A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER DESIGN. 135 ultimate dispersai of a rubble mound left entirely to itself is only a matter of time. The preservation of a mound breakwater necessitates, therefore, a constant repleuishment of material.1 The pitching of seaward slopes with ashlar work, or with massive concrète blocks, goes far to neutralise the destructive action; but the protection afforded is not always complète, and in cases where it has proved effectuai, the result has only been attained by a much greater outlay than could justifiably be assigued to the formation of a simple rubble mound. Efficiency.—The efficiency of a type is, after all, the consideration of greatest importance. Cheap construction and maintenance, though points to be carefully weighed, must inevitably be subservient to the attainment of the object in view. The wall, rising up sheer from a sea bottom below the zone of disturbance with its exposed face vertical, or practically so, receives the wave before any conversion of oscillation into translation can take place. The wave is de- flected upwards, and it falls back and down upon a bed of water too deep to permit of any deleterious influence upon the foundation. On the rubble mound, with face slopes of 1, 2, 3, 4, and 5 to 1, the stroke of the converted wave is delivered with powerful and inimical effect not only as regards the breakwater structure, but also the area which it incloses. The mass of water rushing up the seaward slope eventually falls over the crest, beating down upon the inner face and tending to effect a breach which must ultimately lead to serious results. Furthermore, even if the wave do not surmount the crest of the mound, the undulations of the sea are trans- mitted through the interstices of the stone mass and the harbourage area is kept more or less in a state of agitation. This action will be the more evident «s the stones or blocks are of greater size, involving vacuities of corresponding magnitude. Amid large-sized artificial blocks deposited irregu- larly, the voids will amount to at least 25 or 30 per cent, of the whole; and as these blocks are employed to crown the majority of mound breakwaters, the protective value of the type falls considerably below that of a wall.- In order that a composite breakwater may possess the efficiency of the wall, it is necessary that its superstructure should commence at a depth of at least 5 fathoms, otherwise the back-draught of the waves will exercise an undermining influence upon the rubble foundation. lhe peculiar drawback attaching to this class of breakwaters is that due to irregular settlement, whereby the superimposed wall is liable to be cracked and fissured. This point will receive further notice in the next chapter. From the foregoing remarks, it will be seen that no absolute preference 1 There is, as niight be expected, considérable variation in cost at different localities. The maintenance of the mound at Holyhead is stated to be Is. 3d. per linear foot per annum ; at Genoa it is 7s. ; at Naples 13s. ; while, during a certain period, the Alderney breakwater involved an expenditure of from 25s. to 45s. This, however, was quite an abnormal experience. The renewal of large artificial blocks on the seaward slope of Cette forms an annual charge of 29s. per foot run. H . 2 At Marseiiles it has been found that external waves 3 feet high give rise to fluctua- tions of from 4 to 6 inches witliin the area sheltered by the breakwatei.