Other Afellioa's for Defensizr Works. —The above-named defensive works for the protection of the banks of running, streams may vary consid erably according to circumstances. The planting of flat sloping sides with sod or with willows will answer only in the case of very sluggish streams. Where the slopes are steep, the points below low-water level should be pro tected by means of stones or fascine-mattresscs. Where there is consider able current, recourse may be had to piling loose stone upon a gentle slope, which may eventually be planted with willows should the stones applied for the purpose not be heavy and large enough to answer the purpose un aided. Where an abundance of large stone is at hand, the lower portion of the slope, in the case of streams with swift current, may be protected by a deposit of loose stone, and the upper portion with a wall of masonry (pi. 57, fig. 7). It is the concave stretches of the banks that require facings of this description. Where the requirements of traffic or the value of the shore, etc., render it necessary to provide a steep slope, either sheet-piling (figs. 8, 9) or massive quay-walls (fig. io) are resorted to.
Offensive IVorks.—Offensive works for stream regulation are intended either to " correct " the banks or the bed as quickly as possible, or to cause the stream itself gradually to shape its bed or its banks to the desired form. If, for example, it becomes necessary to contract the bed of a stream, so that, in consequence of the increased velocity of current, de position of sediment at that point shall cease and a certain depth be per manently maintained, jetties are built parallel with the conrse of the stream, and each is connected at the upper end with the shore by a cross dam. The height of such structures must bear a certain relation to the fall of the stream and to the velocity needed for carrying off the detritus. These structures will be covered at high water, and the suspended detritus will be deposited between them and the banks. The process of deposition will be hastened by the construction of several cross-dams and by planting the banks. If the dams are built too high, the cross-section of the stream will be too narrow at high water, and the result will be that the bed at the affected part will be scoured out too deeply, resulting, in turn, in a lower ing of the level in those portions of the stream which lie above. Pio- lire 13 exhibits a cross-section of a structure of this kind whose forma tion is too apparent to require description. The dams are sometimes con structed of fascine-mattresses, the side next the channel being covered with stone.
I'arious Systems of Offensive If'orks.—Where it becomes necessary to deflect the stream from the shore and cause it to deposit material for the formation of a new bank, jetties of fascines or of masonry are built out from the bank into the stream (fig. II). These may be inclined up stream or
down stream. The former are the more effective for the purpose, and the deposit of new material forms more quickly. On the other hand, they are more dangerous to navigation, since vessels are more liable to come into collision with their projecting ends. According to the purpose for which they are designed, the hydraulic structures of this order are distinguished by various names—thus, training-walls or embankments, designed to pro tect the bank; scouring jetties, designed to effect the removal of sand-bars or other obstructions, or to prevent their formation by reason of the in creased velocity of currents which they cause; catch-dykes, such as have for their object to catch and arrest the sediment and cause it to be deposited in situations where it is desired to form new land; deflecting jetties, dams, or weirs, designed to intercept the stream or a part of it and deflect it in another direction; dividing jetties, often established at the confluence of two streams to guard against any irregularities that might occur at such points; finally, dams, which cut off entire branches or close up useless passes or channels. Constructions of the latter description are confined chiefly to streams of magnitude and of great importance as navig-able highways.
E.romple of an Offensive It'ork.—Figure 12 (fi/. 57) gives a view of a scheme of river-engineering- exhibiting the application of the plans above described. The object sought to be accomplished is to close up the chan nel on the left side, and to establish and maintain a navigable channel of uniform cross-section on the right. This is accomplished in the following manner: The shore is first properly protected by embankments at a, and the vegetation on the island is rooted up to the line of the contemplated shore-line. Thereupon, at b, are built the dykes which oppose the dispo sition of the stream to deflect its current into the left channel. This chan nel is now entirely shut off by the construction of a dam at e, and at the same time, that navigation may not be hindered, a navigable channel, a', e, is dredged out througli the sand-bank in the right channel. It is obvious that by these constructions the entire body of the stream will be thrown into the rig-ht channel, with the effect of rapidly carrying away the sand bank by scour, owing to the increased velocity of the current. To guard against the secondary effects of this more rapid flow, the cross-dykes at f, g-, b will be found necessary. The dotted parallel lines exhibit the new channel which the works just described will establish and maintain when their effects have been fully realized.