VARIOUS TYPES OF BREAKWATERS. 173 



3. Concrete blocks, laid in horizontal courses, either through- 

 out the structure or in conjunction with a hearting and base of 

 mass-work. 



4. Mass-work of concrete deposited within temporary frames. 



5. The same, with a base formed of concrete deposited in 

 bags. 



G. Concrete blocks laid in sloping courses. 

 7. Any of the foregoing, with the addition of a wave-breaker 

 of heavy blocks on the weather side. 

 Of the mound type we find 



1. Mounds formed of rubble stone of all sizes, thrown into 

 the sea and allowed to take such slopes as the waves may give 

 them. Such mounds may either be left with loose rubble faces 

 as deposited, or the slopes may be faced with stone pitching, as 

 has been done at Plymouth and Kingstown. 



2. Mounds of concrete blocks thrown together pell-mell, 

 sometimes associated with rubble, and brought up to the level 

 of high water or a few feet above it. Of this type the break- 

 waters at Alexandria and Algiers are examples. 



The combinations of the above-named types are almost 

 infinite, indicating that in the construction of breakwaters, as 

 in that of other engineering works, no one material or method of 

 using it can be considered the best under all circumstances. 



The vertical type of breakwater requires less material per 

 unit of length other things being equal than any other form. 

 It is, therefore, a proper section to adopt where suitable materials 

 for breakwater construction are scarce or costly, provided that 

 other conditions, such as excessive depth of water or a bad 

 foundation, do not overrule its adoption. 



Even where a foundation is questionable, if the depth of 

 water is not too great, the structure may be carried upon pile- 

 work (p. 272), or additional stability may be given by deposit- 

 ing a bed of rubble, as was done at Ymuiden and the Tyne. 

 In either case, wide-spreading aprons should be provided to 

 prevent scour and undermining. 



With regard to the action of waves against a vertical face, 

 waves of oscillation, and also those of translation up to certain 

 limits, rise against such a face to approximately double the 

 height of their crests above the normal water-level, and they are 

 reflected from it, or from a face having a backward inclination, 

 or batter, not exceeding 45 degrees. 



