Chapter IV · THE SILTING AND SCOURING ACTION OF STREAMS
Reading edition
1. Preliminary Remarks.—When flowing water carries solid substances in suspension, they are known as “silt.” Material is also moved by being rolled along the bed of the stream. The difference between silt and rolled material is one of degree and not of kind. Material of one kind may be rolled and carried alternately. The quantity of silt present in each cubic foot of water is called the “charge” of silt. Silt consists chiefly of mud and fine sand; rolled material of sand, gravel, shingle, and boulders. When a stream erodes its channel, it is said to “scour.” When it deposits material in its channel, it is said to “silt.” Both terms are used irrespective of whether the material is silt or rolled material. A stream of given velocity and depth can carry only a certain charge of silt. When it is carrying this it is said to be “fully charged.”
If a stream has power to scour any particular material from its channel, it has power to transport it; but the converse is not true. If the material is hard or coherent, the stream may have far more difficulty in eroding it than in merely keeping it moving. And there is generally a little more difficulty even when the material is soft.
Silting or scour may affect the bed of a channel or the sides or both. The channel may thus decrease or increase in width or—if one bank is affected more than, or in a different manner to, the other—alter its position laterally whether or not it is altering its bed level, and vice versa.
The cross-section of a stream is generally “shallow,” i.e. the width of the bed is greater than the combined submerged lengths of the sides, and the action on the bed is generally greater than on the sides.
Silting and scouring are generally regular or irregular in their action according as the flow is regular or irregular, that is, according as the channel is free or not from abrupt changes and eddies. In a uniform canal fed from a river, the deposit in the head reach of the canal forms a wedge-shaped mass, the depth of the deposit decreasing with a fair approach to uniformity. Salient angles or places alongside of obstructions are most liable to scour, and deep hollows or recesses to silt. Eddies have exceptionally strong scouring power. Immediately downstream of an abrupt change scour is often severe. An abrupt change is one, whether of sectional area or direction of flow, and whether or not accompanied by a junction or bifurcation, which is so sudden as to cause eddies. The hole scoured alongside of an obstruction may extend to its upstream side, though there is generally little initial tendency to scour there. An obstruction is anything causing an abrupt decrease in any part of the cross-section of a stream, whether or not there is a decrease in the whole cross-section, e.g. a bridge pier or spur.
Most streams vary greatly at different times both in volume and velocity and in the quantity of material brought into them. Hence the action is not constant. A stream may silt at one season and scour at another, maintaining a steady average. When this happens to a moderate extent, or when the stream never silts or scours appreciably, it is said to be in “permanent régime,” or “stable.” Most streams in earthen channels are either just stable and no more, or are unstable.
Waves, whether due to wind or other agency, may cause scour, especially of the banks. Their effect on the bed becomes less as the depth of water increases, but does not cease altogether at a depth of 21 feet, as has been supposed. Salt water possesses a power of causing mud, but not sand, to deposit.
Arts. 2, 3, and 6 of this chapter refer to action on the bed of a stream. Action on the sides will be considered in Art. 7.
Weeds usually grow only in water which has so low a velocity that it carries no silt to speak of, but if any silt is introduced the weeds cause a deposit. The weeds also thrive on such a deposit.
2. Rolled Material.—If a number of bodies have similar shapes, and if D is the diameter of one of them and V the velocity of the water relatively to it, the rolling force is theoretically as V2 D2, and the resisting force or weight as D3. If these are just balanced, D varies as V2, or the diameters of similarly shaped bodies which can just be rolled are as V2 and their weights as V6. From practical observations, it seems that the diameters do not vary quite so rapidly as they would by the above law, the weights being more nearly as V5.
Let a stream of pure water having a depth D, and with boulders on its bed, have a velocity V just sufficient to move them very slowly. Any larger boulders would not be moved. Any smaller boulders would move more quickly. Similarly, fine sand would be rolled more quickly than coarse sand. If the velocity of the stream increases, larger boulders would be moved. Streams are thus constantly sorting out the materials which they roll. If the bed is examined it will be found that large boulders exist only down to a certain point, smaller boulders, shingle, gravel, coarse sand and fine sand following in succession.
If the water, instead of being pure, is supposed to contain silt, this may affect its velocity—it is not, however, known to do so—but, given a certain velocity, it is not likely that the rolling power of the stream is much affected by its containing silt.
It is sometimes supposed that increased depth gives increased rolling power, because of the increased pressure, but this is not so. The increased pressure due to depth acts on both the upstream and downstream sides of a body. It is moved only by the pressure due to the velocity.
When sand is rolled along the bed of a stream there is usually a succession of abrupt falls in the bed. After each fall there is a long gentle upward slope till the next fall is reached. The sand is rolled up the long slope and falls over the steep one. It soon becomes buried. The positions of the falls of course keep moving downstream. The height of a fall in a large channel is perhaps 6 inches or 1 foot, and the distance between the falls 20 or 30 feet. A fall does not usually extend straight across the bed but zigzags.
It has sometimes been said that the inclination of the bed of a stream, when high, facilitates scour, the material rolling more easily down a steep inclined plane. The inclination is nearly always too small to have any appreciable direct effect. The inclination of the surface of the stream of course affects its velocity, and this is the chief factor in the case.
A sudden rise in the bed of a stream does not necessarily cause rolled materials to accumulate there, except perhaps to the extent necessary to form a gentle slope. Frequently even this slope is not formed, especially if the rolled material is only sand. The eddies stir it up and it is carried on. The above remarks apply also to weirs or other local rises in the bed.
3. Materials carried in Suspension.—It has long been known that the scouring and transporting power of a stream increases with its velocity. Observations made by Kennedy have shown that its power to carry silt decreases as the depth of water increases (Min. Proc. Inst. C.E., vol. cxix.). The power is probably derived from the eddies which are produced at the bed. Every suspended particle tends to sink, if its specific gravity is greater than unity. It is prevented from sinking by the upward components of the eddies. If V is the velocity of the stream and D its depth, the force exerted by the eddies generated on 1 square foot of the bed is greater as the velocity is greater, and is probably as V2 or thereabouts. But, given the charge of silt, the weight of silt in a vertical column of water whose base is 1 square foot is as D. Therefore the power of a stream to support silt is as V2 and inversely as D. The silt charge which a stream of depth D can carry is as V½. V is called the “critical velocity” for that depth, and is designated as V0.
The full charge must be affected by the nature of the silt. The specific gravity of fine mud is not much greater than that of water, while that of sand is about 1·5 times as great. Moreover, the particles of sand are far larger than the particles of mud. If two streams of equal depths and velocities are fully charged, one with particles of mud and the other with particles of sand, the latter will sink more rapidly and will have to be more frequently thrown up. They will be fewer in number. From some observations referred to by Kennedy (Punjab Irrigation Paper, No. 9, “Silt and Scour in the Sirhind Canal,” 1904), it appears that in a fully charged stream which carried 1/3300 of its volume of a mixture of mud and sand of various grades, sand of a particular degree of coarseness formed only 1/35,000 of the volume of the water, but that when the same stream was clear and was turned on to a bed of the coarse sand it took up 1/15,000 of its volume. It would thus appear that the full charge of silt is less as its coarseness and heaviness are greater. This is in accordance with the laws mentioned above (Art. 2, par. 1). See also Chap. V., Art. 2, last paragraph.
It is probable that fine mud is carried almost equally into all parts of the stream, whereas sand is nearly always found in greater proportion near the bed and, as before remarked, some materials may be rolled and suspended alternately. The charge of mixed silt which a stream can carry is, no doubt, something between the charge which it can carry of each kind separately, but the laws of this part of the subject are not yet fully known. From the observations above referred to, Kennedy concludes that a canal with velocity V0 will carry in suspension 1/3300 to 1/5000 of its volume of silt, according as it is charged with sand of all classes or only with the heavier classes.
Let a stream be carrying a full charge of any kind of silt. Then if there is any reduction in velocity, a deposit will occur—unless there is also a reduction of depth—until the charge of silt is reduced again to the full charge for the stream. The deposit generally occurs slowly, and extends over a considerable length of channel. The heavier materials are, of course, deposited first. If a stream is not fully charged, it tends to become so by scouring its channel. It is generally believed that a stream fully charged with silt cannot scour silt from its channel, or bear any introduction of further silt. This seems to be correct in the main, but the remarks made in the latter part of the preceding paragraph must be taken into consideration.
It has been stated (Art. 2) that a weir or a sudden rise in the bed does not necessarily cause an accumulation of rolled material. It never causes a deposit of suspended material unless it causes a heading up and reduction of velocity to below the critical velocity.