displaced from the face of a sloping wall than from one that is upright. A stone that has become loosened by the repeated percussion of the breaking waves is soon sucked out, and mischief of this kind once commenced, disintegration rapidly sets in. On the sloping walls that protect the sea-dykes in Holland, during a single gale stones have been displaced over an area exceeding an acre in extent; and the cost of maintenance of these sloping walls is always very heavy.

The quantity of material required for facing a sloping wall is as great or greater than that required for constructing a vertical wall in the same position; while the amount required for maintenance and repairs is considerably greater.

A concave form has in some places been used for the face of a sloping wall, but this form has not been found to withstand the action of the waves as well as a straight face. Thus in the seawall made for the reclamation of land in Loch Foyle, on which the lower part of the pitching was given a flat slope, the upper part being made concave, the waves were forced up the flat slope with great velocity, and, accumulating on the more abrupt part, curled over, and in this recoil fell with such force on the flat part of the pitching as to cause breaches in it. Subsequently the concave portion had to be changed to a rectilinear bank, which stood the heavy seas without being breached.

A somewhat similar profile was given to an embankment for carrying the Dublin and Drogheda railway across the Clontary estuary. The bottom part of the slope was given a convex form and the upper made concave, the mean slope of both being at the rate of 3 to 1. It was found that this form did not stand as well as one having a straight slope; the waves, breaking on the concave part, beat down the clay bed and loosened the pitching.1

The destructive action of the waves is generally considered to decrease in proportion to the flatness of the slope, the percussive force of the blow being smaller in proportion as the impinging particles are spread over a greater surface. The most effective angle to be given to the face depends on the nature of the materials to be used. In Holland, slopes varying from 1 in 8 to 1 in 40 are not uncommon where the protection consists of fascines or straw wattling.

Mr. Brunlees, in a paper on sea-embankments given in the 1 Min. Proc. Inst. C. E., vol. xiv.

Minutes of Proceedings of the Institution of Civil Engineers (vol. xiv., 1855), records the result of experiments made by him as to the force required to draw facing bricks out from a bank 4 feet high having various slopes. The fire-bricks used were placed on end 9 inches deep, and the centre brick was extracted from each slope by means of a chain working over a pulley to which a direct pull was gradually applied. The average of the trials gave the following results:

As the result of considerable experience, however, Mr. Brunlees came to the conclusion that a steep slope of, say, 2 to 1, with good protection for the face and heavy pitching, was more effective to resist wave-action than a flatter slope with less depth of pitching.

Most Effective Form of Wall.-A wall having a cycloidal or elliptical form at the bottom dying out into a vertical face at the top, and having a slope conformable to the natural angle of repose of the beach at the bottom, seems to meet the requirements of a wall for coast defence in the best manner, and to suffer from the fewest disadvantages.

A sea-wall is generally placed at or above the line of ordinary high water, and the beach, after the construction of the wall, should be maintained at that level; the wall then is only subject to waves during gales which reach above this level.

The curved form shown in the illustration dies out at the bottom into a slope of about 1 in 10, which is the natural slope of a beach composed of shingle; or coarse sand and shingle mixed after a gale. It therefore leads the water from the beach on to the wall with the least element of disturbance. The elliptical form, being the natural curve of a coast wave, is best adapted to change the direction of the particles of water from the horizontal to the vertical in the easiest manner possible; while the vertical portion at the top does not tend to lead the water over the top of the wall on to the roadway or promenade at the back, and the return water is led back by the curved bottom to the beach by a slope conformable to its natural angle of repose,

while the apron prevents the cutting out of the beach at the toe. The greatest substance of the wall is where the greatest strength is required for the support of the earth at the back, and is disposed in the centre of the curve where the wave-stroke operates on the wall with the greatest violence. In the case of a beach consisting entirely of sand, and where the gradient is much flatter

than on a shingle beach, it may be desirable to extend the toe further seaward by an apron having a slope of 1 in 30, protected at the end by sheet piling.

Forces operating on a Sea-wall. The forces to which a seawall are subjected from waves are: (1) the direct horizontal thrust due to the percussion of the waves, which tends to loosen and disrupt the materials of which the wall is composed; (2) vertical force acting upwards on any projection or rough surface; (3) vertical force acting downwards and tending to disturb the foundation of the wall; (4) the action on the top of the wall due to the fall of a mass of water and the disruption of the material at the back. It is very rarely that a sea-wall fails from general weakness of construction, or is pushed over by the lateral thrust of the earth at the back. The most frequent cause of failure arises from the fall of the water from the return wave on to the beach, by which the material of which it is composed is cut out, loosened, and washed away; or if falling on the back of the wall, the earth is washed out in a similar manner, leaving the wall bare.

Another cause of disruption arises from stones projecting from the face; the waves catch the underside of these and use them as levers to disturb the face of the wall. This has frequently occurred where a bull nose or coping has been made to project over the face of the wall. Mr. Stevenson records that at the harbour of Stonehaven, a projecting string course had to be removed from the sea-wall, to prevent the concussions which took place during storms, and which were so great as to shake the superincumbent masonry. Many instances could be quoted of heavy copings being lifted by the sea, and where it has been found necessary to cut off string courses and other projections.

Concrete, from the smooth surface which it presents, for this reason has an advantage over stone blocks.

Another advantage which concrete possesses is the absence of joints. During storms, when walls are subject to the constant percussive action of the waves, the mortar used in jointing the stones is apt to be disturbed and washed out. The cavities thus made are frequently increased by shellfish. Every blow of the wave on the surface, where these cavities occur, acts in the same manner as an hydraulic ram, and in time forces the face stones outwards and dislocates the interior of the wall. Where the walls

have been made up with rubble in the interior, and the cavities not carefully filled, the water is forced through the joints, and in heavy gales may be seen bursting through the top to a considerable height. The sudden impact of the waves, forcing the water into the wall, compresses the air, forcing it into the smallest cavities, and its expansion assists in disrupting the masonry. Even in well-constructed walls, the necessity of at once repairing defects is for this reason obvious.

In settling the section of a wall, reliance should rather be placed on mass or dead weight than on the cohesion of the material by which the several parts are joined together. The inertia due to mass is constant and unchangeable, while the effect derived from the cohesion obtained from the cementing material is always liable to be disturbed and disintegrated by the constant vibration due to the percussion of the waves.

Effect of Walls on the Beach.-The effect of the construction of walls is generally disastrous to the beach in front, and unless care has been taken to protect the beach, the wall becomes the agent of its own destruction.

At high tides and heavy onshore gales, the waves are thrown

A great

against the wall and suddenly stopped in their course. part of the water is projected upwards to a height depending on the depth of the water in front of the wall, and in falling back cuts out the beach, the loosened material being carried seaward by the return wave. As the beach thus becomes lowered. the depth of the water is increased, and consequently the volume and force of the wave.

A wall that has been designed to withstand the force due to a given height has thus not only the back pressure increased by the greater depth in front, but also the footing is scoured away and the toe left exposed. The base of the wall then becomes in a condition to slide outwards, and the structure to become a ruin. The destructive effect of the waves is also increased when large masses of water driven by the onshore wind falls on the roadway behind the wall and cuts out the earth at its back.

It is frequently the custom, when constructing sea-walls, for promenades to be advanced considerably seaward, and to enclose the bank of shingle and sand which forms the natural protection of the shore. Thus, although a considerable area is gained for the formation of the promenade, the protection which the enclosed beach would afford in moderating the force of the waves on the wall is lost.

One of the most striking examples of the effect of sea-walls in denuding beaches in front of them after their construction is that of the wall erected at Hove. The base of the wall was carried 9 feet below the lowest level to which the beach had ever been known to scour, and it rose 11 feet 6 inches above this level. When completed there was a bed of 9 feet of shingle in front of it. Within a year from the commencement of the wall, and before it was completed, the whole of the shingle bed in front of it was scoured away to within a few inches of the base of the wall, and to prevent it collapsing, sheet piles had to be driven all along the toe. Subsequently, by the erection of groynes, and by the deposit of shingle on the beach from the dredgings at Shoreham, a beach was again accumulated, and has since been maintained.

Near Conway, on the Holyhead road, a sea-wall was constructed of masonry to carry the road round the promontory of Penmaenmawr, of which about 10 feet in height was exposed to the sea at high water, the wall being carried 5 feet below the level of the beach, which consisted of angular fragments of basalt. The face was made to batter at the rate of 1 in 6. In October, 1846, during