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.
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
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.
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
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_.