh p. 39, it will be 

 seen that waves in Peterhead Bay, 2G feet in height, broke in 

 passing the 5J-fathom line. 



There were several features in the design of the Alderney 

 breakwater which, since the days of its design, experience has 

 shown to be objectionable, and which doubtless contributed to 

 its destruction ; but the one just referred to intensifying, as 

 it undoubtedly did, the destructive power of the waves was 

 probably the means of developing other defects, which under 

 different circumstances might have remained latent. 



The failure of this large work has taught engineers many 

 instructive lessons, and the profession is indebted to Mr. L. F. 

 Vernon-Harcourt, M.I.C.E., for having given, both in a paper 

 which he contributed to the Institution of Civil Engineers, 2 and 

 also in his work on " Harbours and Docks," 8 a very full and 

 interesting description of the construction and maintenance of 

 this work, up to the time of its abandonment. 



1 The superstructure along the outer half of the breakwater has been entirely 

 washed away. 



2 Min. Proc. Tnst. C.E., vol. xxxvii. 



8 " Harbours and Docks," Harcourt, Clarendon Press Series. 



