EMBANKMENT. It is often necessary to raise mounds or dykes along the course of rivers to keep them within their channels, and prevent their flooding the lands which lie near them. Many parts of Holland could not be inhabited if the sea were not kept out by strong embankments : and the destruction of a dyke frequently desolates great tracts of country.
The first thing attended to in forming em bankments is to enable them to resist the pressure of the highest floods which are likely to occur, and to prevent the effect of the waves and currents in washing them away. When it is the mere pressure of a column of water which is to be withstood, a simple earthen bank made of the soil immediately at hand, provided it be not of a porous nature, is sufficient. Its form should be a very broad base with sloping sides, and with a flat top, which may serve as a path or even a carriage road. When the dykes are only intended to check the waters at the time when they flow over their natural banks, it is best to raise them at some distance from the liver on each side, and parallel to its course ; because, in sudden floods, the water, having a greater space to flow through, will not rise so high, and will sooner recede.
Where embankments are made against the sea, greater skill is required to resist the force of the waves. If there are materials at hand to lay a bank of stones imbedded in clay, with a broad base, and the sides sloping very gra dually upwards, a very safe barrier may be opposed to the waters. It is not the direct
impulse which is the most destructive ; waves striking against a sloping surface lose their force and rise over it; but it is in returning that they draw the materials with them, and scoop out the foundations. In a place where shingles were usually thrown up by the waves, and the bottom was a strong clay, their re treat has been intercepted by rows of strong piles driven in a line along and parallel to the shore, and covered with boards nailed to them on the land side; in one night the shingles have been thrown over the piles ; and being retained by the boarding, have formed a per. feet wall. In other cases several rows of piles are driven in, and stones thrown into the spaces between them.
Where the land lies very fiat for a conside rable distance from the shore, it is of advan tage to have two complete banks, one within the other ; so that if the outer bank is broken through, the second will keep back the waters until the first can be repaired. The- water which accumulates within the bank's, and is collected in the internal ditch and those which divide the marshes, must be let off occasionally by means of channels and sluices at the time when the tide is out, and the water outside the bank is lower than that which is within it.