DYKE, or DIKE (Dutch, dyk), an artificial mound along the bank of a river or sea shore, erected for the purpose of preventing inundation. The term is from the same root as dig—hence also ditch, or the hollow from which the D. is formed. The French employ the term levee to signify this species of embankment, of which there is a notable example in the levees erected along the Mississippi near New Orleans. The principle on which dykes or leves are formed is very simple. The embankment must be of suffi cient breadth and height to resist the pressure of the water, and must be constructed with that easy slope which will allow the floods to rise 'without any particular impedi ment. This is quite understood in practical engineering. Flowing water must not be abruptly resisted, but suffered to rise gradually and expend itself. It is accordingly of the first consequence, in all attempts to restrain water by embanking, that the mounds should possess not only magnitude, but a very gradual rise in the side which has to resist the impact of the flood. For want of attention to this method of embanking, there has often been much ineffectual dyking of the sea and rivers liable to do damage by flooding.
In no country has the erection of dykes been carried to such a length as the Dutch Netherlands. Consisting to a large extent of low meadow-land, formed of materials brought from Switzerland and Germany by the Rhine, there is a constant liability to be deluged by the several branches of that river previous to their entering the sea. Inspired by a sense of their perilous situation, as well as a naturally industrious and painstaking disposition, the Dutch have for ages been distinguished for their ingenious system of river-embanking; till at length the dykes of Holland are spoken of as almost one of the wonders of the world. While the country generally is guarded against sea
inundations by high mounds of sand or dunes, created by the deposit of light sand blown from the level shores (see DUNE), the interior is secured from the rivers by the system of dykes here referred to. These ramparts are in appearance long green mounds, broad at the base, graduated in their slope, and often of sufficient width to admit of a. canal or road, or both, being formed along the top. To give strength to the fabric, willows are planted and also interwoven like wicker-work on the sides. Carried along the banks of rivers, and in some places along the margin of the sea, as well as crosswise in different parts of Holland, a singular network of embanking is presented, which answers the double purpose of a protection from inundation and a means of having canals, by which superfluous water pumped from'the meadows, or polders, may be run off into the sea. The whole system of dyking is placed under local and general super intendence, at a considerable cost to the public. One of the most gigantic of these dykes is that along the Helder; it measures about 6 m. in length, 40 ft. broad at the summit, which there is a good road, and descends into the sea by a slope of 200 ft., inclined about 40 degrees. Notwithstanding the precautions taken, one or other of the lower branches of the Rhine occasionally overflows its banks and lays a wide dis trict of country under water. One of these inundations took place in the winter of 1860-61, and, committing immense havoc, was the cause of much loss and suffering. A. good example of dyking for the purpose of drainage is shown near Haarlem, where it has facilitated the withdrawal of the Haarlem lake (q.v.).