(Paper No. 3,590.)

"Sea-Coast Defence-Works in the Netherlands."

By HARCO THEODORUS HORA SICCAMA, M. Inst. C.E. THE defence of inhabited land against encroachment by the sea is one of the most ancient of engineering problems. River-deltas, with their soft fertile soils, naturally have been from the earliest times thickly populated; and such alluvial formations, being easily disintegrated, are unable to withstand the scouring action of the sea unless they are protected by some harder and tougher material.

The sea attacks a coast in two ways: by littoral currents, and by waves. The havoc caused by waves may sometimes be very great, but as the ultimate effect of waves is to form a shelving beach, they put a bridle on their own destructive power. This, however, is not the case where littoral currents sweep away the detritus hauled down by the waves from the dry land of the coast-line. The problem of defending a coast against erosion rests on the consideration as to whether the currents or the waves are the cause of the damage, wholly or in part. Where a coast is not exposed to an extended reach of deep sea, as in an estuary, for example, the erosion is solely due to currents, and in order to prevent erosion these currents must be deviated; this is usually done by means of obstacles to the current placed more or less at right-angles to the coast-line. A coast-line exposed to the waves only is free from erosion after the beach is once formed, but if a current also sweeps past, which keeps the sea in front deep, the necessary works are of two sorts, each appropriate to the nature of the attack.

The obvious method of averting the horizontal motion of the water is by means of a more or less vertical surface conveniently formed of rows of piles or boards driven close together in groynes, across the current, or, if waves have to be arrested, parallel to the coast-line. Along the foot of the earthen sea-dams on the Zuiderzee there used to be many, and there are still some places where the turfcovered slope of the "dike" is defended against the surf by two or

three rows of closely-driven piles, parallel to each other and to the sea-dam. With a wide beach in front, and the toe of the dam dry at low-water, this system answers its purpose; but where timber is constantly submerged the maintenance is costly, due to rapid decay and to the damage caused by the Teredo Navalis. Many groynes on the Walcheren coast are constructed partly of timber piles, but the object of these piles, which are often creosoted, is not so much to stem the current as to keep in place the stone blocks pitched in between them on the sea-bottom, to protect it against scour, and to prevent channels forming too near the foot of the adjoining sea-dams.

To replace timber, so liable to destruction, by masonry is not financially feasible where agricultural land has to be defended over long distances, even if the cultivated area which can be laid under contribution for this object is prosperous and extended. Moreover, not only are masonry and brickwork expensive, but soft alluvial formations and many sandy or mixed soils do not afford a foundation of sufficient stability to permit of their use. The problem of rendering the coast-line sufficiently resisting to withstand the wash of water, can, however, be solved in other ways, and by the use of less costly materials. The slope of a dam can be protected efficaciously by layers of seaweed, or rushes and weeds, or even in some cases by straw matting; all these, however, require continual watching and maintenance. Intertwined twigs, and the straight branches of some trees, can withstand greater strains, but by themselves are not lasting. Stone, or similar hard material, alone can be relied upon to remain unaltered for any length of time. Its weight also is in its favour, so long as it is distributed over a sufficiently large surface if it is to rest on a soft soil. These considerations led to the adoption of systems in which stone and vegetable matter are employed in combination. In Japan, soft earth exposed to the wash is covered with long bamboo baskets filled with large pebbles and gravel. By the time the bamboo has decayed, the pebbles have settled down and have become overgrown with weeds, which bind them together.

The country which depends for its very existence on the maintenance of its coasts against erosion by the sea is the Netherlands. The most fertile and prosperous part of this kingdom lies below sealevel, having been, in fact, wrested from the sea by dint of hard. labour and the exercise of considerable ingenuity during many centuries, the dwellers on these low marshes and muddy islands at times taking their land from the sea, and at other times the sea retaking the land, and with it the lives of its inhabitants. At present the balance is in favour of man, but one tidal wave may cause the

whole country, from the grain-covered islands of Zeeland to the rich meadows of Friesland, to be converted into a pestiferous swamp and

a dismal wilderness of mud-flats and tangled rushes. In that country the art of reclaiming land from the sea and defending that which has been reclaimed

has reached its highest development. The construction, repair, and maintenance of dams and dikes constitute there a special trade, with its long-gained experience, its rules and customs, and its technical terms. The methods employed to attain the desired result are not the same in all parts of the country, but are all designed with the common object of overlaying the soft soil of the dams, or the sand forming the beach, with some material which will keep the wash from the underlying formation, and resist erosion. A covering of heavy stones does not, however, afford adequate protection to soft ground or loose sand which is churned up by water unless the stones are laid on some sort of matting, which not only supports them, but also binds them together.

Many varieties of matting are employed, their construction and handling being of considerable importance and constituting a large portion of the total work. The larger and heavier mats are made of interwoven twigs and branches, according to the stresses which they are inThe lightest matting (Fig. 1) consists of a layer of straight branches of willow or similar trees, green and fresh,

tended to resist.

10 feet to 12 feet in length, laid close together, in some cases with the butt-ends pointing down the slope and in others in the reverse position. If the length of the slope is too great to be covered by one length of the branches, as many layers as may be necessary are laid, breaking joint, so that the butt-ends are always uppermost. In some cases the first layer is laid along the top of the dam, but in general the spreading is begun at the toe, the butt-ends pointing outwards towards the water. To keep this covering from floating, pegs are driven at intervals of about 1 foot in horizontal rows about 3 feet apart. A low stout hurdle is then twined round the pegs, close down on the branches, and 6 inches to 8 inches high. Every third or fourth peg is bored through for the insertion of a skewer to prevent the hurdle from sliding up, Fig. 2. Should the slope be soft or of newly-made ground, a layer of sedge, rushes, reeds, or straw is laid underneath the branches, but on old well-settled ground this can be dispensed with. The hurdles are intended not only to keep the matting down, but also to keep in place the ballast of stone or brick with which the whole is overlaid. Such a covering is not intended to withstand a heavy sea, and it can be laid only above low-water level. The weight of stone ballasting laid upon it should not exceed 1 cwt. per square foot, and it requires a good deal of attention and maintenance.

For stronger and more important work, "wiepen" are employed; these are fascines or long faggots, 4 inches to 6 inches in diameter, bound up with osier-twigs at intervals of 1 foot. They can be made of any desired length, the boughs or branches composing them being laid so as to break joint and to form a sort of roll or cable. The spinning of these "wiepen" requires much care, as they must be strong enough to resist considerable stresses and yet sufficiently pliant to be pressed down under a superimposed load on a possibly uneven bottom.

The uses to which these fascines are put are many and various. To cover a slope above low-water level they are laid side by side, at right-angles to the trend of the dam, on a preliminary matting of sedge or straw, and if the body of the dam consists of sand this is first overlaid with a layer of well-rammed clay. Hurdles of stout willow boughs, on stakes, preferably of oak, are run crosswise over the fascines, 3 or 4 feet apart. The lower ends of the fascines are then closed up with short round piles, standing up above the fascines to a height of about 2 feet. The whole is then covered with ballast to the level of the tops of the hurdles, giving a layer 8 or 9 inches in thickness, on which the pitching of rubble is carefully laid. Such a mat can easily support a load of stone 3 feet in thickness. Where

the slope is steeper than 4 to 1 the hurdles would not be strong enough by themselves to keep the stone from slipping down, and rows of piles are then driven, not more than 6 feet apart.

To cover a submerged slope, the fascines are joined together to form a raft of two or three tiers, the fascines of each tier being laid across those of the tier below it, Figs. 3. The fascines in each tier are bound together with osier-twigs, and the successive tiers are also similarly bound to one another. For greater strength the corners are lashed with tarred rope. Over the upper tier are laid hurdles, the stakes of which are driven through the fascines at places where they cross each other. A stout gangway-hurdle surrounds the whole

surface of the raft, and the inner space is divided up into square compartments by means of hurdles crossing each other and 3 to 4 feet apart in both directions. Such a raft or bed is sometimes as large as 80 feet in width by 400 feet in length; larger rafts would be difficult to handle. If a dam has to be built up on a submerged bottom, a bed is towed to the spot and carefully moored. At lowwater the bed is then loaded with clay, preferably sods taken from mud-flats already overgrown with marine vegetation. The centre compartments are filled first, and when the bed sags in the middle and begins to sink, the outside gangways are loaded until the whole settles down, the centre touching the bottom first and the sides