1911 Encyclopædia Britannica/Reclamation of Land
RECLAMATION OF LAND. The boundaries between, sea and land are perennially changing. In many sheltered bays and estuaries the sea is receding, while along other portions of the sea-coast it is continuously encroaching. The same causes operate to produce both results: the rivers carry down with them detritus and sediment from the higher ground; the sea, aided by wind and tide, is always eroding exposed portions of the seaboard; and even such lesser influences as rain and frost assist in disintegrating cliffs composed of softer strata.
The main object of reclaiming land from the sea is to increase the area of ground available for cultivation. Land which has been raised by accretion nearly to high-water level can be shut off from the sea by works of a simple and inexpensive nature, and the fresh alluvial soil thus obtained is generally very fertile.
Accretion in estuaries takes place very slowly under ordinary conditions. Although at any one time the sheltered areas may be large and the deposit of silt fairly rapid, not much permanent accretion will take place owing to the frequent shifting of the channels. Directly, however, a fixed channel is secured by longitudinal embankments or training walls, accretion progresses rapidly and uninterruptedly by the deposit of sediment in the slack-water behind the embankments and at the sides of the estuary; and this is especially the case if the training works are raised to the level of high water, for this has the effect of restricting the greater part of the scour of tide and fresh-water discharge to the one fixed channel. The rate of accretion varies with the shelter of the site and the amount of sediment carried by the water; but by degrees the foreshores, in the upper portion and at the sides of the embanked estuary, are raised sufficiently for samphire to make its appearance, and, later on, a coarse grass. Ultimately the time arrives when the water may be altogether excluded by the construction of enclosing embankments; these must be raised above the level of the highest tide, and should have a flat slope on the exposed side, protected, in proportion to exposure and depth of water, against the face with clay, sods, fascines or stone pitching.
In the intermediate stages of the process outlined above much may be done to promote the growth of accretion, or warping as it is termed, and to ensure the fertility of the reclaimed land. The deposit of warp is accelerated by anything which tends to reduce the flow and consequent scour of the ebb-tide over theforeshore: 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 £10,000; 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 danger of 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 flowing 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 final gap during construction; this is owing to the channel or gully scoured out upon the opening of the breach.
When a breach occurs which cannot be closed in a single tide, the formation of an over-deep gully may to some extent be prevented by enlarging the opening. Breaches in embankments have been closed by sinking barges across the gap, by piling and planking 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 efflux 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,strengthened at intervals by counter forts. The thickness of the wall varies from 11 ft. 6 in. at the base to 3 ft. at the top, and is surmounted by a dressed cornice and coping; the length is 340 yards. The work was constructed, in 1888, at a cost of £10,000, or £29, 8s. per lineal yard.
Walls with almost vertical faces, or slightly stepped, appear to be the best. Unless, however, the foreshore consists of hard rock, or a raised beach maintained by groynes, a wall of this kind should be protected by an apron, in order to prevent the destructive undermining to which such forms of wall are necessarily liable.
Where the coast is fringed with sand dunes, and the beach protected from erosion by a regular series of groynes, as at Ostend (Belgium), the sand dunes, or an embankment for a promenade in front of them, may be sufficiently protected by a simple slope, paved with brickwork or masonry, and having a maximum inclination of two to one. The paving requires to be laid on a bed of clay, rubble or concrete.
Parts of the sea bank at Ostend (see fig. 8) have been carried out beyond high-water mark to gain a strip of land for the esplanade; and these portions have had to be protected from undermining at the toe with piles and planks, and an apron of concrete or pitching, laid on fascines, extending down the foreshore. For the parts above high-water mark a short paved slope, with moderate protection at the toe, has been found sufficient. The top face of these slopes is reflexed so as to protect the esplanade from surf during storms. Sea-Walls are very costly and, while temporarily resisting, do not diminish but actually increase, the erosive action of the sea. In short, sea-walls are a most unsatisfactory type of protective work.
The protection afforded to the coast by groynes is based on a totally different principle, which may be summarized as that of promoting natural accretion by the construction of artificial shelter. Along most coasts there is a littoral drift of sand or shingle; by means of groynes, projecting from the coast-line down the beach, this drift may be intercepted so as to produce accretion to the foreshore, where previously there has been constant erosion. The problem, however, of coast protection by this method presents difficulties. Littoral drift is the product of erosion, and the fate of a large portion of this drift is to be deposited in deep water. Any scheme, therefore, of stopping erosion altogether by means of groynes would be purely chimerical; in the same way, partial failure of groynes, from lack of drift and inability to stop wastage, must be expected in many localities. Another difficulty may be illustrated by the action of such natural projections as Dungeness: this point, by completely arresting the easterly drift of shingle, causes a rapid accretion to the beach on the one side, but a corresponding denudation on the other. The old type of high groyne, erected at Cromer and Hastings, has produced the same undesirable result; moreover, the general effect of groyning certain portions of the foreshore is to render the adjacent unprotected portions more liable to erosion. Nevertheless, the benefit which may be derived locally from suitable groyning is very great. The timber groynes erected between Lancing and Shoreham raised the shingle beach sufficiently to cause high-water mark to recede 85 ft. seawards in the course of a few years.
The eroding action of the river Scheldt in front of Blankenberghe has been arrested by carrying out groynes at right angles to the coast-line, and down to below low water (see figs. 9, 10). These, on the average, are about 820 ft. long and 680 ft. apart: they are made wide, with a curved top raised only slightly above the beach, so as to minimize the scour from currents and wave action, and facilitate the evendistribution of drift over the protected area. They are constructed with a foundation of fascines and concrete, faced with brickwork or stone pitching. The result has been the formation of a gently sloping beach which reduces wave action; such loss, too, as is still occasioned by storms is speedily made good by natural accretion in moderate weather. The Blankenberghe groynes are too expensive a type for ordinary use. The beach at Bridlington, which rests on boulder clay, was rapidly disappearing owing to the increased scour due to the sea-walls.
Accordingly, groynes (see figs. 11, 12) made of 14 ft. × 9 in. × 9 in. pitch-pine piles, and 11 in. × 4 in. planking, were erected along the foreshore. The piles originally projected about 6 ft.; but, to prevent heaping up of sand to windward with denudation to leeward, the planking was never raised more than two strakes above sand-level, fresh planks being added as the sand rose. The south-easterly gales are said to be the most erosive here, and prevalent during the Winter months; on this account the groynes were given an inclination of 10° south of east, that is 10° from the perpendicular. It may be doubted whether this was the best angle, but the result has been very satisfactory. The cost of construction was from 125. 3d. to I8S. per lineal foot.
The sand-banks at the entrance to Poole Harbour have been protected by groynes (see fig. 13) inclined at slightly varying angles, some yielding better results than others. This is a good example of the important work which may be accomplished by groyning. Unprotected, a breach would soon have been effected in these sand-banks; with a double entrance to the bay the present deep channel would have silted up, and Poole Harbour would have been practically destroyed.
It is evident that the efficacy of groynes in collecting drift is proportionate to the distance which they can be carried seawards, and that they should always be extended to out low-water mark; whilst, by raising them only slightly above the beach, the accumulation of drift to leeward is promoted, passage of drift over the obstruction being facilitated the scour of the waves diminished. By this means, and by gradually raising and extending the groynes as the drift accumulates, the general elevation of the beach can be secured. Drift generally travels in both directions along a coast, veering with the wind; thus the prevailing wind determines the preponderating travel of the drift. Groynes are usually constructed at right angles to the shore, but it is believed that increased benefit may be obtained by slightly inclining them to leeward of the prevailing wind. Some engineers have advocated the extension of groynes below low-water mark; and as wood when permanently submerged is specially liable, even when creosoted, to be attacked by the teredo and limnoria, the use of reinforced or ferro-concrete has been suggested as the most suitable material for submarine groyning. These suggestions, however, and many other current theories on groyning, require to be demonstrated by repeated experiments.
For a useful bibliography of the subject see British Parliamentary Reports, Coast Erosion and the Reclamation of Tidal Lands, Cd. 3684, Appendix No. X. pp. 146-158. (L. W. V.-H.)