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For the protection of the Corso and bathing establishment, a retaining wall of rubble masonry was built, having a width of 5 feet at the top and 10 feet at the base, which was carried 3 feet below the mean water-level. This wall was protected by a row of piles 3 feet 6 inches long. The cost was £4 a foot run.

The foot of the cliffs was protected by a breakwater consisting of a double row of piles 3 feet 3 inches apart; the front battering at an angle of 1 in 4, the back row being vertical, the space between the piles being filled with fascines. This breakwater was strutted with timbers driven at an angle of 45 degrees, the heads being secured to the back piles, these being strengthened by longitudinal waling and cross-braces to the front piles. In front of the breakwater, and parallel with it, two rows of piles were driven at distances of 6 feet and 13 feet, and having a plank spiked to them at the bottom for the purpose of diminishing the effect of the backwash. These piles were 10 to 12 feet long, and 8 inches square. The cost of this breakwater was 30s. a foot run.

The general effect was good; the groynes became covered with sand, and a foreshore was formed between the breakwater and the front row of piles (Min. Proc. Inst. C.E., vol. li., 1877).

Nordenay. The face of the bank here is paved with heavy sandstone blocks, laid on concrete to a little above the level of the highest tides, the lower face being concave, with a radius of 14.76 feet, and the lower part convex, and above this brick paving on concrete. The base is protected by a toe 18 feet long, consisting of rows of piling with transverse timbers, the space between being filled in with large blocks of stone, the total thickness being 4 feet.

CHAPTER VI.

GROYNES.

FOR the protection of a shore where cliffs exist to provide the material, and where the object to be attained is only the preservation of the land, shingle-banks are sufficient for the purpose, and a sea-wall is not required.

For the purpose of gathering the shingle and raising the beach, groynes are made use of, placed on the beach transversely to the coast-line.

The system of groyning generally adopted may be divided into three classes

(1) High substantial structures placed at short intervals apart.

(2) Those of a less expensive character, the planking of which rises only a short distance above the beach.

(3) Where reliance is placed on a single substantial structure to accumulate the material over a considerable length of coast.

Groynes in themselves cannot be regarded as a protection, but are serviceable in the effect they produce on the beach. They do not create fresh material, but simply stay the littoral drifting action, and so prevent denudation and assist in accumulation.

Provided they stay the drift of the shingle from the length of the coast to be protected, that is all that can be expected from them. There is, therefore, little or no service to be derived from placing them on a coast where there is no material to collect, and no wasting cliffs to provide fresh supplies, except to prevent the formation of lows on sandy shores or beaches.

Effect of Groynes on Wave-action.-Groynes are of no service in moderating the force of the waves, or preventing them breaking on the beach, but, on the contrary, tend to add to their destructive effect. When the momentum of a wave is suddenly

checked by an obstruction, such as a high groyne, the water is projected upwards, and in falling cuts out and erodes the beach. The retiring wave, flowing back in a considerable volume, carries with it in its undertow this loosened material.

When groynes project much above the beach, and are placed within short distances of each other, the water thrown into the bay between them, owing to its momentum being checked, is forced above its normal height. Owing to this increased elevation, the wave retires with greater velocity than when it has a freer course, and is thus more effective in the removal of material down the slope of the beach.

When waves are driven by the wind obliquely into these short bays, the water eddies round and cuts out the shingle. Owing to the irregularity in the breaking of the waves, due to the obstruction caused by a high groyne, it frequently occurs that the crest of the wave is on one side of a groyne while the trough is on the other, and consequently the water pours over the top like a cataract, and in falling 8 or 10 feet, as in some cases, on the bare beach on the other side, disturbs and cuts out the surface.

Shingle is frequently found heaped up to the top of a high groyne on the windward side, but never accumulates for any permanency on the leeward side; on the contrary, the sea always has the greatest eroding effect at this part, the water running freely up the shingle heaped up on the windward side, and dropping over the top of the groyne on to the bare beach.

With direct on-shore gales shingle is drawn down, and the upper part of the beach becomes denuded. Groynes do not prevent this removal.

Groynes on Sandy Beaches.--On sandy beaches there is not the same amount of drift as where shingle abounds. Groynes, therefore, have much less effect in collecting beach material.

There is, however, a certain amount of sand that is set in motion during the flood tide, and more especially during heavy gales, which is carried by the current and deposited in slack

water.

Where long low jetties have been run out across sandy foreshores, there has in some cases been accumulation on the windward side of these. Thus, on the coast of Holland, where there exists a long flat beach of sand, a considerable accumulation of and has taken place on the west side of the jetties of the harbours

at Ymuiden, and at those at the outlet of the river Maas, as described more fully in Chapter VIII.

The surface of sandy beaches frequently consists of wide ridges and lows, the lowest places generally being found where the waves plunge at H.W.S.T. If low groynes be placed across such beaches, extending from the shore to low water, the action of the flood tides causes the sand off the ridges to work upwards along the groynes towards the shore, filling the hollows, and leaving an even surface for some distance from the shore of hard firm sand lying at an angle of from 1 in 10 to 1 in 15, the surface below this having an incline to low water of from 1 in 50 to 1 in 100. A wet, soft surface may thus be converted, for a certain distance from the shore, by a transposition of material, into dry, firm sands. Examples of this effect will be found in the description of the beaches at Bridlington and Blackpool in Chapter VII.

Distance apart.-No general rule can be laid down as to the space that should intervene between groynes; so much depends on local circumstances that experience of these alone can determine this factor, but on the score of economy and efficiency, it is better to carry out the work tentatively, and add to the number, if experience shows this to be desirable, rather than to place them near together at first.

Where fishing or boating is carried on, and the boats are beached on the shore, a series of low groynes placed near together may be a source of great danger, as it is difficult for the boatmen to ascertain their position and avoid them when the beach is covered at high water.

Groynes placed at short intervals make a very uneven surface, and completely spoil the appearance and amenities of the beach for the purposes to which they are applied at seaside resorts. In fact, groyning was described by Mr. Redman as a plan for cutting up the easy flowing lines of a shingle beach into a multitude of short bays, with a repletion of material on one side and deep water on the other, and by means of which the long easy slope which shingle assumes on a natural beach is changed into a series of short abrupt banks, by which new antagonistic forces are brought into play.

Although practice varies greatly, the rate most generally followed appears to be to make the distance between the groynes equal to their length, and this, to be effective, may be taken as

the distance between high and low water of spring tides. This distance may, however, be increased without detriment to the raising of the beach, while lessening the cost.

At Cooding, near Bexhill, with groynes 250 yards apart, the shingle on the beach accumulated to a height of 20 feet, and high water was driven back 30 yards from the shore.

Under the most favourable conditions, groynes can but be regarded as obstructions to the free use of the beach, their presence only being justified by the necessity for protecting the shore. They should not, therefore, be multiplied more than is absolutely necessary.

As the only use of groynes is to prevent the drift of material, experience shows that, where the length of shore to be protected is short, a single groyne, placed at the leeward end of the length to be protected, and raised from time to time as the beach material accumulates, will answer all the purposes required for protection, without creating the objectionable features above referred to.

The action of the waves in disturbing the shingle in gales is much less destructive on a level line of beach than where it is broken up by numerous groynes projecting above the surface.

The length of coast that may be effectually protected by a single groyne to the best advantage, and without accumulating more shingle than is required for the protection of the coast, may be taken, under normal conditions, at about 600 yards.

In the case of a jetty carried out from the shore for the protection of the entrance to a harbour, and which extends into deep water, the effect will extend to a much greater distance than this, but these structures are of a much more substantial and expensive character than are required solely for coast protection.

Shingle accumulates to the greatest width immediately on the windward side of a groyne, and tapers off in a narrowing line at an angle of about 95 degrees with the lower end of the groyne, the accumulation reaching two-thirds of the way down the groyne. By the time this has been accomplished it will be found, with the distance given, that the beach has become raised to the top of a groyne which reaches to the level of high water, after which future additions will pass over to the lee side. By the time this occurs the shingle-bank under the cliff, for the distance of about 600 yards, will have accumulated to a width sufficient for the protection of the cliff.

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