THE SILTING AND SCOURING ACTION OF STREAMS
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 V^2 D^2, and
the resisting force or weight as D^3. If these are just balanced, D
varies as V^2, or the diameters of similarly shaped bodies which can
just be rolled are as V^2 and their weights as V^6. 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 V^5.
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 V^2 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 V^2 and inversely
as D. The silt charge which a stream of depth D can carry is as V^{½}.
V is called the “critical velocity” for that depth, and is designated
as V_{0}.
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 V_{0} 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.
4. =Methods of Investigation.=--The quantity of silt in water is found
by taking specimens of the water and evaporating it or, if the silt is
present in great quantity, leaving it to settle for twelve hours--an
ounce of alum can be added for every 10 cubic feet of water to
accelerate settlement--drawing off the water by a syphon, and heating
the deposit to dry it. The deposit is then measured or weighed. It is
best to weigh it. If clay is filled into a measure, the volume depends
greatly on the manner in which it is filled in. When silt deposits in
large quantities in a channel, or when heavy scour occurs, the volume
deposited or scoured is ascertained by taking careful sections of the
channel.
[Illustration: FIG. 1.]
Silt is best classified by observing its rate of fall through still
water. A sand which falls at ·10 feet per second is, in India, called
class (·1), and mixed sand which falls at rates varying from ·1 to ·2
feet per second is called class ·1/·2. Fig. 1 shows a sand separator
designed by Kennedy. The scale is ⅛. It has a syphon action, and the
rate of flow can be altered by altering the length of the exit pipe.
Suppose it is desired to measure the sand of class (·10) and all
heavier kinds. The pipe is adjusted so as to give a velocity of ·1 foot
per second to the upward flowing water, which then carries off all silt
of class (·10) or finer. All heavier silt falls into the glass tube.
It can be separated again by being mixed with water and passed through
the instrument again, the velocity of flow through the instrument being
increased.
The quantity of silt present at various depths can be found by pumping
specimens of water through pipes. At each change of depth the pipe,
delivery hose, etc., should be cleaned. Allowance must be made for the
velocity of ascent of the water up the pipe. Suppose this to be 1·4
feet per second. Then the velocity of sand of class (·2) would be 1·2
feet per second, and the quantity of sand actually found in the water
would have to be increased by one-sixth.
5. =Quantity and Distribution of Silt.=--The quantity of silt present
in water varies enormously. Fine mud, even though sufficient to
discolour the water, may be so small in volume that it only deposits
when the water is still, and even then deposits slowly. In the river
Tay, near Perth, the silt was found to be ordinarily 1/10,000 of the
volume of water, and at low water only 1/28,000. In the river Sutlej at
Rupar, near where it issues from the Himalayas, the silt in the flood
season is extremely heavy. Out of 360 observations, made at various
depths, during the flood seasons of four successive years, in water
whose depth ranged up to 12 feet, the silt was once found to be 2·1
per cent. by weight of that of the water. It was more than 1·2 per
cent. on four occasions, and more than 0·3 per cent. (or 3 in 1000)
on sixty-four occasions. Generally about one-half of the silt was
clay and sand of classes finer than (·10), about one-third was sand
of class ·1/·2, and the residue was sand of class ·2/·3. The sand of
the river Chenab is generally coarser than that of the Sutlej. There
are very great differences in the degree of coarseness of river sand.
The sand in any river becomes finer and finer as the gradient flattens
in approaching the sea. Sea sand has been found to be of class (·20).
In the Sirhind Canal, which takes out from the Sutlej at Rupar, the
maximum quantity of suspended silt observed in the four flood seasons
was 0·7 per cent., on one occasion out of 270, and it exceeded 0·3 per
cent. on twenty-five occasions. About 80 per cent. of the silt was clay.
In another part of the paper quoted, it is stated that the silt
suspended in the canal water averaged, during the whole of one flood
season, about 1/1700 of the volume of the water. This would be about
1/1200 by weight. The silt deposited in the bed of the canal, in a
period of a few days, was sometimes as much as 1/1000 of the water
which had passed along, and occasionally as much as 1/500. It was
nearly all sand, only about 3 per cent. being clay. Silt of classes
finer than (·1) gave no trouble, and were to be eliminated in future
investigations. In a canal, as in a river, the sand on the bed becomes
finer the further from the head.
Regarding the distribution of the silt at various depths, in water 5 to
17 feet deep, the quantity of silt near the bed may, when the charge is
heavy and consists of mixed silt, be 1¼ to 3 times that at the surface.
If the charge is fine mud, there is likely to be as much silt at the
surface as near the bed, if sand, there may be none at the surface and
little in the upper part of the stream.
In all cases single observations are likely to show extraordinarily
discordant results; a number of observations must be made at each point
and averaged.
6. =Practical Formulæ and Figures.=--A stream which carries silt
generally rolls materials along its bed. The proportion between the
quantities of material rolled and carried is never known, and this
makes it impossible to frame an exact formula applicable to such cases,
but Kennedy, from his observations on canals fully charged with the
heavy silt and fine sand usually found in Indian rivers near the hills,
arrived at the empirical formula for critical velocities
V = ·84 D^{·64}
The observations were made on the Bari Doab Canal and its branches, the
widths of the channels varying from 8 feet to 91 feet, and the depths
of water from 2·3 feet to 7·3 feet. The beds of these channels have,
in the course of years, adjusted themselves by silting or scouring, so
that there is a state of permanent régime, each stream carrying its
full charge of silt, and the charges in all being about equal. From
further observations referred to above (_Art. 3_, par. 2) it appears
that this kind of silt forms about 1/3300 of the volume of the water,
and that on the Sirhind Canal, sand coarser than the (·10) class,
formed 1/35,000 of the volume of water.
The formula gives the following critical velocities for various
depths:--
D = 1 2 3 4 5 6 7
V_{0} = ·84 1·30 1·70 2·04 2·35 2·64 2·92
D = 8 9 10
V_{0} = 3·18 3·43 3·67.
In Indian rivers not near the hills the silt carried is not so heavy,
and the critical velocities are supposed to be about three-fourths of
the above. Thrupp (_Min. Proc. Inst. C.E._, vol. clxxi.) gives the
following ranges of velocities as those which will enable streams to
carry different kinds of silt. It does not appear that the streams
would be fully charged except at the higher figure given for each case.
D = 1·0 10·0
V = 1·5 to 2·3 3·5 to 4·5 (Coarse sand).
V = ·95 to 1·5 2·3 to 3·5 (Heavy silt and fine sand).
V = ·45 to ·95 1·2 to 2·3 (Fine silt).
It cannot be said that the exact relations between D and V are yet
known, but it is of great practical importance to know that V must vary
with D. The precise manner in which it must vary does not, for moderate
changes, make very much difference. In designing a channel a suitable
relation of depth to velocity can be arranged for, and one quantity or
the other kept in the ascendant, according as scouring or silting is
the evil to be guarded against.
The old idea was that an increase in V, even if accompanied by an
increase in D, _e.g._ simply running a higher supply in a given
channel, gave increased silt-transporting power. In a stream of very
shallow section this is probably correct, for V increases faster than
D^{·64} (_Hydraulics_, CHAP. VI., _Art. 2_). In a stream of deep
section a decrease in D gives increased silt-transporting power.
If the discharge is fixed, a change in the depth or width must be
met by a change of the opposite kind in the other quantity. In this
case widening or narrowing the channel may be proper according to
circumstances. In a deep section widening will decrease the depth
of water, and may also increase the velocity, and it will thus give
increased scouring power. In a shallow section, narrowing will increase
the velocity more than it increases D^{·64}. In a medium section it is
a matter of exact calculation to find out whether widening or narrowing
will improve matters.
If the water entering a channel has a higher silt-charge than can be
carried in the channel, some of it must deposit. Suppose an increased
discharge to be run, and that this gives a higher silt-carrying power
and a smaller rate of deposit per cubic foot of discharge, it does not
follow that the deposit will be less. The quantity of silt entering
the channel is now greater than before. Owing to want of knowledge
regarding the proportions of silt and rolled material, and to want of
exactness in the formulæ, reliable calculations regarding proportions
deposited cannot be made.
The channels in which the observations above referred to were made have
all assumed nearly rectangular cross-sections, the sides having become
vertical by the deposit on them of finer silt; but the formula probably
applies approximately to any channel if D is the mean depth from side
to side, and V the mean velocity in the whole section.
If the ratio of V to D differs in different parts of a cross-section,
there is a tendency towards deposit in the parts where the ratio is
least, or to scour where it is greatest. There is a tendency for the
silt-charge to adjust itself, that is, to become less where the above
ratio is less, but the irregular movements of the stream cause a
transference of water among all parts, and this tends to equalise the
silt-charge.
Dubuat gives the following as the velocities close to the bed which
will enable a stream to scour or roll various materials. The bed
velocity is probably less than the mean velocity in the ratio of about
·6 to 1 in rough channels, and about ·7 to 1 in smooth channels:--
Gravel as large as peas ·70 feet per second
” ” French beans 1·0 ” ” ”
” 1 inch in diameter 2·25 ” ” ”
Pebbles 1½ inch in diameter 3·33 ” ” ”
Heavy shingle 4·0 ” ” ”
Soft rock, brick, earthenware 4·5 ” ” ”
Rock of various kinds 6·0 ” ” ”
and upward.
The figures for brick, earthenware, and rock can apply only to
materials of exceedingly poor quality. Masonry of good hard stone
will stand 20 feet per second, and instances have occurred in which
brickwork has withstood a velocity of 90 feet per second without injury
so long as the water did not carry sand and merely flowed along the
brickwork. If there are abrupt changes in the stream, causing eddies,
or if there is impact and shock, or if sand, gravel, shingle, or
boulders are liable to be carried along, velocities must be limited.
7. =Action on the Sides of a Channel.=--It has been seen that the laws
of silting and scour on the bed of a channel depend on the ratio of the
depth to the velocity. The same laws probably hold good in the case of
a gently shelving bank, so that here again V ought to vary as D^{·64}.
The velocity near the angle where the slope meets the water surface
seems to decrease faster than D^{·64}. At all events, silt tends to
deposit in the angle and the slope to become steep.
When the slope is steep the law seems to be different, the tendency for
deposit or scour to occur on the bank depending on the actual velocity
without much relation to the depth. The velocity very near to a steep
bank is always low relatively to that in the rest of the stream. Thus
there is often a tendency for silt to deposit on the bank, especially
in the upper part, and for the side to become vertical except for a
slight rounding at the lower corner. A bank may receive deposits when
the bed may be receiving none, and it may have a persistent tendency
to grow out towards the stream. The growth of the bank is generally
regular, the line of the bank being preserved, but it may be irregular,
especially if vegetation, other than small grass, becomes established
on the new deposits.
When scour of the sides of a channel occurs it may occur by direct
action of the stream on the sides near the water-level, or by action
at or near the toe of the slope, which causes the upper part of the
bank to fall in. Such falling in is generally more or less irregular,
and the bank presents an uneven appearance. The fallen pieces of bank
may remain, more or less intact, especially if they are held together
by the roots of grasses, etc., where they fell, and prevent further
scour occurring along the toe of the slope. Falling in of banks is most
liable to occur in large streams and with light soils. It may be caused
by the waves which are produced by steamers and boats or, especially in
broad streams, by wind. The action on the banks at bends is discussed
in _Art. 8_.
Thus in designing a channel according to the principles laid down in
_Art. 6_, the question of action on the sides of the channel has to
be dealt with as follows. Whether or not the velocity is to be low,
relatively to the depth, _i.e._ whether or not deposit on the bed
is more likely to occur than scour, care can be taken not to make it
actually too low, and not to make it actually too high, particularly if
the soil is light and friable. With ordinary soils a mean velocity of
3·3 feet per second in the channel is generally safe as regards scour
of the sides. Any velocity of more than 3·5 feet per second may give
trouble. A velocity of less than 1 foot per second is likely to give
rise to deposit on the sides.
In channels in alluvial soils the falling in of banks is sometimes said
to occur more when the stream is falling than at other times. This has
been noticed on both the Mississippi and the Indus. The cause has been
said to be the draining out of water which had percolated into the
bank, the water in flowing out carrying some sand with it. The effect
of this cannot however be great.
8. =Action at Bends.=--At a bend, owing to the action of centrifugal
force and to cross-currents caused thereby, there is a deposit near the
convex bank and a corresponding deepening--unless the bed is too hard
to be scoured--near the concave bank. The water-level at the concave
bank is slightly higher than at the convex bank. The greatest velocity
instead of being in mid-stream is nearer the concave bank.
As the transverse current and transverse surface slope cannot commence
or end abruptly, there is a certain length in which they vary. In
this length the radius of curvature of the bend and the form of the
cross-section also tend to vary. This can often be seen in plans of
river bends, the curvature being less sharp towards the ends.
When once a stream has assumed a curved form, be it ever so slight,
the tendency is for the bend to increase. The greater velocity and
greater depth near the concave bank react on each other, each inducing
the other. The concave bank is worn away, or becoming vertical by
erosion near the bed, cracks, falls in, and is washed away, a deposit
of silt occurring at the convex bank, so that the width of the stream
remains tolerably constant. The bend may go on increasing, and it often
tends to move downstream.
[Illustration: FIG. 2.]
In fig. 2 the deep places are shown by dotted lines. Along the straight
dotted line there is no deep place. Such a line would be used for a
ford. At low water it becomes a shoal. This is the chief reason why a
tortuous stream at low water consists of alternate pools and rapids.
It is sometimes said that deep water occurs near to a steep hard bank.
Such deepening is due to bends or obstructions which give the current a
set towards the bank, or it is due to irregularities in the bank which
cause eddies. In a straight channel with even and regular banks there
is no such deepening.
When a bend has formed in a channel previously straight, the stream
at the lower end of the bend, by setting against the opposite bank,
tends to cause another bend of the opposite kind to the first. Thus
the tendency is for the stream to become tortuous and, while the
tortuosity is slight, the length, and therefore the slope and velocity,
are little affected; but the action may continue until the increase
in the length of the stream materially flattens the slope, and the
consequent reduction in velocity causes erosion to cease. Or the stream
during a flood may find, along the chord of a bend, a direct route
with, of course, a steeper slope. Scouring a channel along this route
it straightens itself, and its action then commences afresh. Short
cuts of this kind do not, however, occur so frequently as is sometimes
believed. In some streams the bends acquire a horse-shoe shape and the
neck becomes very narrow and short cuts may then occur. Otherwise they
are not common. V increases only as √(S), and if the country is covered
with vegetation it is not easy for a stream to scour out a new channel.
The effect of bends on the velocity of a stream is not well understood.
In case of a bend of 90° the increased resistance to flow when the bend
is absolutely sudden (a sudden bend is known as an “elbow”) amounts
perhaps to V^2/(2_g_). Whether it is greater or less in the case of
a gentle bend of 90° is not known. In the case of a pipe there is a
certain radius which gives a minimum resistance (_Hydraulics_, CHAP.
V.). The increased resistance at a bend is due partly to the fact that
the maximum velocity is no longer in the centre of the stream, and
partly to the fact that the velocities at the different parts of the
cross-section have to be rearranged at the commencement of the bend and
again at its termination. Thus the effect of a bend of 45° is a good
deal more than half of that of a bend of 90°. Two bends of 45°, both
in the same direction, with a straight reach between them, will cause
more resistance than a single bend of 90° with the straight reach above
or below the bend. If the two bends of 45° are in different directions
the resistance will be still greater. A succession of sharp bends may
produce a serious effect, amounting to an increase in roughness of
the channel. A succession of gentle bends, of any considerable angle,
cannot of course occur within a moderate length of channel.
When there is head to spare there is clearly no objection to bends,
except that the bank may need protection. At a place where the bank has
in any case to be protected, _e.g._ at a weir, there is no objection to
an elbow.
9. =General Tendencies of Streams.=--Since the velocity is greater
as the area of the cross-section is less, a stream always tends
to scour where narrow or shallow, and to silt where wide or deep.
The cross-section thus tends to become uniform in size. Suppose
two cross-sections to be equal in size but different in shape. The
velocities of the two sections will be equal. The tendency of the bed
to silt will (_Art. 6_) be greater at the deeper section and, when
silting has occurred on the bed, the section will be reduced and there
will be a tendency to scour at the sides. Thus the cross-sections tend
to become also uniform in shape. If a bank of silt has formed in a
stream, the tendency is for scour to occur. There is also a tendency
for silt to deposit just below the point where the bank ends. Hence a
silt bank often moves downstream.
Owing to the tendency to scour alongside of, or downstream of,
obstructions (_Art. 1_), it is clear that a stream constantly tends to
destroy obstructions.
There is an obvious tendency for silt to deposit where the bed slope
of a stream flattens, and for scour to take place where it steepens
(_Hydraulics_, figs. 16 and 17, pp. 24 and 25), and thus the tendency
is for the slope to become uniform.
In a natural stream flowing from hilly country to a lake or sea, the
slope is steepest at the commencement and gradually flattens. There
is thus a tendency for the bed to rise except at the mouth of the
stream. This rising tends to increase the slope and velocity in the
lower reaches, and this again enhances the tendency, described in the
preceding article, of the stream to increase in tortuosity.
When a silt-bearing stream overflows its banks the depth of water on
the flooded bank is probably small and its velocity very low, and a
deposit of silt takes place on the bank. When the deposit has reached
a certain height it acts like a weir on the water of the next flood,
which flows quickly over it and, instead of raising it higher, deposits
its silt further away from the stream. In this way a strip of country
along the stream gradually becomes raised, the raising being greatest
close to the stream. The country slopes downwards in going away from
the stream. In other words, the stream runs on a ridge. If the bank
becomes raised so high that flooding no longer occurs, the raising
action ceases, but if, as is likely in alluvial country, the bed of the
stream also rises, the action may continue and the ridge become very
pronounced.
Some rivers have very wide and soft channels which are only filled
from bank to bank in floods, if then. The deep stream winds about in
the channel, and the rest of it is occupied by sandbanks and minor
arms. The winding is the result of the velocity being too great for
the channel. The streams, especially the main stream, constantly shift
their courses by scouring one bank or the other. Now and then the main
stream takes a short cut, either down a minor arm or across an easily
eroded sandbank. This is a very different matter from a short cut
across high ground. The sandbanks receive deposits of silt in floods,
but are constantly being cut away at the sides. Such rivers frequently
erode their banks to an extraordinary extent. The Indus sometimes cuts
into its bank 100 feet or more in a day, and it may cut for half a mile
or more without cessation. The tortuosity of such a stream increases as
it gets nearer the sea. The actual length of the Indus in the 400 miles
nearest the sea is 39 per cent. greater than its course measured along
the bank. In the reach from the 600th to the 700th mile from the sea,
the difference is only 3 per cent. For a detailed description of some
such rivers, see _Punjab Rivers and Works_.
Sometimes general statements are made regarding silting or scour in
connection, for instance, with a stream which is confined between
embankments or training walls, or has overflowed its banks or is held
up by a weir. It is impossible to say that any such condition, or any
condition, will cause silting or scour, unless the velocity depth and
silt charge are known.