TIDES. 



59 



The cost of the rubble-mound 

 type of breakwater is, however, 

 affected by range of tide more than 

 any other. This will be apparent 

 on reference to the accompanying 

 diagram. On comparing the sec- 

 tional areas of the several mounds 

 there indicated with that of mound 

 A which represents the section 

 that would be required in a tideless 

 sea it will be seen that even so 

 moderate a rise of tide as 7 feet, in 

 the case of a breakwater con- 

 structed in water having a depth 

 of 40 feet at low water, would 

 entail an addition of 66 per cent, to 

 the quantity of material employed, 

 A range of 15 feet would increase 

 the quantity 2J-fold, and nearly 

 four times as much material would 

 be required in constructing such a 

 breakwater where the range of tide 

 was 25 feet. 



In tideless seas, the area of 

 breakwater face exposed to the 

 wave-stroke is constant, and the 

 work may be designed accordingly ; 

 but in the case of a structure 

 situated where a range of tide 

 exists, the additional area of face 

 brought under the wave-stroke 

 necessitates, so to speak, a corre- 

 sponding extension of the armour. 



The cost of building work 

 within the tidal range is usually 

 much in excess of that incurred in 

 building above high-water mark, 

 seeing that the workmen employed 

 upon such work are unable to 

 work steadily at it, and addi- 

 tional sea-risk is incurred, which 



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