New Water-Line - Operations
Inundations can be made on any ground which is below the level of the intersecting or surrounding watercourses. Foremost among the inunable grounds are polders and reclaimed land, which can be put under water by merely stopping the mills which keep them dry. This plan, however, occupies much time, and may fail altogether in dry weather. Hence it was generally necessary to conduct water to the ground to be inundated by previously prepared sluices, and if by this meant water is not admitted fast enough, the dykes may be pierced. The latter measure has the disadvantage that it takes from the defender the command of the water, and inundates more ground than is required. Ground situated alongside of the watercourses which are kept in by dykes may also be inundated, but only when, from natural or artificial causes, the water is high.
The water for making the inundations is drawn from the large rivers; the small rivers; the sea; the rains and springs on the polder land. Of these the most useful is the first. The principal rivers, the Rhine, Maas, and Scheldt, though varying in height, always contain a large supply of water, and are naturally more to be trusted than the small rivers, especially when these are fed by local rains, and are consequently often dry. The sea as a source of water supply is practically inexhaustible, but the use of salt water for inundations is attended with the disadvantage that it causes serious damage to the cultivated land. Spring and rain water can only be used when the supply exceeds the evaporation.
Under the most favorable circumstances an inundation may be supplied with water from more than one source ; and it may frequently happen that the site of inundations is already covered with water from atmospheric causes, so that in such a case the flooding of the ground would be partly carried out before the sluices were opened.
To answer its purpose thoroughly a military inundation should be neither fordable nor navigable, and, therefore, it should have a fixed depth. In clay and marsh land a small depth is sufficient, as the ground soon becomes so soft and slippery as to be impracticable for troops. Ground intersected with watercourses also need only be so far covered as to conceal the latter. A firm, sandy soil, on the other hand, requires a greater depth of water to be impracticable, and it is then likely to become navigable.
Absolutely unfordable inundations are rarely to be met with. Small detachments of Infantry in loose order would be able to eross most inundations, if not too broad, though perhaps with some loss. Flat-bottomed boats also would bo able to navigate almost any inundation. A navigable inundation is always preferable to one that is only fordable on account of the difficulty it imposes on the enemy of collecting boats, and the loss of time thus occasioned. In 1672 the Dutch posted boats manned with infantry on the inundations, and others with light guns on the water courses. Leiden was relieved in 1574 by Boisot's flotilla, which crossed the navigable inundations and came to the relief of the place. On the other hand, in 1787, the Prussians crossed the Haarlem lake in ships, and penetrated the Amsterdam position. The defensive measures taken on this occasion were, liowever, very defective, so that the example is not worth much.
The depth generally considered sufficient for inundations ranges between 6 inches and 20 inches. The limits are necessary on account of the possible differences of level of the area to be inundated. A disadvantage of a deep inundation is that it must necessarily cover a much greater amount of ground than one of less depth, and, moreover, requires more water, and therefore more time to complete.
The breadth of an inundation is limited by certain conditions. It is considered that the area should be as narrow as possible, on account of the damage to the land, the amount of water, and the time necessary for completing it. On the other hand, a broad inundation is more of an obstacle than a narrow one. The nature of the ground also affects the breadth not less than the depth. In any case, however, it is considered that the breadth should be such that the enemy cannot bring an overwhelming fire of artillery to bear on the defender's side.
In order to regulate the depth and breadth of the inundations, so that they may be neither fordable nor navigable, it is generally necessary to divide the area into basins by throwing up cross dykes (komkeeringen), and to give each of these a different water level. Where such dykes do not exist already, temporary dykes must be thrown up. The defects of these basins are twofold. In the first place, the komkeeringen form approaches, and in the second, the enemy may cut the dykes and draw the water from one basin into the other, when the difference of level is considerable. The komkeeringen must, therefore, not only be made unavailable as approaches, but must also be protected against any.attempt to cut them.
The quantity of water necessary to form an inundation depends on the nature of the ground and the time of year. In summer, when the ground is dry, the most water is required, and sandy ground naturally absorbs more water than clay or marsh. It is therefore found needful to multiply the dimensions of the inundation by a co-efficient which varies with the nature of the ground and the state of the weather.
It has been suggested that during a frost, the water should be partly run off every day, or every other day, so as to prevent the ice in the ditches from bearing. This plan, however, appeared better in theory than in practice, for it would occupy much time to execute, and the ditches would then be no longer obstacles, as they could be seen and avoided, or crossed by planks, &c Another proposed preventive is to keep the water at all times running into and out of the inundations. This, however, would only be successful in preventing the ice from bearing.if the stream was rapid, for the rivers in Holland are frozen almost every year. Other measures have been proposed, such as changing the water level constantly, and breaking the ice with machines.
Salt water inundations are, of course, not likely to become practicable, except in very severe weather, but they injure the ground, especially if it is clay, to such an extent that it does not recover for years. Grass land, however, is not so much affected by salt water, and in some parts the sea is let in every year.
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