41* 



' 



SEA DEFENCES. 



Within UMM few years, bowerw, noon ha* been Men to modify 

 thew eonaluaion* ; and in the course of the part year (1860) the 

 rtMnrvbm of 1 >r. G. r. Wallicb, on the bd of the North Atlantic, have 

 chown that animal life, in various form* ol foraminifen and rsdiaU, if 

 not of higher group*, exist* t va*t depth*. It ought, iudeed, to have 

 been *ran long *go, tht the uniform distribution of pressure effected 

 l.j the water, would neoeoarUj preclude the destruction which it was 

 raaaonaMn to infer that the partial accumulation of pressure would 

 ; 



The proportion which exist* between the wn nd land has contri- 

 buted to maintain the productive pown of the earth. If that pro- 

 portion were materially changed, it* productive power* wouM lie 

 changed alao. The *ca, by mean* of the vapour* continually rising 

 from it* surface, supphos the atmo*]ihere with sufficient moisture for 

 the su|-port of organic life. Countries which do not partake of the 

 benefit* derived from this source, and which arc not refreshed by rain 

 or dew, are uninhabitable and destitute of all kinds of vegetation. 

 Those part* of the earth which are farthest from the sea are much lees 

 fertile and populous than those which, owing to their greater vicinity 

 to it. receive a larger supply of moisture from this great source. The 

 ea contribute* also considerably to the advancement of civilisation. 

 At the first view it seem* to constitute an insuperable obstacle to the 

 communication between nations who inhabit countries widely apart 

 from one another ; but the ingenuity of men ha* converted the ocean 

 into the most frequented high road on the globe. The easy commu- 

 nication which is thus established between nations at great distances 

 from one another, has perhaps more than any other circumstance con- 

 tributed to improve the condition of tho human race. It is at least 

 certain, that all those nations which have acquired any considerable 

 degree of civilisation inhabit countries cither contiguous to the sea or 

 at no great distance from it. 



On the subjects of this article, four works may be particularised, 

 which, of course among many others, may be consulted with advan- 

 tage: Sir John F. W. Herschel's 'Physical Geography;' Admiral 

 Smyth's ' Mediterranean ;' Sir John Richardson's ' Polar Regions ;' and 

 The Physical Geography of the Sea, and its Meteorology,' by Captain 

 Maury, the American hydrographer. The lost is an immense repository 

 nf facts, relating chiefly, however, to tho Atlantic and its marine depen- 

 dencies ; and many views entertained by the author are not wholly 

 accepted by British hydrographers and meteorologists. 



i liKKKXCES. When the tidnl action, 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 f the materials at hand ; the various conditions to be 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 shore* of the granitic 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 

 tho 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 efficient 

 sea defence consists of a solid masonry wall, built of the largest and 

 hardest stones which can be obtained, and finished with a curvilinear 

 batter of about 1 in 40 from the sea. Colonel Emy, and the French, 

 Sianish, and German engineers sometimes make the upper parts of 

 their curvilinear defence walls continue 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 bo. thrown outwards, instead of breaking 

 over the wall. It is to be suspected, however, that the repercussion 

 produced by the wave falling upon the toe of the wall is even more 

 injurious than the effect of the water falling at the back or in.-iil- ; 

 and tho best hydraulic engineers at the present day are in the habit of 

 ererting their sea-walls with continuous hatters in the same direction, 

 and of protecting their in-shore faces by good paving on tho 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 boa 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 observation* made at Cherbourg and Algiers, it would seem that 

 the power of wave* to communicate horizontal motion sometimes 



attains a* much as 291 Ib*. per foot superficial. The spray has at 

 time* been dashed over the Bell Rook, a height of 117 feet; ami I.rd 

 Adah- states, that he ha* measured on the coast of Ireland, rolling 

 breakers of ISO 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 waves 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 1 i .1 

 Adair, it would be almost impossible for human skill to contend with 

 the forces of nature ; but 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 a* to enable them to resist tho action of the sea. 



It may be gathered from the preceding remarks, that if it be thus 

 necessary to guard against the tendency of breaking wave* to produce 

 Literal displacement, and to remove tho backing of a defence wall, it 

 must be equally important to guard agaiust the tendency to under- 

 mine the footings on the seaward face produced by the receding w.i\ >, 

 or the undertow a* it is technically called. This object is usually 

 effected in the more moveable strata, by constructing an apron, or in 

 fact, a specie* of flat wall, presenting the same inclination as the 

 natural line of the fore 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 large 

 blocks of marble, or of granite, dovetailed and cramped together at an 

 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 trass 

 mortar in wooden frames. The choice of the system to be adopted in 

 any particular cose 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 2'2, are 

 susceptible of being displaced by the force of the waves. 



These remarks with respect to defence-walls on the shores com, 

 of resisting rocks apply equally, or rather d fortiori, to the defences 

 required for formations of a softer nature. The action of the sea npju 

 them is, however, of a rather more complicated nature than that which 

 takes place on granitic, or crystalline shores; for it will in man;. 

 be found that at the feet of limestone, or clay cliffs, there are large 

 deposits of shingle composed of the detritus of harder rocks, carried 

 forward by the force of the currents, and of the tides, according to 

 peculiar laws. If this shingle should only advance slowly it may tend 

 to protect the surface of the shore, by forming as it were a matloss on 

 which the waves might break without affecting the subjacent rocks ; 

 but if, on the other hand, the shingle should have any tendency to 

 lateral displacement, it will materially assist the abrading act 

 the littoral currents by the friction it exercises at the base of the cliffs. 

 The outlines of a limestone, or of a clay, shore often bear traces of this 

 action; and they exhibit cliffs which are nearly vertical in their 

 upper portions, but hollowed out about the zone of tidal action ; 

 at their feet there is usually a shingly beach, in which the stones 

 have been abandoned by the receding waves according to the specific 

 gravities of the stones themselves, or, in other words, the largest 

 pebbles are always at the top of the beach. The defence of shores 

 of this description may either be effected by walls with aprons, as 

 before-mentioned, or by the construction of stonework, or of timber, 

 groynes, placed at right angles to the advance of the shingle, in order 

 to fix the latter. Groynes are in fact dwarf-walls, projecting more or 

 less above tho face of the beach, and continued to about the lin of 

 low water; and they act by retaining the advancing shingle in tho 

 re-entrant angle, until it has accumulated to such an extent as 

 carried round the point, or over the top of the groyne. In great 

 storms the groynes are often laid bare, and there is danger of their 

 being then undermined; and, indeed, the actions to which tl; 

 exposed are so violent, that it may fairly bo laid down as a rule that, 

 it is preferable to form them of cheap, temporary materials, than to 

 form them in a permanent manner. 



The consolidation of shores of soft alluvial formations may !>e 

 effected in various manners, according to their outlines under tho 

 action of tho currents. If the natural inclination should happen to 

 form a very small angle with the horizon from tho line of the low- 

 water, it would generally bo desirable to crown the top of ti, 

 by a vertical wall, of either maxonrv or of timber. In Holland tin 

 arc executed of earthwork, sometimes with rather an abrupt tdn* 



i - the sea, which talus is protected by to 



the slope is made with a much flatter angle, and ( Uro or three parallel 

 rows of piles, standing up above the face of tin 

 the purpose of breaking tho force of the waves ; and at otl 

 of the earthwork is protected by a species of wall composed of l.innl! 

 of reeds, fascines, Ac. The hearts of these embankments ore f 

 with reeds and fascines, and especial care is taken to attach them firmly 

 to the subsoil, as well as to secure an efficient system of back drainage. 

 Many instances occur in which the banks of the more protected 

 polders of Holland and Flanders are constructed in the same manner 



