means, inevitably produces some corresponding change at another which can seldom be foreseen, and for which the remedy applied may prove but a new cause of harm. Thus, a bar removed from one point is found gradually to form lower down; one bank protected from the current's force transfers its action to the opposite one, on any increase of volume from freshets, widening the bed, and frequently giving a new direction to the channel. Owing to these ever varying causes of change, the best weighed plans of river improvement sometimes result in complete failure.

741. In forming a plan for a river improvement, the principal objects to be considered by the engineer, are, 1st, The means to be taken to protect the banks from the action of the current. 2d, Those to prevent inundations of the surrounding country. 3d, The removal of bars, elbows, and other natural obstructions to navigation. 4th, The means to be resorted to for obtaining a suitable depth of water for boats, of a proper tonnage, for the trade on the river.

742. Means for protecting the banks. To protect the banks, either the velocity of the current in-shore must be decreased so as to lessen its action on the soil; or else a facing of some material sufficiently durable to resist its action must be employed. The former method may be used when the banks are low and have a gentle declivity. The simplest plan for this purpose consists either in planting such shrubbery on the declivity as will thrive near water; or by driving down short pickets and interlacing them with twigs, forming a kind of wicker-work. These con structions break the force of the current, and diminish its velocity near the shore, and thus cause the water to deposite its finer particles, which gradually fill out and strengthen the banks. If the banks are high, and are subject to cave in from the action of the current on their base, they may be either cut down to a gentle declivity, as in the last case; or else they may receive a slope of nearly 45°, and be faced with dry stone, care being taken to secure the base by blocks of loose stone, or by a facing of brush and stone laid in alternate layers.

743. Measures against inundations. At the points in the course of a river where inundations are to be apprehended, the water-way, if practicable, should be increased; all obstructions to the free discharge of the water below the point should be removed; and dikes of earth, usually termed levées, should be raised on each side of the river. By increasing the water-way a temporary improvement only will be effected; for, except in the season of freshets, the velocity of the current at this point will be so much decreased as to form deposites, which, at some future day, may prove a cause of damage. In confining the water between levées, two methods have been tried; the one consists in

leaving a water-way strictly necessary for the discharge of freshets; the other in giving the stream a wide bel. The Po in Italy and the Mississippi present examples of the ormer method; the effect of which in both cases has been to raise the bed of the stream so much that in many parts the water is habitually above the natural surface of the country, leaving it exposed to serious inundations should the levées give way. The other method has been tried on the Loire in France, and observation has proved that the general level of the bed has not sensibly risen for a long series of years; but it has been found that the bars, which are formed after each freshet, are shifted constantly by the next, so that when the waters have subsided to their ordinary state, the navigation is extremely intricate from this cause. Other means have been tried, such as opening new channels at the exposed points, or building dams above them to keep the water back; but they have all been found to afford only a temporary relief.

744. Elbows. The constant wear of the bank, and shifting of the channel towards the concave side of elbows, have led to various plans for removing the inconveniences which they present to navigation. The method which has been most generally tried for this purpose consists in building out dikes, termed wingdams, from the concave side into the stream, placing them either at right angles to the thread of the current, or obliquely down stream, so as to deflect the current towards the opposite shore

α

A

Fig. 174-Represents a section of the timber wing-dams on the Po, formed of plank nailed

on the inclined pieces of the ribs.

ab and be, inclined faces of the dam, the first making an angle of 63o, and the second of 230 with the horizon.

d and e, pieces of the rib.

f and g, horizontal pieces connecting the ribs.

Wing-dams are usually constructed either of blocks of stone of crib-work formed of heavy timbers filled in with broken stone or of alternate layers of gravel and fascines. Within a few years back, wing-dams, consisting simply of a series of vertical frames, or ribs, (Fig. 174,) strongly connected together, and covered on the up-stream side by thick plank, which present a broken inclined plane to the current, the lower part of which is less steep than the upper, have been used upon the Po, with, it is stated, complete success, for arresting the wear of a bank by the current. These dams are placed at some distance above the point to be protected, and their plan is slightly convex on the up-stream side.

Wing-dams of the ordinary form and construction are now regarded, from the experience of a long series of years on the Rhine, and some other rivers in Europe, as little serviceable, if not positively hurtful, as a river improvement, and the abandonment of their use has been strongly urged by engineers in France.

The action of the current against the side of the dam causes whirls and counter-currents, which are found to undermine the base of the dam, and the bank adjacent to it. Shallows and bars are formed in the bed of the stream, near the dam, by the débris borne along by the current after it passes the dam, giving very frequently a more tortuous course to the channel than it had naturally assumed in the elbow. The best method yet found of arresting the progress of an elbow is to protect the concave bank by a facing of dry stone, formed by throwing in loose blocks of stone along the foot of the bank, and giving them the slope they naturally assume when thus thrown in.

745. Elbows upon most rivers finally reach that state of development in which the wear upon the concave side, from the action of the current, will be entirely suspended, and the regimen of the river at these points will remain stable. This state will depend upon the nature of the soil of the banks and bed, and the character of the freshets. From observations made upon the Rhine, it is stated that elbows, with a radius of curvature of nearly 3000 yards, preserve a fixed regimen; and that the banks of those which have a radius of about 1500 yards are seldom injured if properly faced.

746. Attempts have, in some cases, been made to shorten and straighten the course of a river, by cutting across the tongue of land that forms the convex bank of the elbow, and turning the water into a new channel. It has generally been found that the stream in time forms for itself a new bed of nearly the same char acter as it originally had.

747. Bars. To obtain a sufficient depth of water over bars, the deposite must either be scooped up by machinery, and be

conveyed away, or be removed by giving an increased velocity to the current. When the latter plan is preferred, an artificial channel is formed, by contracting the natural way, confining it between two low dikes, which should rise only a little above the ordinary level of low water, so that a sufficient outlet may be left for the water during the season of freshets, by allowing it to flow over the dams.

If the river separates into several channels at the bar, dams should be built across all except the main channel, so that by throwing the whole of the water into it the effects of the current may be greater upon the bed.

The longitudinal dikes, between which the main channel is confined, should be placed as nearly as practicable in the direction which the channel has naturally assumed. If it be deemed advisable to change the position of the channel, it should be shifted to that side of the bed which will yield most readily to the action of the current.

748. In situations where large reservoirs can be formed near the bar, the water from them may be used for removing it. For this purpose an outlet is made from the reservoir, in the direction. of the bar, which is closed by a gate that turns upon a vertical axis, and is so arranged that it can be suddenly thrown open to let off the water. The chase of water formed in this way sweeping over the bar will prevent the accumulation of deposites upon it. This plan is frequently resorted to in Europe for the removal of deposites that accumulate at the mouth of harbors in those localities where, from the height to which the tide rises, a great head of water can be obtained in the reservoirs.

749. In the improvement of the mouths of rivers which empty into the sea through several channels, no obstruction should be placed to the free ingress of the tides through all the channels. If the main channel is subject to obstructions from deposites, dams should be built across the secondary channels, which may be so arranged with cuts through them closed by gates, that the flood-tide will meet with no obstruction from the gates, while the ebb-tide, causing the gates to close, will be forced to recede through the main channel, which, in this way, will be daily scoured, and freed from deposites by the ebb current. The same object may be effected by building dams without inlets across the secondary channels, giving them such a height that at a certain stage of the flood-tide, the water will flow over them, and fill the channels above the dams. The portion of water thus dammed in will be forced through the main channel at the ebb.

750. When the bed is obstructed by rocks, it may be deepened by blasting the rocks, and removing the fragments with the as*sistance of the diving-bell, and other machinery.

751. In some of our rivers, obstructions of a very dangerous character to boats are met with, in the trunks of large trees which are imbedded in the bottom at one end, while the other is near the surface; they are termed snags and sawyers by the boatmen. These obstructions have been very successfully re moved, within late years, by means of machinery, and by propelling two heavy boats, moved by steam, which are connected by a strong beam across their bows, so that the beam will strike the snag, and either break it off near the bottom, or uproot it Other obstructions, termed rafts, formed by the accumulation of drift wood at points of a river's course, are also found in some of our western rivers. These are also in process of removal, by cutting through them by various means which have been found successful.

752. Slack-Water Navigation. When the general depth of water in a river is insufficient for the draught of boats of the most suitable size for the trade on it, an improvement, termed slack-water, or lock and dam navigation, is resorted to. This consists in dividing the course into several suitable ponds, by forming dams to keep the water in the pond at a constant head; and by passing from one pond to another by locks at the ends of the dams.

753. The position of the dams, and the number requisite, will depend upon the locality. In streams subject to heavy freshets, it will generally be advisable to place the dams at the widest parts of the bed, to obtain the greatest outlet for the water over the dam. The dams may be built either in a straight line between the banks and perpendicular to the thread of the current, or they may be in a straight line oblique to the current, or their plan may be convex, the convex surface being up stream, or it may be a broken line presenting an angle up stream. The three last forms offer a greater outlet than the first to the water that flows over the dam, but are more liable to cause injury to the bed below the stream, from the oblique direction which the current may receive, arising from the form of the dam at top.

754. The cross section of a dam is usually trapezoidal, the face up-stream being inclined, and the one down-stream either vertical or inclined. When the down stream face is vertical, the velocity of the water which flows over the dam is destroyed by the shock against the water of the pond below the dam, but whirls are formed which are more destructive to the bed than would be the action of the current upon it along the inclined face of a dam. In all cases the sides and bed of the stream, for some distance below the dam, should be protected from the action of the current by a facing of dry stone, timber, or any other construction of sufficient durability for the object in view.