CHAPTER V

METHODS OF INCREASING OR REDUCING SILTING OR SCOUR

1. =Preliminary Remarks.=--Most important works which affect the régime of a stream have some effect on its silting or scouring action, but this is not generally their chief object. Such works will be dealt with in due course, and the effects which they are likely to produce on silting or scouring will be mentioned. In the present chapter only those works and measures will be considered whose chief object is to cause a stream to alter its silting or scouring action. It does not matter, so far as this discussion is concerned, whether the object is direct, _i.e._ concerned only with the particular place where the effect is to be produced, or indirect, as, for instance, where a stream is made to scour in order that it may deposit material further down the stream. The protection of banks from scour is considered in CHAP. VI. Dredging is dealt with in CHAP. VIII.

2. =Production of Scour or Reduction of Silting.=--Sometimes the silt on the bed of a stream is artificially stirred up by simple measures, as, for instance, by scrapers or harrows attached to boats which are allowed to drift with the stream, or by means of a cylinder which has claw-like teeth projecting from its circumference and is rolled along the bed, or by fitting up boats with shutters which are let down close to the bed and so cause a rush of water under them, or by anchoring a steamer and working its screw propeller. It is thus possible to cause a great deal of local scour, but the silt tends to deposit again quickly, and it is not easy to keep any considerable length of channel permanently scoured. The system is suitable in a case in which a local shallow or sandbank is to be got rid of and deposit of silt a little further down is not objectionable. It may be suitable in a case in which the bed is to be scoured while a deposit of silt at the sides of the channel is required, especially if some arrangement to encourage silt deposit at the sides is used (_Art. 3_, par. 4; also CHAP. VI., _Art. 3_).

Holding back the water by means, for instance, of a regulator or movable weir, and letting it in again with somewhat of a rush, will, if frequently repeated, have some effect in moving silt on in the downstream reach. Regarding the upstream reach, it has been remarked (CHAP. IV., _Art. 3_) that a weir does not necessarily cause silt deposit. If, in a stream which does not ordinarily silt, a regulator or movable weir causes, when the water is headed up, some silt deposit, the cessation of the heading up not only removes the tendency to silt, but the section of the stream, at the place where the deposit occurred, is less than elsewhere, and there is thus a tendency to scour there. If a regulator is alternately closed and opened, no permanent deposit of much consequence is likely to occur.

A stream may be made to scour its channel by opening an escape or branch. This causes a draw in the stream, and an increase in velocity for a long distance upstream of the bifurcation (_Hydraulics_, CHAP. VII., _Art. 6_). This procedure is sometimes adopted on irrigation canals. The escape is generally opened in order to reduce the quantity of water passing down, but it may be opened solely to induce scour or prevent silting. The floor of the escape head is usually higher than the bed of the canal, but this does not interfere with operations except at low supplies. It may (CHAP. IV., _Art. 2_) have some effect on the quantity of rolled material passed out of the escape.

If there is a weir in the river below the off-take of the canal, and if the escape runs back to the river and thus has a good fall, the scouring action in the canal may be very powerful.

If the main channel has a uniform slope throughout, the slope of its water surface is greater upstream of the escape than downstream of the escape, and there is thus an abrupt reduction of velocity and possibly a deposit of silt in the main channel below the escape. This may or may not be objectionable. In the case of an irrigation canal, it is far less objectionable than deposit in the head of the canal. The best point for the off-take of any escape or scouring channel depends on the position of the deposits in the main channel. The off-take should be downstream of the chief deposits, but as near to all of them as possible. A breach in a bank acts of course in the same way as an escape.

A stream of clear water when sent down a channel will scour it if the material is sufficiently soft. In the case of the Sirhind Canal, it has already been mentioned (CHAP. IV., _Art. 3_), that when the river water became clear after the floods the proportion of coarse sand, _i.e._ sand above the (·10) class, carried by the canal water was about 1/15,000 by volume. This was in the period from 22nd September to 7th October. From 8th to 23rd October the proportion averaged 1/32,000 from 24th October to 8th November 1/44,000, and from 9th to 24th November 1/85,000. The reason of this reduction was that the comparatively clear water kept picking up the sand from the bed and moving it on, the finer kinds being moved most quickly. As the coarse sand left on the bed became less in quantity, the water took up less. It appears, however, that the water also picked up some clay which was left, and that the total suspended silt in November was 1/9000 of the water. All the observations mentioned in this paragraph appear to have been made at Garhi, 26 miles from the head of the canal.

3. =Production of Silt Deposit.=--Works or measures for causing silt deposit may be undertaken in order to cause silt deposit in specific places where it will be useful, or in order to free the water from silt. Sometimes both objects are combined.

If a stream can be turned into a large pond or low ground--a bank being built round it if necessary--it can be made to part with some or all of its silt whether rolled or suspended. Even if the pond is so large that the velocity becomes imperceptible, the whole of the suspended matter will not deposit unless it has sufficient time, but the matter which remains in the water is likely to be extremely small in amount. The silting up of marshes, pools, borrow-pits, etc., is now being effected, or should be effected, in places where mosquitoes and malaria are prevalent.

In the upper or torrential part of a stream, a high dam, provided with a sluice and a high-level waste weir, may be built across it. The space above the dam becomes more or less filled with gravel, etc. This has been done in Switzerland (_Min. Proc. Inst. C.E._, vol. clxxi.). In the U.S.A. long weirs have been built in order to stop the progress of detritus from gold mines. Such detritus was liable to choke up rivers and damage the adjoining lands. The detritus from hill torrents can also be reduced by afforestation of the hill sides.

When a stream is in embankment--irrigation channels are frequently so--the bank can be set back (fig. 3), and suspended silt will then deposit on the berms. The object of this arrangement is generally to create very strong banks in low ground. A similar plan can be adopted when the berm is only slightly below the water-level and even when it is only occasionally submerged. In this case the deposit of a small bank of silt along the edge of the berm next the stream will prevent the access of fresh supplies of silt-bearing water to the parts further away. Gaps should be cut in the bank of silt at intervals, and cross banks made to form “silting tanks,” as shown in fig. 4. The inlets to the tank should be large, and the outlets small, so that the water in the tank may have little velocity. It is not, however, correct to have the outlet so small--unless the water contain very little silt--that there is very little flow through the tank. The tanks will generally be silted up most quickly by allowing a good flow through them, even though only a small proportion of the silt in the water is deposited. Regular banks arranged to form tanks on the above principle can be made behind the original banks of a canal in cases where the original banks were not, for any reason, set back.

When a channel is made in low ground and the excavation is not sufficient to make the banks, borrow-pits can be dug in the bed of the channel. Such pits should not be long and continuous, but wide bars should be left so that a number of short pits will result. These pits will trap rolled material as well as suspended silt. The object in this case is to free the water from silt and to reduce the size of the channel and thus reduce the loss of water from percolation.

On the Indus, where it has a strong tendency to shift westwards, long earthen dams or groynes are run out from the west bank across the sandbanks. One object is to cause silt deposit, and so increase the quantity of material which the river will have to cut away, but whether this result is achieved is doubtful. The sandbanks receive deposits in any case. A groyne may increase the deposit on its upstream side, but it cuts off the flood water from its downstream side and so reduces the deposit there.

4. =Arrangements at Bifurcations.=--At a bifurcation, as where a branch takes off from a canal, it is possible to reduce the quantity of rolled material entering the canal by raising its bed or constructing a weir or “sill” in its head. This arrangement may have great effect in excluding boulders, shingle, or gravel. As regards rolled sand, it has much less effect than might be expected (CHAP. IV., _Art. 2_). If the canal is reduced in width (fig. 5) there will be eddies below the bed level of the branch. They will stir up the sand and some of it will enter the branch. If the canal is not reduced in width, eddies will be produced in the surface water, and they will affect the bed.

The above remarks apply also to the case of a canal taken off from a river when there are no works in the river.

5. =A Canal with Headworks in a River.=--In the case of a canal taking off from a river and provided with complete headworks, it is possible to do a great deal more. The case of the Sirhind Canal, already referred to (CHAP. IV., _Arts. 5_ and _6_), is a notable example. The canal (fig. 6) is more than 200 feet wide, the full depth of water 10 feet, and the full discharge about 7000 cubic feet per second. In 1893 when the irrigation had developed, and it became necessary to run high supplies in the summer--July, August, and part of September--the increase in the silt deposit threatened to stop the working of the canal. In the autumn and winter, say from 25th September to 15th March, the water entering the canal is clear and much of the deposit was picked up by it, but not all. In the five years 1893 to 1897 inclusive, the following remedial measures were adopted. Increased use was made of the escape at the twelfth mile. This did some good, but there was seldom water to spare. In 1893 to 1894 the sill of the regulator was raised to 7 feet above the canal bed, and it was possible to raise it 3 feet more by means of shutters. This had little effect. The coarsest class of sand was ·4, and the velocity of the water, even of that part of it which came up from the river bed and passed over the sill, was over 2 feet per second, so that all sand was carried over. In 1894 to 1895 the divide wall, which had been only 59 feet long, was lengthened to 710 feet, so as to make a pond between the divide wall and the regulator,[8] but probably the leakage through the under-sluices was often as much as the canal supply, and the water in the pond was thus kept in rapid movement and full of silt. The canal was closed in heavy floods. This did some good, but probably the canal was often closed needlessly when the water looked muddy but contained no excessive quantity of sand. The above comments on the measures taken were made by Mr Kennedy when chief engineer. The above measures did not reduce the silt deposits, but the scour in the clear water season improved, probably because higher supplies were run owing to increased irrigation. The deposit in the upper reaches of the canal, when at its maximum about the end of August of each year, was generally more than twenty million cubic feet. From the year 1900 a better system of regulation was enforced, the under-sluices being kept closed as much as possible, so that there was much less movement in the pond and much less silt in its water. By 1904 the deposit in the canal had been reduced to three million cubic feet, and no further trouble occurred.

During the period from 20th September 1908 to 10th October 1908 the quantity of silt in the canal above Chamkour (twelfth mile) decreased from 19,325,800 cubic feet to 12,477,600 cubic feet. The quantity scoured away was 6,848,200 cubic feet. During this period no silt entered the canal. The quantity which passed out of the reach in question in suspension was 4,183,660 cubic feet, so that 2,664,540 cubic feet of material must have been rolled along the bed. The rolled material was 64 per cent. of the suspended material. During this period the Daher escape, in the twelfth mile, was open, and the mean velocity in the canal just above the escape was about 4 feet per second, the depth of water being about 10 feet. The velocity near the escape was thus greater than the critical velocity for mixed silt (CHAP. IV., _Art. 6_), and even a long way up the canal it would be in excess of the critical velocity. The water seems to have carried about 1/1800 of its volume of silt. Whether the above proportions of rolled to suspended matter would hold good in a fully charged stream flowing with the critical velocity it is not easy to say.

As silt deposits in the pond, the velocity of the water in it, along the course of the main current towards the canal, increases and eventually the water begins to carry coarse sand dangerous for the canal. In order to ascertain when this state of affairs has been reached, two methods of procedure are possible. One is to frequently test specimens of the water in the pond along the course of the main current and see when it contains more than 1/15,000 of its volume of coarse sand. This plan would be troublesome and liable to error, and is rejected by Kennedy, who suggests that the depth and velocity of the water in the pond be frequently observed along the course of the main current. As soon as the velocity exceeds the critical velocity for mixed silt, it is time to close the canal and open the under-sluices and scour out the deposit from the pond. The period in which most silt is believed to have been deposited in the canal is the spring and early summer, say from 15th March to 1st July. This is the time when the snows are melting and the river water is clear. It can then carry more sand than in the rains--1st July to 15th September,--when it is muddy.

Kennedy also suggests that some under-sluices should be provided at the far side of the river, _i.e._ at the right-hand side of the weir. It would then be possible, by opening them, to let floods pass without interfering with the pond.

The two spurs or groynes, shown in the plan, were constructed in 1897 so as to cause the stream to flow along the face of the canal regulator and not allow deposits to accumulate there. The depth of silt deposited in a great part of the pond amounted at times to 8 or 10 feet.

6. =Protection of the Bed.=--It is possible to afford direct protection from scour to the bed of a stream by constructing walls across it, but unless the walls are near together the protection will not be effective. An arrangement used in some streams in Switzerland consists of tree trunks secured by short piles and resting on brushwood. But as long as the walls are not raised above the bed they cannot entirely stop scour, unless extremely close together. If raised above the bed they form a series of weirs.

The weirs must be so designed that the depth of water in a reach between two weirs is great enough to reduce the velocity down to the critical velocity, or less. The fall in the water surface at each weir being very small, the discharge over the weir can be found by considering it as an orifice extending up to the downstream water surface, and the head being the fall in the surface at the weir.

To stop scour of the bed by direct protection without raising the water-level, the bed can be paved, a plan adopted in artificial channels with very high velocities. The paving can be of stones, bricks, or concrete blocks. The Villa system of protection, which has been used in Italy, France, and Spain, consists of a flexible covering laid on the bed. Prisms of burnt clay or cement are strung on several parallel galvanized iron wires, which are attached to cross-bars so as to form a grid a few feet square. The grids are loosely connected to one another at the corners, and the whole covering adjusts itself to the irregularities of the bed (_Min. Proc. Inst. C.E._, vol. cxlvii.).

The special protection or paving required in connection with weirs and such-like works is considered in CHAP. X., _Arts. 2_ and _3_.