A treatise on Civil Engineering
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| From "A Treatise on Civil Engineering" by Dennis H Mahan, published in 1890 (that's almost 120 years ago)...
830. River Improvements. There is no subject that falls within the province of the engineer's art, that presents greater difficulties and more uncertain issues than the improvement of rivers. Ever subject to important changes in their regimen, as the regions by which they are fed are cleared of their forests and brought under cultivation, one century sees them deep, flowing with an equable current, and liable only to a gradual increase in volume during the seasons of freshets; while the next finds their beds a prey to sudden and great freshets, which leave them, after their violent passage, obstructed by ever shifting bars and elbows. Besides these revolutions brought about in the course oi years, every obstruction temporarily placed in the way of the current, every attempt to guard one point from its action by any artificial 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. 831. 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. 2nd. Those to prevent inundations of the surrounding country. 3rd. 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. 832. The erosion of the banks of rivers is often the main cause of changes in their regimen, such as the formation of sand banks, opening new channels, change of thalweg. This erosion, moreover, is very prejudicial to adjoining property, and should be strongly opposed. Various means for attaining this end are used. Among these are rip-rap, or random stone, paving the banks, plantations, timber revetment, coverings of fascines, and finally, projecting works called wingdams, spurs, jetties, or dykes. 1st. Rip-rap. Rip-rap, or random stone, carefully put down and properly kept up, is very effective. It may be economical to use it when stone can be easily obtained. A trench should be dug at the foot of the bank and carried below any expected scour. The stone should then be thrown in, with the smallest pieces at the bottom, and increasing in size to the top. The blocks on top should be heavy enough to prevent their being washed away by the current. 2nd. Paving. Pavements, or stone revetments, may be either of dry stone or of stone laid in hydraulic mortar. Dry stone has the advantage of readily adapting itself to the shape of the ground, but it has also the great disadvantage of falling in entirely in case the foot should be attacked. With good hydraulic masonry a thinner pavement and smaller stones may be used, and when a scour takes place at the foot, the upper part of the revetment forms an arch which may stand by itself until repairs are made. Whatever system be followed, the foot of the pavement should be well protected with rip-rap and the foundation should be put down with great care. 3rd. Plantations. Plantations also give a good protection, especially for the upper part of the banks. The common water willow is excellent for this purpose. It grows quickly and spreads rapidly. Willow sticks, about a foot and a half long and an inch in diameter, are stuck a foot into the ground along the bank. At the end of three or four years the plantation will have become a small wood, which will break the current. If it be desired to make ground, the larger branches toward the water may be cut half through, the upper end pointed and stuck in the ground. This growth becomes very thick, and the water within it almost stagnant. At each rise of the river a certain amount of deposit takes place, and the new ground is quickly made. On the Ohio it has been found that willows do not grow below the six-feet stage; consequently they afford no protection below that limit. They are mainly useful to prevent the wash of waves and steamboats. 4th. Timber revetments. These are only mentioned because they may serve a good purpose in an emergency for some threatened point, but for general work they are too perishable to be of much value. 5th. Fascines. In some countries a great deal has been done for the protection of banks by means of revetments of fascines. The elements of fascine work are pickets, fascines of large and small size, saplings, fascine rafts or mattresses, hurdles, reeds and straw. In France, on the Rhine, fascines have been employed to a great extent. This system is now but little used, on account of the cost of materials and labor, which have both increased very much; on the other hand, transportation has now become easy and cheap. Were works like those on the Rhine to be built to-day, large use would doubtless be made of masonry and rip-rap, as these could be easily carried by rail. 6th. Wing-dams. For the protection of the banks of the Rhine projecting fascine works were formerly used. These were called wing-dams, spurs, or jetties, and were placed normal to the direction of the current. It was found that they were frequently undermined by the scour at the outer end, and in some cases, when not very securely rooted to the bank, the river cut around behind them. The result of experience in the use of wing-dams has led to their abandonment to a great extent. However, it must be acknowledged that wingdams have given good results on several rivers where they have been properly built and arranged, and where care has been taken to give them an inclination in the direction of the current. On the Ohio river a number of dykes, or wingdams, nearly parallel to the current, have been built with excellent results. |