WORKS FOR THE PROTECTION OF BANKS
1. =Preliminary Remarks.=--The protection of a length of bank from
scour may be effected by spurs, which are works projecting into the
stream at intervals, or by a continuous lining of the bank. A spur
forms an obstruction to the stream (CHAP. IV., _Art. 1_), and when
constructed, or even partly constructed, the scour near its end may
be very severe, even though there may be little contraction of the
stream as a whole. If the bed is soft a hole is scoured out. Into
this hole the spur keeps subsiding, and its construction, or even its
maintenance, may be a matter of the greatest difficulty. A high flood
may destroy it. If it does not do so, it may be because the stream has,
for some reason, ceased to attack the bank at that place. A continuous
lining of the bank is not open to any objection, and is generally
the best method of protection. Spurs made of large numbers of rather
small trees, weighted with nets filled with stones, have been used on
the great shifting rivers of the Punjab which swallowed up enormous
quantities of materials. The use of spurs on such rivers has now, in
most cases, been given up. If L is the length of a spur measured at
right angles to the bank, the length of bank which it protects is about
7 L--3 L upstream and 4 L downstream,--but the spur has to be strongly
built, and its cost is, in many cases, not much less than the expense
of protecting the whole bank with a continuous lining.
Whatever method is adopted, a plan, large enough to show all
irregularities, should always be prepared, and the line to which it is
intended that the bank shall be brought marked on it.
[Illustration: FIG. 7.]
Sometimes natural spurs exist as, for instance, where a tree projects
into a stream or has fallen into it, and the holes between the spurs
may be deep, so that a continuous protection would be expensive. Or
there may be trees standing in such positions that, if felled, they
will be in good places for spurs. In cases such as the above, spurs may
be suitable even in a stream with a soft channel.
Regarding the use of spurs or groynes for diversion works or for
reducing the width of a stream, see CHAP. VII., _Art. 1_, and CHAP.
VIII., _Art. 3_.
2. =Spurs.=--A spur may be made of--
(_a_) Loose stone, which may be faced with rubble above low-water
level (fig. 7).
(_b_) Layers of fascines weighted with gravel or stones.
(_c_) Earth or sand closely covered with fascines.
(_d_) A double line of stakes with fascines or brushwood laid
between them (fig. 8).
(_e_) A single line of stakes with planking or basket work on its
upstream side, or with twigs or wattle laid horizontally and passed
in and out of the stakes, as in fig. 20.
(_f_) A single tree with the thick end of the trunk on the bank and
with stakes, if necessary, to prevent the current from moving it.
(_g_) A number of small trees heaped together and weighted with
nets full of stones.
(_h_) A layer of poles and over them a layer of fascines on which
are built walls of fascine work so arranged as to form cells or
hollow rectangular spaces which become filled with silt.
(_i_) Large fascines running out into the stream and having their
inner ends staked to the bank while the outer ends float, other
fascines being added over them and projecting further into the
stream, and the whole eventually sinking.
[Illustration: FIG. 8.]
Combinations of the above are also used, for instance, (_d_) or (_e_)
may be used for the upper portion, the foundation being (_a_) or (_c_).
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
[Illustration: FIG. 11.]
Instead of running out at right angles to the bank a spur may be
inclined somewhat downstream. This somewhat reduces the eddying and
scour round the end. The ends of a system of spurs should be in the
line which it is intended that the edge of the stream shall have
(fig. 9). The tops of short spurs are usually above high flood level.
Sometimes spurs are made to slope downwards (fig. 10), and they then
cause less disturbance of the water and less scour than if built to the
form shown by the dotted line. Such spurs are sometimes combined with
a low wall running across the bed of the stream, the whole forming a
“profile” of the cross-section to which it is intended to bring the
channel. Regarding such walls, see CHAP. V., _Art. 6_. When a spur is
long it may have small subsidiary spurs (fig. 11) to reduce the rush of
water along it; or its end may have to be protected in the same manner
as the advancing end of a closure dam (CHAP. VII., _Art. 2_).
The following is a curious case of misconception of the action
of spurs. In 1909 the river Indus was eroding its right bank and
threatening to destroy the town of Dera Ghazi Khan. A clump of date
palms formed a promontory and resisted erosion to some extent. A
suggestion was made--by an engineer of eminence who had formerly been
consulted in the case--to the effect that the date palms be removed,
the reason given being that they caused disturbance and scour. On this
principle spurs would have to be made not to protect a bank but to
cause it to be eroded.
3. =Continuous Lining of the Bank.=--The lining or protection of a
bank may be of stone or brick pitching (figs. 12 and 13), loose stone
(fig. 14), fascines (fig. 15), turfing, plantations, brushwood, or of
other materials laid on the slopes. Before protecting a bank it is
best to remove irregularities and bring it to a regular line. This can
generally be done most easily by filling in hollows, but sometimes it
is done by cutting off projections. It is also necessary to make the
side slope uniform. Where the slope is as shown by the dotted lines in
figs. 12 to 14, filling in can be effected, but cutting away the upper
part of the slope is also feasible. Such cutting away has been proposed
as a remedy in itself in cases where the steep upper part of the slope
was falling in, but it is not much of a remedy.
Stone pitching may rest, if boats are required to come close to the
bank, on a toe wall of concrete, as in fig. 13,[9] or otherwise on a
foundation of loose stone, as in fig. 12. When concrete is used the
bed is dredged to such a depth as will provide against undermining by
scour. Sloping boards attached to piles are placed along the front face
and the concrete is thrown in under water. The slope of stone or brick
pitching is usually from 2 to 1 to 1 to 1, but it may be as steep as
½ to 1. The earth behind the pitching must be well rammed in layers.
In order to prevent the earth from being eaten away by the water which
penetrates through the interstices of the stone or brick, a layer, 3
to 6 inches thick, of gravel or ballast is placed over the earth and
rammed. When loose stone is used, dredging is not necessary, but the
stone is allowed to gradually sink down and more is added at the top. A
certain proportion of the stones should be of large size.
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
When fascining is used, long twigs are made into bundles and tied up
at every 2 feet so as to form fascines about 4 to 6 inches thick, and
these are laid on the slopes and secured by pegs driven in at short
intervals, between the fascines.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
Sometimes the pitching or loose stone is not carried up to the top
of the bank, or even up to high flood-level, and the bank above the
pitching is protected by turfing--the pieces of turf being placed on
edge normally to the slope if very steep (fig. 14) or laid parallel to
the slope if it is not very steep--or, above ordinary water-level, by
plantations of osiers or willows which obstruct the water and tend to
cause silting, and whose roots bind the banks together.
[Illustration: FIG. 16.]
Another method of using fascines is to lay them on the slopes with
their lengths normal to the direction of the stream. The upper end of
a fascine is above low water, and the lower end extends down to the
bed of the stream. Sometimes large ropes made of straw, or rough mats
made of grass, are laid on the slopes and pegged down, or mattresses of
fascines are laid on the slopes and weighted with stones.
[Illustration: FIG. 17.]
A deep recess in the bank (fig. 16) can be filled in, before the
protection is added, with earth well rammed. On the Adige the filling
material consisted (fig. 17) of faggots filled with stones, small cross
dams being made at intervals, as shown by the dotted lines, to arrest
flood water and cause it to deposit silt. At the back of the berm,
poplar or willow slips were planted, and these grew up and their roots
held the bank together. This system succeeded well.
A method of protection which is suitable when the water contains
much silt is what is known in India as bushing. Large leafy branches
of trees are cut and hung, as shown in fig. 18, by ropes to pegs.
They must be closely packed so as not to shake. At first they require
looking after, but silt rapidly deposits and the branches become fixed
and no longer dependent on the ropes. If the work is carefully done,
the result is a smooth, regular, and tenacious berm, as per dotted line
in the figure.
[Illustration: FIG. 18.]
[Illustration: FIG. 19.]
Another method, used on canals, is to make up the bank with earth and
to revet it with twigs or reeds, as shown in fig. 19. The foundation
must be taken down well below bed-level, otherwise the work may slip.
This kind of work cannot be done except when the canal is dry.
If the bank consists of sand or of very sandy soil, it must in any
case have a flat side slope such as 3 to 1. If the sand is in layers
alternating with firm soil, it is a good plan to dig out some of the
sand and to replace it with clods of hard earth.
Staking (fig. 20) may be used, the stakes being one or two feet apart
from centre to centre, and long twigs laid horizontally being passed
in and out of the stakes, or bushing filled in behind the stakes. But
bushing alone is cheaper and nearly as good.
[Illustration: FIG. 20.]
For protecting the banks of the Indus it has been proposed (_Punjab
Rivers and Works_, CHAP. IV.) to use trees in exactly the same manner
as bushing, the trees being grown in several rows parallel to the river
so that whenever the river, by eroding its bank, comes up to the lines
of trees the first row will fall in. The first row would be chained to
the second, which would take the place of the pegs used in bushing. The
other rows would remain as a reserve.
The Villa system of bed protection (CHAP. V., _Art. 6_) has also been
successfully used for bank protection on the Scheldt, and on the
Brussels-Ghent Canal, the prisms being about 10 × 10 × 4 inches, and
having overlapping joints. The bands of prisms are placed in position
by a boat, the bands unrolling over a drum. The boat is provided with
an oscillating platform carrying rollers at its end. A thin layer of
gravel is laid over the bank and is pressed down by the rollers before
the prisms are laid on it (_Min. Proc. Inst. C.E._, vol. cxxxiv., and
vol. clxxv.).
In the case of the river mentioned in CHAP. XI., _Art. 3_, where
extremely high velocities were met with, cylindrical rolls of
wire-netting were made, each 50 feet long and 5 feet in diameter, and
filled with boulders. These rolls can be used for bank protection. The
netting was made by wires 6 inches apart, crossing each other at right
angles and tied together at the crossings by short pieces of wire.
[Illustration: FIG. 21.]
On ship canals a berm (fig. 21) is frequently made a few feet below the
water-level. It serves as a foundation for the pitching, which need
not usually extend down to more than 5 feet below the water-level.
Below that the wash has little or no effect on the banks. On ordinary
navigation canals a similar berm is sometimes made--one or two feet in
width and a foot or less below the water-level--and rushes are planted
on it.
Sometimes a bank has been protected by a kind of artificial weed,
consisting of bushes or branches of trees attached to ropes. The end
of the rope is fastened to the bank and the weeds float in the stream
alongside the bank.
To protect a bank from ice, which exercises an uplifting force on
pitching, use has been made of a covering of a kind of reinforced
concrete consisting of slabs of concrete with wires embedded in it,
and fastened to the bank by wires, 20 inches long, running into the
bank, these wires being embedded in mortar so as to act like stakes.
[Illustration: FIG. 22.]
4. =Heavy Stone Pitching with Apron.=--On the great shifting rivers of
India a system of bank protection is adopted, consisting of a pitched
slope with an apron (fig. 22). The system is used chiefly in connection
with railway bridges or weirs, but it has been used in one instance,
that of Dera Ghazi Khan, for the protection of the bank near a town.
When, as is usual, the flood-level is higher than the river bank, an
artificial bank is made. In any case the bank is properly aligned. The
pitching has a slope of 2 to 1, and consists of quarried blocks of
stone loosely laid, the largest blocks weighing perhaps 120 lbs. The
apron is laid at the time of low water on the sandbank or bed of the
stream. If necessary, the ground is specially levelled for it. It is
intended to slip when scour occurs. The following dimensions of the
apron are given by Spring (_Government of India Technical Paper_, No.
153, “River Training and Control on the Guide Bank System,” 1904).
The probable maximum depth of scour can be calculated as explained
in CHAP. XI., _Art. 3_. If this depth, measured from the toe of the
slope pitching is D, and if T is the thickness considered necessary
for the slope pitching, then the width of the apron should be 1·5 D,
and its thickness 1·25 T next the slope and 2·8 T next the river. It
will then be able to cover the scoured slope to a thickness of 1·25 T.
This thickness is made greater than T because the stone is not likely
to slip quite regularly. The thickness T should, according to Spring,
be 16 inches to 52 inches, being least with a slow current and a
channel of coarse sand, and greatest with a more rapid current and fine
sand; but since the sand is generally finer as the current is slower,
it would appear that a thickness of about 3 feet would generally be
suitable. Under the rough stone there should be smaller pieces or
bricks. Along the top of the bank there is generally a line of rails
so that stone from reserve stacks, which are placed at intervals along
the bank, can be quickly brought to the spot in case the river anywhere
damages the pitched slope.
For the special protection to banks required near weirs and similar
works, see CHAP. X., _Arts. 2_ and _3_.