DIVERSIONS AND CLOSURES OF STREAMS
1. =Diversions.=--When a stream is permanently diverted the new course
is generally shorter than the old one, and the diversion is then often
called a cut-off. The first result of a cut-off is a lowering of
the water-level upstream and a tendency to scour there, and to silt
downstream of the cut-off. Fig. 23 shows the longitudinal section of a
stream after a cut-off _A B_ has been made. The bed tends to assume the
position shown by the dotted line. If both the diversion and the old
channel are to remain open, the water-level at the bifurcation will be
lowered still more, and the tendency to scour in the diversion will be
reduced.
[Illustration: FIG. 23.]
If the material is soft enough to be scoured by the stream, it is often
practicable to excavate a diversion to a small section and to let it
enlarge itself by scour. This operation is immensely facilitated if
the old channel can be closed at the bifurcation. The question whether
the scoured material will deposit in the channel downstream of the
diversion must be taken into consideration; also the question whether
the diversion will continue to enlarge itself more than is desirable.
The velocity in the diversion will be a maximum if its section is
of the “best form,” _i.e._ if its bed and sides are tangents to a
semicircle whose diameter coincides with the water surface, but this
may not (CHAP. IV., _Art. 6_) be the section which will give most
scour. In order to prevent the enlargement of the diversion taking
place irregularly, the excavation can be made as shown in fig. 24,
water being admitted only to the central gullet. The side gullets
should not be quite continuous, but unexcavated portions should be left
at intervals, so that if the water in scouring out the channel breaks
into the gullet, it will not be able to flow along it until it has
broken in all along.
[Illustration: FIG. 24.]
If a diversion is made, not with the object of lowering the water-level
but merely in order to shorten the channel, the increased velocity
caused by the steepened slope may be inconvenient. In this case a weir
or weirs can be added (CHAP. VIII., _Art. 4_).
If the water contains sufficient silt to enable the abandoned loop to
be silted up within a reasonable time, it may be desirable to do this.
The silting up may, for instance, increase the value of the land. The
loop should be closed at its upper end. Water entering the lower end
will cause a deposit there. When the lower end is well obstructed by
silt, the upper end should be opened.
The set of the stream, due, for instance, to a bend at the point
where a diversion takes off has very little to do with the quantity
of water which goes down the diversion. The only effect of the set of
the stream is a slight rise of the water-level as compared with the
opposite bank. Similarly, the angle at which the diversion takes off
is only of importance in giving, in some cases, a velocity of approach
whose effect is generally small. The distribution of the water between
the diversion and the old channel really depends on their relative
discharging capacities. If the required quantity of water does not flow
down a diversion it can be dredged.
Sometimes a long spur is run out to send the water towards the off-take
of a diversion. The effect of this is very small--it merely causes a
set of the stream,--unless its length is so great that it amounts to
something like a closure dam. It is sometimes said that it is easier
to lead a river than to drive it. This remark is probably based on the
fact that spurs, such as those under consideration, generally produce
little effect, whereas the excavation of a diversion or the deepening
of a branch by dredging it, is more likely to produce some result.
There is, however, no certainty about this. Sometimes too much is
expected of such channels. Calculations are not always made as to the
scouring power of the stream, nor is account always taken of the fact
that as the cut scours its gradient flattens.
2. =Closure of a Flowing Stream.=--The closure of a flowing stream by
means of a dam is usually attended with some difficulty and sometimes
with enormous difficulty. There may be little trouble in running out
dams from both banks for a certain distance, but as soon as the gap
between the dams becomes much less than the original width of the
stream, the water on the upstream side is headed up and there is a rush
of water through the gap, which tends to deeply scour the bed and to
undermine the dams. The smaller the gap becomes the greater is the rush
and scour.
The closure is most easily effected at or near to the place where the
stream bifurcates from another. Then, as the gap decreases in width,
some of the water is driven down the other stream and it does not rise
so much. Eventually all the water goes down the other stream, and
the total rise is only so much as will enable this other stream to
carry the increased discharge. If the closure is not effected near a
bifurcation, the rise of the water will go on even after the closure is
completed, and it will not cease, unless the water escapes or breaks
out somewhere, until it has risen to the same level as that to which it
would have risen if the closure had been at the bifurcation, or perhaps
not quite to the same level, since there may still be a slight slope in
the water surface and a small discharge which percolates through the
dam. Sometimes in such a case it is possible to arrange for temporary
escapes or bifurcations, which will be shallow and therefore easily
closed, after the main closure has been completed.
A closure is, of course, far more easily effected where the bed is hard
than where it is soft. Very often it is best to close temporarily at
such a place or near a bifurcation, even if the permanent dam has to be
elsewhere, and then to construct the permanent dam in the dry channel,
or in the still water, and remove the temporary one or cut a gap in it.
Generally the best method to adopt in a closure is to cover the bed
of the channel beforehand--unless it is already hard enough--with a
mattress or floor, such that it cannot be scoured as the gap closes.
A floor may consist of a number of stones or sandbags dropped in from
boats or by any suitable means, and placed with care so that there
shall not be gaps or mounds. Sandbags should be carefully sewn up. A
mattress may be made of fascines laid side by side and tied together,
floated into position, weighted and sunk. Even a carpet made of matting
or cloth and suitably weighted has sufficed in some cases. If the
scour is likely to be such that stones or sandbags will be carried
away, the stones may be placed in nets, baskets, or crates. Sandbags
may also be placed in nets. Probably the long rolls of wire-netting
filled with stones, described in CHAP. VI., _Art. 3_, would be better
than anything, and the diameter could be reduced somewhat. The floor or
mattress need not usually extend right across the stream. It must cover
a width much greater than--perhaps twice as great as--the width of the
gap is likely to be when scour begins. Its length, measured parallel
to the direction of the stream, must be such that severe eddies in
the contracted stream will have ceased before the stream reaches its
downstream edge. It need not extend to any considerable distance
upstream of the line of the dam.
The dams when started from the banks can generally be of simple earth
or gravel, or loose stones, but before they have advanced far they
will probably require protection at the ends by stones, or by staking
and brushwood, or by fascines. As soon as the dams have advanced well
onto the mattress and their ends have been well protected, it is best
to cease contracting the stream from the sides and to contract it
from the bottom by laying a number of sandbags across the gap so as to
form a submerged weir. In this way the rush of water is spread over
a considerable width of the stream. The weir is then raised until it
comes up above water. Leakage can be stopped by throwing in earth, or
gravel, or bundles of grass on the upstream side. Sometimes it is best
to construct the mattress over the whole width of the stream, and to
effect the closure entirely by a weir, carrying each layer right across
before adding another. The banks of the stream, if not hard, can be
protected by sandbags, stones, staking or fascining.
The chief cause of failures of attempts to close flowing streams is
neglect to provide a proper floor or mattress. The stones or other
materials may be of insufficient weight or not closely laid, or the
extent of the floor may be insufficient. In a soft channel and deep
water loose stones in almost any quantity may fail unless a mattress of
fascines is laid under them. Another cause of failure is running short
of materials, such as sandbags. Allowance should be made for every
contingency, including making good any failure of parts of the work.
Enormous sums of money have been wasted, and vast inconvenience, loss
and trouble incurred, in futile attempts to close breaches in banks, or
gaps in dams.
Sometimes the gap is closed by sinking a barge loaded with stones, or
by sinking a “cradle” or large mattress made of fascines, taken out to
the site by four boats, one supporting each corner, and then loaded
with stones and sunk. Another method is to run out a floating mattress
of fascines from one side of the gap to the middle and sink it, then
to proceed similarly on the other side, and so on.
An excellent plan, when it can be adopted, is to have more than one
line of operations, so that the heading up of the water is divided
between them.
In India closures of streams having depths of 6 or 8 feet are effected
by means of rough trestles made from trunks of small trees and placed
at intervals in the stream like bridge piers, one leg of the trestle
inclined upstream and one downstream. Each pair of adjacent trestles
is connected by a number of rough, horizontal poles. Against these are
placed bundles of brushwood. Earth is at the same time collected and
is rapidly added at the last. The chief danger is the undermining of
the bed by scour. This is prevented by driving in stakes and placing
brushwood against them. Closures of small channels or of breaches in
the banks of canals are effected by means of staking and brushwood.
Where dangerous breaches are liable to occur, it is a good plan to have
a barge, fitted up with a small pile-driver and carrying a supply of
sheet piles, ready at a convenient spot.
Hurdle dykes, first used on the Mississippi, were employed on the
Indus in 1902 to close partially the main channel of the river.
There were to be three dykes, each dyke consisting of three lines of
very long piles--some were 60 feet long,--driven into the bed of the
stream, which was to be protected with mattresses made of fascines and
extending right across it, with their heads above flood-level. The
idea was not to wholly stop the flow of the water, but to obstruct it
so much that silt would deposit, the channel become choked up, and the
water find a course down another channel. The work was begun in March
1902, and was in progress in May of the same year when an unusually
early flood put a stop to it. The dykes had at this time advanced
considerable distances from the right bank of the stream, but none
had been completed. Two dykes out of the three were for the most part
carried away. The river, however, took a new course, starting from a
point far upstream, the western channel became a creek, and the remains
of the dykes were soon embedded in silt.
In any case in which the provision of a proper mattress has been
omitted, or when the mattress has been destroyed, or when a breach has
occurred in an embankment, whenever, in short, it is evident that the
gap cannot be closed until some other escape for the water is provided,
it may be possible to provide such an escape by cutting partly through
the dam or embankment on the downstream side at another place, and
thoroughly protecting the place and extending the protection downstream
and away from the dam or embankment. The water can then be let in, and
the closure of the old gap attempted. If a closure is effected, the
protected gap can then be closed. Sometimes it may be desirable to make
such a protected gap beforehand and with deliberation.
Dams for closing streams which are dry can be made similarly to flood
embankments (CHAP. XII., _Art. 7_). Sand does very well, provided it is
protected by a covering of clay or by fascining.
3. =Instances of Closures of Streams.=--In 1904 the Colorado River
broke into the Salton Sink--a valley covering 4000 square miles.
Unsuccessful attempts were made to close the stream by two rows of
piles with willows and sandbags between them, by a gate 200 feet long,
supported on 500 piles, and by twelve gates each 12 feet wide. A
“rock-fill” dam was then constructed on a mattress 100 feet wide and
1·5 feet thick. The river, which was 600 feet wide, broke through, but
was stopped by the construction of three parallel rock-fill dams in the
gap (_Min. Proc. Inst. C.E._, vol. clxxi.).
[Illustration: FIG. 25.]
At the site of the railway bridge over the river Tista in Bengal, it
was necessary to close the main stream (fig. 25), which flowed at the
left side of the channel, while the bridge had been built at the right.
The bed was of sand, width 500 feet, depth 6 feet, and discharge 3700
cubic feet per second. The first attempt to close the stream was made
at M N, a floor of stone 200 feet long, 20 feet wide, and 2 feet
thick, being laid in the middle of the stream, and dams of earth,
sandbags, and stones being run out from each bank. As the gap decreased
in width the bed was torn up and the work failed. The heading up was 3
feet 9 inches. It was recognised later that the site should have been
at the bifurcation higher up, and that the stone floor should have been
laid on a mattress.
In the next working season the dams C D and E F G were made. The dam C
D was of earth. Two walls, each consisting of a double line of bamboos
with the spaces between the lines filled with bundles of grass weighted
with earth, were run out 50 feet in advance of the earthwork near the
lines of the toes of the slopes. Along the line of the upper wall a
mattress of broken bricks 10 feet in width, and 1 foot thick, was laid,
and was kept 50 feet in advance of the wall. A total length of 1000
feet of embankment was made in five months and pitched on its upstream
side. The end was strongly protected by a mass of stone. The embankment
F G was of earth. The dam E F consisted of three lines of piles driven
10 feet into the bed. A mattress weighted with stones extended for 20
feet upstream of the dam and 40 feet downstream. A gap of 150 feet was
left at D E, and was not protected by a floor of any kind. A channel,
parallel to F G and extending to K, had been dug to a width of 200
feet. During the floods the heading up at D E was about 2·5 feet, and
the water was 30 feet deep. The line E F was greatly damaged and was
repaired. The cut F G K gradually enlarged, and by the end of the
floods more water was going down it than down the main stream. The gap
D E was finally closed by means of a line of bamboos and grass, the
bed being protected by a carpet, 100 × 50 feet, made of common cloth
weighted with sandbags. The success of the operations turned on the
scouring out of the cut F G K. It is remarkable that the gap D E did
not become wholly unmanageable in the floods (_Min. Proc. Inst. C.E._,
vol. cl.).