EMBANKMENTS AND DRAINS.
prevent the incursions of the sea as well as of streams, structures called " dykes " or " embankments " are built. The height to which such works must be carried will depend upon the object they are designed to serve. Dykes that are to protect the adjacent lands from inundation by the highest flood-waters of spring must be carried up to a considerable height, as compared with those intended simply as a protec tion against ordinary high waters. The latter are employed to permit the overflow of the adjacent lands during tbe season of the spring floods, that they may be benefited by the deposition of a layer of the fertilizing river mud. By providing the high dykes with suitable irrigating drains and sluices, throug-h which the water may be delivered when and in such quan tity as it may be required, the same object may be accomplished. The principal embankment may, as occasion requires, be reinforced by others farther back, which will serve to confine the inundated tract within nar rower limits should the main embankment be broken through.
Defecis of the the land adjacent to and outside of the embankments retains its original level, the level of the beach which is sub ject to overflow and the river-bed proper are constantly rising, owing to the deposition of sediment. As the water-surface must necessarily rise cor respondingly, it is evident that the embankments will in time cease to afford adequate protection. The obvious remedy is to build the embankments higher, but this is attended with danger of disastrous inundations should the walls be breached at any point. For some 'seven hundred miles along its course the Mississippi River is prevented from overflowing the adjacent lowlands by a line of embankments called " levees." (See p. 292.) A large portion of Holland is similarly protected, as is likewise the valley of the Po. In some cases the elevation of the embankments in consequence of the constant rise of the river-bed from the deposition of sediment has been carried to such an extreme that the bottom of the river-bed lies many feet above the level of the surrounding plain.
Construe-lion of .Embankments.—The crown of the embankment should be at least 2 feet above the highest water-level. The breadth of the crown should be at least 3 feet; and if it is intended to serve as a roadway, it should be considerably broader. 'file inner face (that on the stream-side)
should have a gentle slope, to enable it better to withstand the force of the current. Ramps are arranged at intervals upon the inner slope, so that wagons may easily reach the roadway on the crown of the structure.
Stability of insure the stability of embankments demands constant vigilance. Should any symptom of weakness appear or a slight leak be detected, it must receive immediate attention, or a breach may be made through which the water would rush, destroying a portion of the embankment and causing a destructive inundation. Stich disasters have occurred repeatedly in various parts of the world where this system of defensive works is in use. Notable instances are the repeated inundations of the valley of the river Po, the flooding of the valley of the Theiss, by which the town of Szegedin and a vast tract of snrrounding country have suffered extensive devastations, and the constantly-recurring inundations of the' lowlands of the 1\lississippi Valley, by which thousands of square miles of cultivated lands and nuinerous towns and cities are annually sub jected to great damage.
Tributary Streams: the embankments built along the course of a river shall serve effectively their intended purpose of protecting the adjacent lowlands from inundation, it is manifest that the banks of its tributary streams must also be similarly guarded as far back from their points of confluence as the inflnence of the flood-water of the main stream is likely to be felt. The banks of small confluents are at times lined with masonry, and the stream is led into the main one by means of a covered drain of masonry built through the embankment. To control such outlets and to prevent the flooding, of the land behind the embankment by the high water of the main stream, they arc provided with sluice-gates or self-acting valves or doors. The flood-waters, as a rule, rim off rapidly, so that the penned-up tributaries, which during this period are liable to overflow their banks, will soon resume their free discharge through the sluices. To drain at low water the lowlands adjacent to the stream, the drainage-sluices may be pierced through the main embankment, their outlets being pro vided with some convenient form of gate or door, so that it may be opened or closed, as may be found necessary.