Page:EB1911 - Volume 22.djvu/972

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foreshore: thus considerable advantage will accrue from placing rows of faggots or sods across the lines of flow; and banks, enclosing the higher portions of the foreshore, may often be constructed so as materially to increase the period of stagnation, near high tide, of the silt-bearing water upon the lower adjacent foreshore. The light, fertilizing alluvium only deposits in shallow water at high tide, and where there are no tidal currents. The final enclosure, therefore, should not be effected until this deposit has taken place. The enclosing works, also, should be so carried out that increasing shelter may favour the deposits of this alluvium during construction. A final and rapid deposit can sometimes be effected by making sluices in the banks: the turbid water is admitted near high tide, and retained until the whole of its silt has been deposited, the clear water being allowed to escape slowly towards low tide. Premature enclosure must be guarded against; it is more difficult, the cost greater, the reclaimed land is less fertile and, being lower, less easy to drain.

The practice of reclaiming land in British estuaries is a very ancient one. The Romans effected reclamations in the Fen districts; the enclosing of Sunk Island in the Humber was begun in the 17th century, and now produces an annual revenue of something like £ro,000; large reclamation sin the Dee estuary took place in the 18th century; and, in recent times, works have been carried out in the estuaries of the Seine the Ribble and the Tees.

In the reclamation of land adjoining the sea-coast, sites where accretion is taking place are obviously the most suitable. Marsh lands adjoining the sea, and more or less subject to inundation at high tides, can be permanently reclaimed by embankments; but these, unless there is protection from sand dunes or a shingle beach, require to be stronger, higher, with a less steeply inclined and better protected slope than is required in estuaries. The width of the bank will generally prevent percolation of water at the base; but if there is any dangerof infiltration, owing to unsuitability of material, a central core of puddled clay or a row of sheet-piling should be employed. Waves over topping the bank will quickly cause a breach, and produce disastrous results; the height of the bank must, therefore, be calculated to meet the case of the severest on-shore gale coinciding with the highest spring tide. Undermining, caused by the recoil of waves on the beach, is liable to occur in exposed sites; this may be prevented by a line of sheet-piling along the outer toe of the bank.

Sea-coast embankments should not generally be constructed farther down the foreshore than half-tide level, as the cost of construction and maintenance would increase out of all proportion to the additional area obtained. It is, as a rule, more economical to reclaim a large area at one time, instead of enclosing it gradually in sections, as the cost varies with the length of embankment; it is, however, more difficult to effect the final closing of a bank, where a large area is thus reclaimed, on account of the greater volume of tidal-water fiowing in and out of the contracted opening. The final closing of a reclamation embankment is best accomplished'by leaving a fairly wide aperture, and by gradually raising a level bank across its entire length. The enclosed area may be left full of water to the height of the unfinished bank, or the tide-water may be allowed to escape and enter again by sluices in the finished sections. The embankments in Holland are closed by sinking long fascine mattresses across the opening; these are weighted with clay and stone, and effectually withstand the scour through the gap; the two terminal slopes of the finished sections are similarly protected.

There are many examples of sea-coast reclamation: Romney marsh was enclosed long ago by the Dymchurch wall (see fig. 1), and a large portion of Holland has been reclaimed from the sea by embankments (see fig. 2); the reclamation bank for the Hodbarrow iron mines (see fig. 3) illustrates the use of puddled clay to prevent infiltration.-The

repair of a breach effected in a completed reclamation embankment is a more difficult task than that of closing the I

final gap during construction; this is owing to the channel or gully scoured out upon the opening of the breach. When a

Fro. 1.-Sea-wall at Dymchurch.

breach occurs which cannot be closed in a single tide, the formation of an over-deep gully may to some extent be prevented

FIG. 2.-Dutch Reclamation Embankment.

by enlarging the opening. Breaches in embankments have been closed by sinking barges across the gap, by piling and planking

Fig. 3.-Reclamation Bank for the Hodbarrow Iron Mines. up, by lowering sliding panels between frames erected to receive them, and by making an inset wall or bank round the breach. By the last-mentioned method the new connecting bank can be formed on solid ground, and the necessary width of opening obtained to obviate excessive scour during the influx and efliux of the tide over the bank while it is being raised.

The gradual drying of reclaimed land lowers the surface some two or three feet; the land therefore becomes more liable to inundation after reclamation than before. Accordingly, it is most important to prevent breaching of the bank by promptly repairing any damage caused by storms; and if a breach should occur, it must be closed at the earliest possible opportunity. The protection of the coast-line from encroachment by the sea is a matter of considerable importance and great difficulty: the more rapid the erosion, the more exposed must be the site; and, consequently, the more costly will be the construction and maintenance of protective works. These are of two kinds: sea-walls or banks, and groynes.

Upright sea-walls with some batter on the face have been constructed along the frontage of many sea-side towns, with the double purpose of making a promenade or drive, and of affording protection to the town. A very sloping and also a curved batter breaks the stroke of the wave by facilitating its rising up the face of the wall, but the force of the recoil is correspondingly augmented. A wall with a vertical face offers more direct opposition to a Wave, minimizes the tendency to rise, and consequently the recoil; while a stepped face tends to break up both the ascending and recoiling wave in proportion to the recession of the steps, but there is a corresponding liability to displacement of the blocks composing the wall. The concrete sea-walls erected in front of Hove, Margate, and the north cliff at Scarborough (see figs. 4, 5, 6) exhibit straight, stepped, and curved forms of batter. The curvature of the last-named wall, though diverting the coil at its base, did not prevent erosion of the shale bed on which it was founded, and a protective apron in front of the toe had to be added subsequently.

The Beaconsfield sea-wall at Bridlington (see fig. 7) is stepped and slightly curved; it has a stone face with concrete backing,