SEA DEFENCES. When the tidal actiou, or littoral currents attack a sea-shore, it becomes necessary to protect the latter from the gradual abrasion thus produced. The mode in which the defences thus required are constructed, must depend not only on the peculiar form of destructive action to be guarded against, but also on the nature of the materials at hand ; the various conditions to bo observed may, therefore, be the most conveniently noticed in the course of the description of some of the most important works of the kind actually executed.
On rocky shores of the granitio and crystalline formations, the sea acts principally by its direct impact, or by the tidal currents, although, no doubt, the effect of the constant humidity, and of the splashing of the waves, must produce a destructive action on the constituent parts of the rocks. In these cases, and especially when deep water comes close in-shore, the most economical, and in all cases the most efficicut sea defence consists of a solid masonry wall, built of the largest and hardest stones which tan be obtained, and finished with a curvilinear batter of about 1 in 40 from the sea. Colonel limy, and tho French, Spanish, and German engineers sometimes make the upper parts of their curvilinear defence walls continuo beyond the axis of the curve, so that at the top the curve again comes towards the sea; and they adopt this construction because they believe that, in consequence of the projection of the top, the crown of the wave striking and running up the face of the wall would be, thrown ontwards, instead of breaking over the wall. It is to be suspected, however, that the repercussion produced by the wave falling upon the toe of tho wall is em-en more injurioiss than the effect of the water falling at the back or inside; and the best hydraulic engineers at the present day are in the habit of ereethig their sea-walls with continuous batters in the same direction, and of protecting their in-shore faces by good paving on the top of the backing, and by good drainage. It, would be impossible to attach too much importance to the necessity for providing against the effect of any sudden Interference with the translation of a deep sea wave, especially when it has been driven for a short distance over an inclined fore shore, and has been exposed to the action of winds or currents able to increase the velocity of its motion. In some cases, the velocity has even been observed to attain as much as 70 feet per second; and from observations made at Cherbourg and Algiers, it would seem that the power of waves to communicate horizontal motion sometimes attains as much as 291 lbs. per foot superficial. The spray has at
times been dashed over the Bell Rock,* height of 117-feet ; and Lord Adair states, that he has measured on the coast of Ireland, rolling breakers of 150 feet in height. The quantity of water a wave of this description might carry over the crown of a wall would be enormous ; and though there may be few positions wherein they occur, even smaller wavce are capable of producing effects of a very serious nature, If the water they furnish were not prevented from washing away the backing of the structure on which they might break. On the shores actually exposed to the shock of such waves as those described by Lord Adair, it would be almost impossible for human skill to contend with the forces of nature; hut fortunately, these waves are only to be met with on what are known as iron-bound coasts, where the character of the rocks is such as to enable them to resist the action of the sea.
It may be gathered from tho preceding remarks, that if it be thus necessary to guard against the tendency of breaking waves to produce lateral displacement, and to remove the backing of a defence wall, it must bo equally important to guard against the tendency to under mina the footings on the seaward face produced by the receding wave, or the undertow as it is technically called. This object is usually effected in the more moveable strata, by constructing an apron, or in fact, a species of flat wall, presenting the same inclination as the natural lino of the foro shore ; and built with every precaution to ensure the stability of its face, by the introduction of longitudinal and of transverse ties, and by protecting the edges by continuous piling. At the Plymouth breakwater the sea slope was paved with largo blocks of marble, or of granite, dovetailed and cramped together at nu enormous cost ; at Cherbourg, the sea slope was protected by huge blocks of rubble masonry set in Portland cement ; and on the Dutch coast, where the angle of inclination of the shore is extremely flat, the apron walls of sea defences are often made of brickwork set with tress mortar in wooden frames. The choice of the system to be adopted in any particular case must depend on the maximum violence of the waves there observed ; but it may be worth while here to mention that in exposed situations on the shores of the ocean, blocks of nearly 500 cubic feet, of a material possessing a specific gravity of are susceptible of being displaced by the force of the waves.