and Cherbourg, was the most suitable form. The majority of the evidence, however, preponderated in favour of an upright wall, and it was contended by the witnesses who were in favour of this form that the percussive effect of waves was far less violent on an upright wall than on a sloping face, on which the waves were changed from undulations to breakers.

This evidence has been justified by the manner in which the Admiralty Pier at Dover has weathered all the gales which have occurred since its construction half a century ago, as compared with the constant damage which occurred after every gale for many years to the breakwaters at Cherbourg and Plymouth, and the great cost incurred in repairs; and also the destructive effect of the waves on the wall at Alderney, the completion of which was finally abandoned.

There seems, however, to be a consensus of opinion that, for deep-water purposes, an upright wall resting on a sloping base is more subject to the destructive effect of the waves than either of the other two.

The different effect of upright and sloping walls on waveaction may be best realized by watching the sea break with violence in a storm on a sloping breakwater; while within a short distance, where the slope terminates at the entrance into the harbour in an upright wall, and where the depth of the water is great, the waves only rise and fall with gentle undulations.

A distinction must be drawn as to the action to which walls constructed parallel with the coast for protective purposes are subject as compared with those built for breakwaters. The former have to act as retaining walls to the land behind, and to contend with the pressure of the earth at the back. These walls at one period are free from any pressure in front, while at other times they are subject to the impact of waves which have broken on the shallow shore in front; whereas the walls of breakwaters are not subject to land pressure, and have water on both sides, and when built in deep water, the waves which come in contact with them on the outside have more the character of waves of undulation than of progression, unless the profile of the wall is such as to change the character of the wave.

The force to be encountered by sea-walls for coast protection consists (1) of the statical pressure due to the head of water brought against the wall by the upward projection of the waves; (2) the percussive force due to the momentum of a large mass of

water moving with considerable velocity being brought suddenly to a stop; (3) the pressure of the earth at the back of the wall. In a tidal sea the point of impact of the waves on the wall is constantly varying as the tide rises and falls.

In designing a sea-wall, the object to be sought is to produce such a form that the wave shall come in contact with it at a point where it can do the least harm, and afterwards expends its force over as wide a surface as possible; and that the force of the wave shall be equalized instead of being concentrated at any one point.

The strongest part of the wall should be at the point where the wave changes from the horizontal to the vertical position.

As far as practicable, the waves coming in contact with the wall should be reflected, and not break on it.

Obstructions to the free movement of the waves, and projections leading to shocks and vibration, should be avoided.

The water should not be guided upwards in a direction which leads, on descending, to its falling behind the wall; nor should the form be such as to concentrate the falling mass at the foot of the wall.

An example of the injury done by the water being thrown upward and falling on the road behind the wall is afforded by an incident that occurred on the Dublin and Kingstown Railway. The waves striking the parabolic curve of the retaining wall were projected upward during a storm, and, falling like a cascade on the granite pavement, loosened and lifted blocks 2 feet square, weighing half a ton.

These conditions continue to attach to walls which batter or leave the vertical until a slope of 45 degrees is attained. With flatter slopes the wave breaks.1

With walls situated above the level of low water, as in the case with most sea-walls for cliff or shore protection, it is difficult to meet all these conditions. The wave which comes in contact with a wall of this character as the tide rises, reaches it more or less in a broken state, and the water is projected forward on to the wall with the momentum due to its mass and velocity, the effect increasing as the water deepens with the rising tide.

The effect of a vertical wall under these conditions in projecting the water upwards is to concentrate its action at that part of the wall where the resulting consequences are most serious. The effect of the water, whether that thrown upwards and falling on

'Scott Russell on Breakwaters, Min. Proc. Inst. C.E., vol. vi., 1347.

G

the back of the wall, or that falling on the beach, tends to cut out the material and weaken the wall.

As an upright wall batters from the vertical, the force of the wave-stroke is decreased in proportion to the cube of the sine of the slope; but, on the other hand, the greater the batter the more the water is led by the slope landwards, and the greater the quantity that falls on the surface at the back of the wall.

Upright walls possess an advantage in their construction, owing to the materials used being placed to the greatest advantage, the weight of the superincumbent mass assisting in keeping the lower stones, where the greatest stress is, in their places. They tend more to reflect the waves than to break them. They also offer no resistance to the upward stroke of the wave.

Walls having a concave batter with the top overhanging have been frequently adopted, with the design of throwing the water off the wall in such a direction that it shall meet the incoming wave and so neutralize its effect. This result is not attained in practice. It frequently happens that a coalescence takes place between the two waves, and the mass of water thus thrown on the wall is increased; and this form of wall has not been found to prevent the erosion of the beach by the falling water.

A wall concave throughout its whole face is also open to the objection that the materials of which it is composed are not used to the best advantage. This form involves the disposition of a great mass of material at the upper part of the wall where the least horizontal wave-force is exerted. The projection of the upper part of the curve also offers a large surface to the upward stroke of the wave. If built of masonry, this surface, having a form like the intrados of an arch, is badly adapted to resist the upward thrust. It is impossible, also, to devise such a curve that shall dispose the form to the best advantage at the varying level of the tides.

In the experiments conducted by Mr. Stevenson with the marine dynamometer, it was shown that the maximum vertical force of the upward stroke of the wave on a wall, the top of which was 23 feet above the water, tending to lift that part of the wall with which it came in contact, was upwards of 1 ton per square foot, while the greatest horizontal force at the same level never exceeded 28 lbs.1

1 "Design and Construction of Harbours," by T. Stevenson. A. & C. Black, Edinburgh. 1874.

Walls having steps have been advocated, with the idea of breaking up the force of the wave as it strikes the wall, and so preventing its being projected upwards, and also with the object of catching the water on the steps as it falls, and thus minimizing its effect on the beach.

A wall of this description offers the great objection of affording a broken surface for the waves to act on, and, from the stones not being placed directly over each other, the advantage of the superincumbent weight which is obtained in a vertical wall of the usual description is lost.

The wall along the bay on the west of the harbour at Margate, of which an illustration is given, has been built on this principle, and it has been claimed that this wall has withstood the effect

of gales when the wall on the east side of the harbour, which has a straight face, was destroyed. This wall, however, is in a sheltered position and less exposed to the violence of the waves The than the one that fell, with which it has been compared. water in front of it at H.W.S.T. is not more than from 4 to 7 feet in depth.

The promenade wall at Bridlington is stepped, each course of the stones projecting in front of the one above, a distance varying from 4 inches at the top to 8 inches in the bottom courses.

It is also furnished at the top with a nosing or cornice projecting a foot, which, however, is curved on the bottom side. The parapet of this wall is 26 feet above the beach, and 20 feet above high water, and is exposed to heavy seas, the depth at H.W.S.T. against the wall being 6 feet.

H.W.S.T.

Beach

BRIDLINGTON.

FIG. 8.

Sloping Walls.-These have been adopted to a much greater extent in Holland than in this country. The wall at Dymchurch and the older wall or "hulking" at Blackpool are merely the outcome of gradual attempts to conserve existing earthen walls than of original design. The new promenade wall at Blackpool, hereafter described, is a combination of a sloping base with an upright wall at the top, and is the result of intentional design.

The walls both at Dymchurch and Blackpool have resulted in continual scour of the beach, necessitating renewal of the pitching, and have been a constant source of expense.

A flat sloping wall necessarily extends further seaward than an upright wall, and consequently feels the effect of the wave at an earlier and for a longer period. The sloping wall at Dymchurch has a face exposed to the waves varying from 80 to 140 feet. Directly the wave comes in contact with the slope it impinges on it and breaks with violence, forcing the water into all the interstices; and an advancing and retreating oscillation parallel to the slope is set up, which has a tendency to remove any of the stone pitching that tends to stop its progress. The retreating wave down the slope is tripped up by the oncoming wave, causing it to break more heavily than it would otherwise have done, and is as destructive in its action as the one advancing.

On a sloping wall the stones have only their own individual weight to keep them in their place, whereas in an upright wall they are aided by the weight of all the stones above them to resist any lifting action. A single stone is thus more easily