PROPOSED IMPROVEMENTS IN IRRIGATION CANALS
1. Preliminary Remarks 158
2. Reduction of Losses in the Channels 159
3. Modules 162
APPENDICES
A. Divide Wall on Lower Chenab Canal 169
B. Specification for Maintenance of Channels 171
C. Specification for Maintenance of Masonry Works 174
D. Watching and Protecting Banks and Embankments 175
E. Specification for Bushing 178
F. Escapes 180
G. Gauges 183
H. Gibb’s Module 186
K. Kennedy’s Gauge Outlet 193
INDEX 196
PREFACE
When _River and Canal Engineering_ was written it was decided to omit
Irrigation works and to deal with them separately because the subject
interests chiefly specialists.
The present book deals with the principles which govern the design and
management of Irrigation works, and it discusses the Canals of Northern
India--the largest and best in the world--in detail.
Some years ago a number of rules for designing distributaries were
framed, at the request of the Punjab Government, by the late Colonel S.
L. Jacob, C.I.E., R.E., and comments on these rules were obtained from
many experienced engineers and recorded. The author has had the
advantage of reading all these opinions. Generally the weight of opinion
on any point agrees with what most experienced engineers would suggest,
and direct conflicts of opinion scarcely occur. Important papers have
been printed by the Punjab Irrigation Branch on Losses of Water and the
Design of Distributaries, on the great Triple Canal Project, on Gibb’s
Module, on Kennedy’s Gauge Outlet, and on the Lining of Watercourses.
These papers are not always accessible to engineers, and the chief
points of interest in them are not, in most cases, discernible at a
glance. Such points have been extracted and are given in this book.
E. S. B.
CHELTENHAM, _May_ 20_th_, 1913.
IRRIGATION WORKS.
CHAPTER I.
INTRODUCTION
1. =Preliminary Remarks.=--The largest irrigation canals are fed from
perennial rivers. When the canal flows throughout the year it is called
a “Perennial Canal.” Chief among these are the canals of India and
particularly those of Northern India, some of which have bed widths
ranging up to 300 feet, depths of water up to 11 feet and discharges up
to 10,000 c. ft. per second. Other large canals as for instance many of
those in Scinde, Egypt and the Punjab, though fed from perennial rivers,
flow only when the rivers are high. These are called “Inundation
Canals.” Many canals, generally of moderate or small size, in other
countries and notably in the Western States of America, in Italy, Spain,
France and South Africa, are fed from rivers and great numbers of small
canals from reservoirs in which streams or rain-water have been
impounded. Sometimes water for irrigation is pumped from wells and
conveyed in small canals. In Australia a good deal of irrigation is
effected from artesian wells. Irrigation works on a considerable scale
are being undertaken in Mexico and the Argentine. In this book,
irrigation works of various countries are referred to and to some extent
described, but the perennial canal of Northern India, with its
distributaries, is the type taken as a basis for the description of the
principles and methods which should be adopted in the design, working
and improvement of irrigation channels and it is to be understood that
such a canal is being referred to where the context does not indicate
the contrary. Any reader who is concerned with irrigation in some other
part of the world will be able to judge for himself how far these
principles and methods require modification. The branches and
distributaries--all of which are dealt with--of a large perennial canal
cover all possible sizes.
CHAPTER II. of this book deals with the design of canals and CHAPTER
III. with the working of canals but as the two subjects are to some
extent interdependent, they will both be dealt with in a preliminary
manner in the remaining articles of the present Chapter. CHAPTER IV.
describes the Punjab Triple Canal Project.[1] CHAPTER V. deals with
certain proposed improvements in the working of canals.
[1] The latest example of canal design.
2. =General Principles of Canal Design.=--The head of a canal has to be
so high up the river that, when the canal is suitably graded, the water
level will come out high enough to irrigate the tract of land concerned.
If a river has a general slope of a foot per mile and if the adjoining
country has the same slope and is a foot higher than the water level of
the river, and if a canal is made at a very acute angle with the river,
with a slope of half a foot per mile, the water level about two miles
from the canal head will be level with the ground.
The headworks of the canal consist of a weir--which may be provided with
sluices--across the river, and a head “regulator,” provided with gates,
for the canal. There are however many canals, those for instance of the
inundation canal class, which have no works in the river and these may
go dry when the river is low. They usually have a regulator to prevent
too much water from going down the canal during floods. If a canal is
fed from a reservoir the headworks consist simply of a sluice or
sluices.
A canal must be so designed as to bring the water to within reasonable
distance of every part of the area to be irrigated. Unless the area is
small or narrow the canal must have “branches” and “distributaries.” A
general sketch of a large canal is given in Fig. 1. On a large canal,
irrigation is not usually done directly from the canal and branches. It
is all done from the distributaries.
[Illustration: FIG. 1.]
From each distributary “watercourses” take off at intervals and convey
the water to the fields. A small canal, say one whose length is not more
than 15 miles or whose discharge is not more than 100 c. feet per
second, may be regarded as a distributary and the word distributary will
be used with this extended meaning.
It is not always the case that the whole tract covered by a system of
canal channels is irrigated. In the case of a canal fed from a river,
the land near the river is often high or broken and the main canal runs
for some distance before it reaches the tract to be irrigated. Again,
within this tract there are usually portions of land too high to be
irrigated. Those portions of the tract which can be irrigated are called
the “commanded area.”
The channels of a large irrigation system should run on high ground. In
the case of a distributary, this is necessary in order that the
water-courses may run downhill, and since the water in the canal and
branches has to flow into the distributaries, the canal and branches
must also be in high ground. Another reason for adopting high ground is
that all the channels should, as far as possible, keep away from the
natural drainage lines of the country and not obstruct them. Also a
channel in high ground is cheapest and safest. When a channel is in low
ground it must have high banks which are expensive to make and liable to
breach. Every tract of country possesses more or less defined ridges and
valleys. When the ridges are well defined, the irrigation channels,
especially the distributaries, follow them approximately, deviating
slightly on one side or the other from the very top of the ridge in
order to secure a more direct course. If any part of a ridge is so high
as to necessitate deep digging the channel does not necessarily go
through it. It may skirt it and return to the crest of the ridge further
on, especially if this arrangement shortens the channel or at least does
not lengthen it much. A channel also goes off the ridge sometimes when
adherence to it would give a crooked line. Of course all the
channels--canals, branches and distributaries--have to flow more or less
in the direction of the general slope of the tract being dealt with.
[Illustration: FIG. 2.]
The alignments of the channels do not, however, depend exclusively on
the physical features of the country. Centrality in the alignment is
desirable. It will be shown (CHAP. II. Art. 10) that a distributary
works most economically when it runs down the centre of the tract which
it has to irrigate. It is better to have short watercourses running off
from both sides of a distributary than long watercourses from only one
side. The same is true of a branch; it should run down the centre of its
tract of country. Again the angles at which the channels branch off have
to be considered. If branches were taken off very high up the canal and
ran parallel to and not far from it, there would be an excessive length
of channel. But neither should the branches be so arranged as to form a
series of right angles. In the case shown in Fig. 2 the size of the
main or central canal would of course be reduced at the point A. By
altering the branches to the positions shown in dotted lines their
length is not appreciably increased while the length A B is made of the
reduced instead of the full size. Moreover the course B C is more direct
than BAC and this may be of the greatest importance as regards gaining
the necessary command. When a channel bifurcates, the total wet border
always increases and there is then a greater loss from absorption. The
water is always kept in bulk as long as possible. If the alignment of a
branch is somewhat crooked it does not follow that straightening
it--supposing the features of the country admit of this--will be
desirable. It may increase the length of distributaries taken off near
the bends. It will be shown (CHAP. II. Art. 10) that a distributary
ought, when matters can be so arranged, to irrigate the country for two
miles on either side of it, and watercourses should be two or three
miles long. A distributary need not therefore extend right up to the
boundary of the commanded area but stop two or three miles from it.
Generally it is not desirable to prolong a distributary and make it
“tail” into another channel (CHAP. II. Art. 3). A distributary, like a
canal, may give off branches.
None of the rules mentioned in the preceding paragraph are intended to
be other than general guides, to be followed as far as the physical
features of the country permit, or to assist in deciding between
alternative schemes. It may for instance be a question whether to
construct one distributary or two, between two nearly parallel branches.
The two-mile rule may enable the matter to be decided or it may
influence the decision arrived at as to the exact alignments of the
branches. The flatter the country and the less marked the ridges the
more the alignment can be based on the above rules. Sometimes, as in the
low land adjoining a river, the ridges are ill defined or non-existent
and the alignment is based entirely on the above rules. The rule as to
following high ground need not be adhered to at the tail of any
distributary if all the land to be irrigated at the tail is low and if
there is a deep drainage line or other feature of the country such as to
preclude the possibility of an extension of the distributary. Possible
extensions should always be considered. In hilly districts an irrigation
canal may have to run in sidelong ground along the side of a valley.
In flat valleys, owing to the land nearest the river having received
successive deposits of silt in floods, the ground generally slopes away
from the river and a canal can irrigate the low land even if taken off
at right angles to the river. But to irrigate the high land near the
river and the land where it rises again towards the hills or watershed,
a canal taking off higher up the river is necessary. Of course much
depends on whether the canal is to irrigate when the river is low or
only when it is high, and whether or not there is to be a weir in the
river. In Upper Egypt, it is common for a high level canal taking off
far upstream, to divide into two branches, one for the land near the
river and one for the land towards the watershed, and for both branches
to be crossed--by means of syphons--by a low-level canal which irrigates
the low ground. Similar arrangements sometimes occur on Indian
inundation canals.
Regulators are usually provided at all off-takes of branches. In the
case of a channel taking off from another channel many times its own
size there is generally only the head regulator of the smaller channel
but in other cases there is a regulator in each channel below the
bifurcation. Thus, when the number of bifurcating channels is two it is
called a double regulator. Regulators, with the “falls”--introduced to
flatten the gradients when the slope of the country is too steep--and
drainage crossings and the bridges, provided at the principal roads,
constitute the chief masonry works on a canal. At a fall, mills are
often constructed or the fall may be used for electric power.
Regarding curves and bends in channels, it is explained in _River and
Canal Engineering_ that, as regards increased resistance to flow and
consequent tendency to silt deposit, curves of fair radius have very
little effect, that a curve of a given angle may perhaps have the same
effect whether the radius is great or small but that if the radius is
large a succession of curves cannot be got into a short length, that a
succession of sharp curves in a short length may have great effect,
amounting to an increase of N in Kutter’s co-efficient, that a single
sharp curve has not much effect, that the chief objection to such a
curve is the tendency to erosion of the bank, that at a place where the
channel has, in any case, to be protected, as for instance just below a
weir or fall, there is no objection to the introduction of a sharp bend
and that such bends, in fact right-angled elbows, exist without any evil
effects at many regulators when the whole supply is being turned into a
branch. It is remarkable that on perennial canals no advantage is ever
taken of the last mentioned fact. Cases undoubtedly occur, though
somewhat infrequently, in which the most suitable and cheapest
arrangement would be to give a canal an abrupt bend at a fall. In order
to reduce eddying, the bend need not be an absolute elbow but can be
made within the length of the pitching which would be curved instead of
straight. This is frequently done on inundation canals, without the
slightest drawback, even when there is no fall, the pitching at bridges
being utilised. A pitched bend can be made anywhere.
When a river floods the country along its banks as in parts of Egypt and
of the Punjab, it is generally necessary to construct marginal
embankments before irrigation can be introduced. The canal may take off
at a point where flooding does not occur or it may pass through the
embankment.[2] If it passes through at a point where flooding occurs, a
masonry regulator is constructed to prevent the floods from enlarging
the gap and breaking into the country.
[2] For detailed accounts of such embankments and canals see _Punjab
Rivers and Works_ (Spon) 1912.
A large canal is provided, so far as is practicable, with “escapes” by
means of which surplus water may be let out. Surplus water occurs
chiefly after rain. At such times the demand for water may suddenly be
reduced and if there were no escapes there would probably be serious
breaches of the banks before there was time for the reduction of water,
effected at the head of the canal, to take effect lower down. There is
usually an escape at some point in the main line, preferably at a point
where it divides into branches, and this escape runs back to the river.
There may also be escapes near the tails of the longest branches. These
escapes may discharge into drainages or into reservoirs formed by
running a low embankment round a large area of waste land.
The drainage of the whole tract irrigated by a canal must be carefully
seen to. The subsoil water level of a tract of country is nearly always
raised by an irrigation canal. The rise near to a canal or distributary
is due to percolation from the channel and is inevitable.[3] The rise at
places further away, if it occurs, is due to over-watering or to neglect
of drainage. Immense damage has been done by “water-logging” of the soil
when irrigation water has been supplied to a tract of flat country and
the clearance and improvement of the natural drainages has not been
attended to. Any drainages crossed by the banks of the irrigation
channels should be provided with syphons or aqueducts or else the
drainage diverted into another channel. Very frequently the main line of
a canal--whether great or small--in the upper reaches near the hills,
has to cross heavy drainage channels or torrents and large and expensive
works are required for this.
[3] But see Chap. V. as to reduction of percolation.
Near the head of a canal and of every branch and distributary, there is
an ordinary gauge which shows the depth of water and is read daily. The
gauge near the head of a main canal is generally self-registering.
The principles sketched out in this article are those generally followed
in the designs of modern canals. They have by no means been followed in
all cases. In some of the older Indian canals both the canal and the
distributaries ran in low ground. Water-courses took off direct from the
canals, and the irrigation did not generally extend far from the canal.
In fact long distributaries were impracticable because they would have
run into high ground. The banks of the channels obstructed drainages and
caused pestilential swamps. Most canals of this type have been
abolished since the advent of British rule and replaced by others
properly designed. Some badly designed canals however, mostly of the
inundation class, still exist but in very dry tracts where drainages are
of little consequence.
3. =Information Concerning Canals.=--Nearly all canal irrigation is done
by “flow,” the water running from the water-courses onto the fields, but
a small proportion is done by “lift.” This is done in the case of high
pieces of land, the lifting being usually done by pumps or, in the east,
by bullocks or by manual labour.
Irrigation generally consists in giving the land a succession of
waterings, one previous to ploughing and others after the crop is sown,
each watering being of quite moderate depth. On inundation canals in
India the waterings for the summer crop are thus effected but for the
winter crop the land is deeply soaked during the flood season and is
afterwards ploughed and sown. In Upper Egypt this system is emphasised,
the water flowing into vast basins, formed by dykes, where it stands for
some time and, after depositing its silt, is drained off.
Until recent times the whole of the irrigation of Egypt was basin
irrigation. In Lower Egypt the construction of the Nile barrage led to
the introduction of canals which take off at a proper level and their
working is not restricted to the period when the river is in flood. In
Upper Egypt most of the irrigation is still basin irrigation but the
canals taking off above the Assiut barrage form a notable exception. By
means of the Assouan dam which crosses the Nile, the water during the
latter part of the flood season and after the floods are over, i.e. from
November to March, is ponded up and a vast reservoir formed and the
impounded water is let down the river in May and June.
In some of the older irrigation canals of India the velocity was too
high and the channels have since had to be remodelled and the crests of
weirs raised or new weirs built. The more recent canals are free from
grave defects of this kind but every canal undergoes changes of some
kind and finality has never yet been quite attained.
On some Indian irrigation canals made about 30 years ago, great sums of
money were wasted in making the canals navigable. There is nothing like
enough navigation to pay for the extra cost. The idea has now been quite
given up except as regards timber rafting from upstream. This requires
no curtailment of the velocity in the channels. The requirements of the
irrigation and navigation were always in conflict. The mere fact that
branches have to be worked in turns is enough to prevent navigation
succeeding.
In India the water used for irrigation is paid for, not according to the
volume used but according to the area irrigated. The volume used in any
particular watercourse is not known. The areas sown are measured.
Certain kinds of crops use up more water than others and the charges are
fixed accordingly.
In the canals which have their headworks among the mountains of Western
America there are frequent tunnels and syphons and the canals often run
in steep sidelong ground. There are great lengths of tunnel and syphon
in the Marseilles and Verdun Canals and there are long tunnels in the
Periyar Canal in Madras and in the Upper Swat Canal in the North West
Frontier Province of India.
The Tieton Canal, Washington, U.S.A., traverses steep sidelong ground
which would be liable to slip if a large cutting were made. The
cross-section of the channel is a circle, 8-ft. 3¹⁄₂-ins. in diameter,
with the upper part removed, so that the depth is 6 feet. It is made of
reinforced concrete 4 inches thick and the sides are tied together by
iron bars which run across the channel above the water. In the Santa Ana
Canal the channel consists for 2¹⁄₂ miles of a flume made of wooden
“staves.”
A canal constructed in Wyoming, U.S.A., after taking off from a river,
passes through a tunnel into another valley and is turned into another
stream which thus becomes the canal. This is said to save loss of water
by percolation. The stream is winding while a canal could have been made
straighter. There may, owing to the ground near the stream being
saturated, have been less loss of water at first than there would have
been in the artificial channel but, owing to the smaller wetted area,
there would probably have been an eventual saving in adopting the
latter. The real advantage of adopting the natural stream was probably a
saving in the cost of construction. (_Min. Proc. Inst. C.E._ Vol.
CLXII.)
Irrigation from canals which are supplied from reservoirs differs in no
respect from that from other canals. The principles on which reservoir
capacities should be calculated and earthen and masonry dams constructed
are given in _River and Canal Engineering_. Sometimes, as for instance
when a reservoir becomes seriously reduced in size owing to silt
deposit, the water is run off after the bed of the reservoir has been
soaked, and crops are grown on the soaked soil.
The distribution of the water of a canal as between the main channel and
the branches, is effected by means of the regulators at the
bifurcations. When the supply is ample and the demand great, the
channels may all be running nearly full. When the demand exceeds the
supply, the water may be reduced proportionately in each branch but this
may result in the water of a branch being too low to give proper
supplies to the distributaries or some of them, and in the water of a
distributary not commanding the higher ground. Moreover it violates the
principle of keeping the water in bulk as far as possible. It is more
usual to give each branch full supply, or a certain large fraction of
the full supply, in turn, and similarly with the distributaries.
The method of distribution from a distributary to the watercourses
varies. In many modern canals there is, at each watercourse head, a
sluice which is adjusted at frequent intervals according to the supply
and the demand. One method, which is excellent because it fulfils in the
highest degree the principle of keeping the water in bulk, is to have
very large watercourses and, by means of regulators which are built at
frequent intervals, to turn the whole of the water of the distributary
into a few watercourses at a time, beginning with those nearest the head
of the distributary and working downstream. But a system which seems
eminently suitable may be impracticable because of local circumstances.
In India, any such arrangement would need an army of officials and would
lead to unbounded corruption.
In India the water from a distributary enters the watercourses through
“outlets” which are small masonry tubes passing through the banks of the
distributary. There is no easy way of closing these outlets or at least
of keeping them closed if the cultivators choose to open them, but it is
easy to close a whole distributary and so regulate the supply. This is
the chief reason why watercourses in India do not usually take off
direct from the canals.
The presence of silt in the water of a river from which a canal is drawn
is often spoken of as being a great evil. If it is an evil at all it is
a very mixed evil. The deposits of silt in the channels have been
enormously reduced by the application of scientific principles of
design. The clayey silt which remains in the water and reaches the
fields, brings to them greatly increased fertility.
In India the fertility of the soil is often reduced or destroyed by the
formation on the surface of the ground of an efflorescence called “reh.”
It consists of various salts or compounds of sodium and occurs chiefly
where there is an impervious layer of subsoil. The salts exist as an
ingredient of the upper soil. This becomes saturated with rain or canal
water and as the water evaporates the salts are left on the surface.
Remedies are drainage, or flooding the soil and running the water off,
or deep tilling, or chemical treatment with lime or gypsum. (_Indian
Engineering, 8th Jan., 1910_).
The inundation canals of the Punjab have been described in _Punjab
Rivers and Works_. All descriptions and remarks in the present book
regarding Indian canals must be assumed to refer to perennial canals
unless the contrary is stated or implied.
4. =Losses of Water.=--When water flows or stands in an earthen channel
or tank, or is spread over a field, losses occur from evaporation,
percolation and absorption. Of these, absorption is by far the most
important and, unless the contrary is stated or implied, it will be
taken to include the others. The losses by evaporation are very small.
The loss by evaporation from the surface of the water, even in the hot
season in India when a hot wind often blows, does not exceed half an
inch in 24 hours and on the average in India is only about a tenth of an
inch in 24 hours.
[Illustration: FIG. 3.]
Percolation and absorption are described as follows by Beresford in
_Punjab Irrigation Paper_, No. 10, “The Irrigation Duty of Water.”
Percolation consists in flow through the interstices of boulders,
shingle, gravel or coarse sand. The flow is similar to that in pipes.
The water percolating into the soil from a channel, extends downwards
and spreads outwards as it descends. None of it goes upwards. In fine
sand and ordinary soil the interstices act like capillary tubes. The
water is absorbed as by a sponge and it remains in the soil by virtue of
capillarity. Owing to the combined action of capillarity and gravity the
water spreads in the manner shown by the dotted lines in Fig. 3. The
amount of absorption from a channel will be greater the greater the area
of the wetted surface. In a high embankment with narrow banks, the
absorption ceases when the water reaches the outer slopes, except in so
far as it is evaporated from the slopes. Moreover high embankments are
generally in clayey soil. If banks of sand are constructed on a layer of
clay (Fig. 4.) and well rammed, the absorption ceases as soon as the
banks are saturated and the channel then holds water as well as any
other except for evaporation from the outer slopes, but if the bed and
subsoil are also of sand the absorption of the water will be far
greater. Absorption ceases when the water extends nearly down to the
level of the subsoil water, i.e., to a point where the effect of
absorption from above plus gravitation is equal to the effect of
absorption from below minus gravitation. If a bottle is filled with
water and a small sponge jammed into the neck and the bottle turned
upside down, the sponge becomes saturated but no water will be given
out. But if a dry sponge is placed in contact with the wet one it will
absorb moisture until saturated.
[Illustration: FIG. 4.]
It is known that the loss of water is greatly influenced by the nature
of the soil. When water is turned into a dry channel or onto a field,
the loss is at first great. It decreases hourly and daily and eventually
becomes nearly constant, tending to reach a fixed amount when the water
extends down to nearly the level of the subsoil water. Observations made
by Kennedy on loamy fields near the Bari Doab Canal in India showed that
on a field previously dry the rate of absorption is given by the
equation
_y_ = ·0891_x_^{·86.}
Where _y_ is the depth of water absorbed in feet and _x_ is the time in
hours. The observations extended over eight days. Denoting by _c_ the
depth of water in feet absorbed in one hour, it was found that on a
field on which no rain had fallen for two months, _c_ was ·04 to ·05 but
on the second watering of the crop about a month later _c_ was ·02 to
·03 and about the same on a third watering. It was found that at the
first commencement the rate of absorption was much affected by the state
of the surface of the ground but that the effect was only temporary. The
losses were found to be as follows:
+------+--------------+---------------+
| DAY. |LOSS PER DAY. | LOSS PER HOUR.|
| | | (_c_) |
+------+--------------+---------------+
| | Feet. | Feet. |
| 1st | 1·36 | ·057 |
| 2nd | 1·13 | ·047 |
| 3rd | 1·07 | ·046 |
| 4th | 1·02 | ·043 |
| 5th | ·96 | ·041 |
| 6th | ·90 | ·037 |
| 7th | ·80 | ·033 |
| 8th | ·77 | ·032 |
| | Total 8·01 | |
+------+--------------+---------------+
In the eight days the total loss was almost exactly eight feet.
The losses by absorption in the various channels of certain canals has
been estimated to be as follows:--
---------------+-------+----------+-------+-----------+---------------
Channel. | Nature|Mean depth| Value | Loss per | Remarks.
| of | of water | of | Million |
| soil. | in | (_c_) |square feet|
| | Channel.| | of wetted |
| | | | surface. |
---------------+-------+----------+-------+-----------+---------------
| | Feet. | | c. ft. |
| | | | per sec. |
Main Line Upper|Shingle| 6 |·035 | 9·7 }|
Bari Doab Canal|and | | | }|
|Sandy | | | }|
|Soil | | | }|
| | | | }|Fairly reliable
Main Line |Sandy | 7 | | 9·0 }|estimates based
Sirhind Canal |Soil | | | }|on discharge
| | | | }|observations.
Branches Upper |Loam | |·0079 | 2·2 }|
Bari Doab Canal| | | | }|
| | | | }|
Branches |Sandy | | | 5·2 }|
Sirhind Canal |Soil | | | }|
| | | | |
Distributaries |Loam | |·012[4]|2·3 to 4·4}|
Upper Bari | | | | (average }|
Doab Canal | | | | 3·3) }|
| | | | }|
Distributaries |Sandy | | | 5 to 12 }|
Sirhind Canal |Soil | | | (average }|
| | | | 8·0) }|Somewhat rough
| | | | }|estimates.
Watercourses |Loam | |·015[4]| 3·3 to 20}|
Upper Bari | | | to | (average }|
Doab Canal | | |·045[5]| 9·4) }|
| | | | }|
Watercourses |Sandy | | | 7 to 60 }|
Sirhind Canal |Soil | | | (average }|
| | | | 22) }|
---------------+-------+----------+-------+----------+----------------
[4] When the channel was in continuous flow.
[5] Maximum value when flow was intermittent.
Some information as to losses of water is also given in CHAPTER IV. Art.
2.
The relative losses of water in the channels of the Upper Bari Doab
Canal were as follows:--
Relative Loss.
In main line and branches 20
In distributaries 6
In watercourses 21
Used in the fields 53
---
Total 100
The reasons for the great variation in the value of _c_ are not properly
known. The depth of water is not likely to have much influence on it. It
is well known that the fine silt carried by the water tends to render
the channels watertight when it deposits. The canals and branches
receive either no deposits or deposits consisting chiefly of sand. The
distributaries, especially in their lower reaches, receive deposits of
fine silt which is only occasionally cleared away. The watercourses
receive similar deposits but they are very frequently cleared out by the
cultivators. This is perhaps the reason why the rate of loss of water in
the watercourses is nearly three times as great as the rate of loss in
the distributaries of the same canal. On the Sirhind canal the
distributaries have more branches than on the Bari Doab canal and the
watercourses are smaller. This accounts for the different relative
losses in the two cases. The sandy nature of the soil on the Sirhind
canal accounts for the general higher value of _c_ on that canal.
The following formula has been deduced as giving the loss by absorption
on a Punjab Canal.
WL
P = 3·5 √d ---------
1,000,000
Where P is the loss by absorption in c. ft. per second in a reach whose
length is L, width (at water level) W and depth d. According to the
formula the loss per million square feet is 10·5 c. ft. per second when
d is 4 ft. and 7 c. ft. per second when d is 2 feet, These figures do
not agree with those in the preceding table and it is clear that there
are not yet sufficient data from which to construct a formula.
The first steps taken on the Bari Doab Canal, and subsequently on other
canals, to reduce the losses of water, consisted in the reduction in the
number of watercourses. This will be referred to again (CHAPTER II. Art.
9). Further steps will be considered in CHAPTER V.
5. =Duty of Water.=--The number of acres irrigated annually by a
constant discharge of 1 c. ft. of water per second is called the “duty”
of water. In India on perennial canals the duty may be as much as 250 or
even 300 acres. On inundation canals which flow for only five months in
the year and are situated in tracts of scanty rainfall and light or
sandy soil, the duty may be only 70 acres. The duties of most existing
canals whether in India or elsewhere, are known only approximately. The
duty is calculated on the average discharge entering the canal at its
head less the water which is passed out at escapes. It thus includes all
losses of water. The duty varies not only as between one canal and
another but on the same canal from year to year. It depends on the
character of the soil, a sandy soil requiring more water than a clayey
soil. It also depends on the rainfall. A moderate amount of rain causes
the canal water to go further, but heavy rain may enable some crops to
do without canal water or may permit of the concealment of canal
irrigation. The duty also depends on the kind of crops grown, on the
losses in the channels by absorption and on the quantity of water
available. A liberal supply of water leads to carelessness in the use,
but a very restricted supply is largely wasted owing to the shortness of
the “turns” or rotational periods of flow in the different channels.
There is an obvious connection between the duty of water and the total
depth of the water, known in India as “delta,” given to the fields.
Calculations are much simplified, while still being accurate enough for
all practical purposes, by assuming that the number of seconds, (86,400)
in a day is twice the number of square feet, (43,560) in an acre.
Assuming this to be the case a discharge of 1 c. ft. per second for a
day gives 2 acre-feet, i.e., it will cover an acre of ground to a depth
of 2 feet in a day; and in six months it will cover 100 acres to depth
of 3·65 feet. In Northern India the year is divided into two halves in
each of which a crop is grown and the duty is calculated for each crop.
In this case, if the flow of a canal has been continuous, a duty of 100
acres per cubic foot of its mean discharge per second, corresponds to a
total depth of 3·65 feet over the area irrigated. Generally the flow in
the half-year has not been continuous. In other countries, and in India
on canals other than the perennial canals, the periods of flow vary a
great deal. The duty cannot be calculated from delta or _vice versa_
until the period of flow is stated.
The daily gauge-readings and daily discharges corresponding to them,
having been booked, the discharges are added up. The total, divided by
the number of days on which the canal has been running, gives the
average daily discharge. Suppose that during the “kharif” or summer crop
which is considered to last from 1st April to 30th September or 183
days, the canal was closed for 13 days and that the total of the daily
discharges on the remaining 170 days comes to 850,000 c. ft. per second.
The average daily discharge is 5,000 c. ft. per second. Suppose the
kharif area irrigated to be 500,000 acres, the kharif duty is 100 acres.
To find delta the total of the daily discharges has to be multiplied by
the number of seconds in a day and divided by the number of square feet
in an acre (these figures are, as already stated, very nearly in the
ratio of two to one) and divided again by the number of acres irrigated.
Thus, in the above case, delta is very nearly 850,000 × 2/500,000 or 3·4
feet. For comparing the results of one canal or one year with another,
delta is the more convenient figure to take. As soon as the areas
irrigated by the canals are known for any crop, the Chief Engineer of
the province issues a statement of the value of delta for each perennial
canal and compares them with those for previous years. The value of
delta for the Punjab canals ranges from 3 to 4 feet for the kharif and
from 1·8 to 2·1 feet for the “rabi” or winter crop. Individual canals
vary greatly, the worst having nearly twice as high a figure as the
best. The differences are due to the causes already mentioned.
Although the figures of duty take no account of the number of days a
canal was closed, they are the most convenient standard for judging
generally of the work likely to be done by a projected canal. It will
readily be seen that figures of duty are not exact and are only an
approximate guide. The delta figures are, on the perennial canals of the
Punjab, also worked out for each month of the crop, the volume of water
used from the beginning of the crop up to the end of the month being
divided by the area irrigated up to the end of the month. But when
irrigation is in full swing, some little delay occurs in booking the
fields. Moreover the same field is watered a number of times during the
crop and much depends on whether waterings have just been given or are
just about to be given. The figures are useful to some extent for
comparison. The figures for the rabi crop of 1908-09 were as follows,
the figure for March being the final figure for the crop.
Up to end of Oct., Nov., Dec., Jan., Feb., March.
Progressive value of delta. 1·69 1·34 1·29 1·47 1·71 2·05.
One great principle to be followed in order to obtain a high duty is to
restrict the supply of water. A cultivator whose watercourse is always
running full may waste great quantities of water, but if he knows that
it is only to run for a few days out of a fortnight he will use the
water carefully. It is not, of course, meant that the water kept back is
run into escapes and wasted. It goes to irrigate other lands. The
available supply of water should be spread over as large an area of land
as just, and only just, to suffice.[6] Other methods of improving the
duty are the reduction in the number of watercourses, the apportionment
of the sizes of outlets, watercourses and distributaries to the work
that they have to do, careful attention to the distribution of the water
and the prevention of wastage due to carelessness.
[6] A system of lavish supply is in most cases likely to lead to harm
by water-logging of the soil or its exhaustion by over-cropping or to
raising of the spring level and injury to the public health.
The following information concerning duties is taken from Buckley’s
Irrigation Pocket Book:--
+---------------------+-------------+-----------+
| PLACE. | RABI DUTY. |KHARIF DUTY|
+---------------------+-------------+-----------+
| | Acres | Acres |
| | | |
|Upper India |135 to 237[7]| 49 to 120 |
|(Punjab and | | |
|United Provinces) | | |
| | | |
|Lower Chenab Canal[8]|133 to 134 | 47 to 88 |
|(Punjab) | | |
| | | |
|Bengal | 56 to 130 | 57 to 113 |
| | | |
|Bombay | 85 to 118 | 58 to 159 |
+---------------------+-------------+-----------+
[7] Occasionally as low as 98 or even 62.
[8] The most recent canal.
The period of flow in each case would be six months or less.
The average rabi duties on the Lower Chenab and Upper Bari Doab Canals,
in the Punjab, calculated on the discharges at the distributary heads,
for periods of 3 and 5 years respectively, ending March, 1904, were 208
and 263 acres respectively, but in the latter case 11 per cent. of the
area received only “first waterings.” For the kharif the figures are 100
and 98 respectively.
In Italy the duty is 55 to 70 acres, in Spain from 45 to 205 acres, in
the Western States of America generally 60 to 150 acres. In South
California the duty is 150 to 300 acres, when, as is usual, surface
irrigation is employed, but 300 to 500 acres with subsoil irrigation,
the water being delivered in a pipe below ground level (CHAPTER V.)
In basin irrigation in Egypt the duty is 20 to 25 acres, but the period
of flow is only 40 days. The basins are flooded to about 3 feet in
depth.
6. =Sketch of a Project.=--The tract of country to be dealt with in an
irrigation project may be limited either by the natural features of the
country, by its levels, by the quantity of water available or by
financial considerations. If the tract is small or narrow, and
particularly if it is not very flat, it may be obvious that there is
only one line on which the irrigation channel can conveniently be
constructed but in any considerable scheme a contour plan of the whole
tract is absolutely necessary. The surveys for such a plan are expensive
and take time and it is desirable, as far as possible, to settle
beforehand the area over which they are to extend. This may be done to
some extent by the examination of any existing levels and of the tract
itself. Very high, sandy or swampy ground, whether occurring at the edge
of the tract or in the middle of it, may have to be left out. The
remainder, as already mentioned, is called the commanded area. When land
occupied by houses or roads or which is very much broken, or which for
any reason cannot be irrigated, has also been deducted, the balance is
the “culturable commanded area.”
Either before or after the culturable commanded area has been
approximately ascertained, the proportion of it which is to be irrigated
must be settled. This depends on local circumstances. In India the
supply of water is calculated on the supposition that a fraction,
generally from ¹⁄₃ to ³⁄₄, of the culturable commanded area will be
irrigated each year. The rest will be lying fallow or be temporarily out
of use or be used for crops which do not require canal irrigation. The
restriction of the area is necessary either because the supply of water
is limited or in the interests of the people. Too liberal a supply of
water tends, as already stated, to over cultivation, and exhaustion and
water-logging of the soil.
The next step is to estimate the duty and the discharge of the canal and
then to fix its main dimensions. In Northern India the duty in the rabi
is higher than in the kharif. It may be 200 acres in the rabi and 100
acres in the kharif. Local circumstances determine which crop has the
greater area. Suppose that it is estimated that both will be equal. Then
the total annual area for which water is to be provided must be divided
by two and this gives the kharif area. During the kharif there is
usually an ample supply of water and the kharif mean supply of the canal
is based on the kharif area and the kharif duty. The full supply is not
run all through the crop because the demand fluctuates, the demand being
greatest when all the crops have been sown and when there is no rain,
but from experience of other canals the ratio of the kharif full supply
to the kharif mean supply can be estimated. The ratio is generally about
1·25. On the kharif full supply depends the size of the channel, every
channel being constructed so as to carry a certain “full supply” or
maximum discharge and the top of the bank being made at such a height
that there shall be a sufficient margin or “free-board” above the “full
supply level.” The canal runs full provided that there is a sufficient
supply in the river or that the water level of the river is high
enough--this last condition referring to canals which have no weir in
the river--and provided also that there is a sufficient “demand” for the
water. At other times a canal runs with less than full supply. This
generally occurs throughout most of the rabi, the supply of water in the
river being then restricted. The distributaries are generally run full
or ³⁄₄ths full, some being closed, in turn, to give water to the others.
In the case of a country where there is only one crop in the year, the
average discharge of the canal can be found by dividing the area by the
estimated duty. The F.S. discharge can be assumed to bear such a
relation to the average discharge as may be found by experience to be
suitable. On some Indian inundation canals the F.S. discharge is taken
as twice the average discharge.
The F.S. discharge of the canal having been arrived at, the alignments
of the canal and branches are next sketched out on the contour plan and
certain tracts and discharges are assigned to each branch. The gradients
can be ascertained from the levels of the country and the cross-section
of the channel can then be sketched out. If the velocity is too great
for the soil “falls” can be introduced. The above procedure will enable
a rough idea to be formed of the cost of the earthwork of the scheme.
The cost of the headworks and masonry works and distributaries can be
best estimated by obtaining actual figures for existing works of similar
character, the distributaries being reckoned at so much per mile. The
probable revenue which the canal will bring in will depend upon the rate
charged for the water and the cost and maintenance, matters which can
only be determined by local considerations based on the figures for
existing canals.
The masonry works on a canal consist of the headworks and of bridges,
regulators and drainage crossings. The principles of design for such
works have been dealt with in _River and Canal Engineering_. It is of
course economical to make a bridge and fall in one. If the off-take of a
distributary is anywhere in the neighbourhood the fall should of course
be downstream of it. The positions of the falls should be fixed in
accordance with these considerations. If the longitudinal section is
such that the position of the fall cannot be much altered, it may be
feasible to divert a road so that the bridge may be at the best site for
the fall. In the case of a railway crossing, a skew bridge is often
necessary. In the case of a road crossing it may be feasible to
introduce curves in the road but here also a skew bridge is often
necessary.
CHAPTER II.
THE DESIGNING OF A CANAL.
1. =Headworks.=--In the design of head works no very precise rules can
be laid down. Some general ideas can however be given as to the chief
points to be attended to and some general and approximate rules stated.
In every case a large scale plan of the river is of course required and
also a close examination of it and study of its character. An attempt to
forecast its action is then possible. Gauge readings for several years
and calculations of discharges are of course necessary. If the bed of
the river, in course of time, rises upstream of the weir or scours
downstream of it, a large amount of protection to the bed and banks will
become necessary. Some description of headworks and weirs, with a plan
of the headworks of the Sirhind Canal, India, has been given in _River
and Canal Engineering_, CHAPTERS IV. and X. Remarks regarding the
collection of information for such works are given in CHAPTER II. of the
same work. It is also explained how, by keeping the gates of the
under-sluices closed, a “pond” is formed between the divide wall and the
canal head so that heavy sand deposits in the pond and does not enter
the canal. By closing the canal and opening the under-sluices the
deposit is scoured away.
The best site for the headworks of a canal depends on the stability and
general character of the bed of the river but in deciding between any
two proposed sites, the question of the additional cost of the canal, if
the upper site is adopted, has to be taken into account. Such cost may,
in rugged country, be considerable.
In the case of Indian perennial canals, the head is often close to the
hills where the river bed is of boulders and shingle and fairly stable,
but it is often at a distance from the hills and in such cases a gradual
rise in the bed of the river, even in the absence of a weir, is more
probable than scour. Such a rise may necessitate a raising of the crest
of the weir and of the bed of the canal.
[Illustration: FIG. 5.]
In the general arrangement of a headworks a great deal depends on local
conditions. Sometimes the river runs in a fairly straight and defined
channel and the weir can then be run straight across it. Sometimes, as
in the case of the Ganges Canal, there is a succession of islands and
various short weirs are required in the different channels. At the heads
of the Eastern and Western Jumna Canals, the river, on issuing from the
hills, widens out (Fig. 5.) and the weir is built obliquely and not in
a straight line. Its crest is higher at the east than at the west side.
There are under-sluices at both sides. The upstream end and west side of
the island are revetted. The old head of the Western Jumna Canal, as
shown in the figure, existed long before the advent of the British, and
a temporary weir, made of gabions filled with stones, was constructed
across the river every year during the low water period and swept away
during the floods. To have carried the weir along the line shown dotted,
the head of the Western Jumna Canal being of course brought up to it,
would apparently have been feasible and cheaper, but the off-take would
have been in shallow water because of the curve in the river, and there
would have been no current along the face of the head regulator of the
canal.
The level of the floor of the under-sluices is generally about the same
as that of the bed of the canal. The sill--made to exclude shingle and
sand as far as possible--of the canal head regulator may be 3 feet
higher and the crest of the weir 6 to 9 feet higher. The top of the weir
shutters is 1 to 2·5 feet above the F.S. level of the canal which may be
5 feet or more above the bed of the canal. If the weir is provided with
falling shutters the width of the waterway of the under-sluices may be
about ¹⁄₁₂th of the width of the waterway of the weir alone, otherwise
about ¹⁄₈th.
In nearly all cases the weir has a flat top and flat slopes both
upstream and downstream. In a case where the river bed is of sand, the
depth of water on the crest of the weir in floods may be 15 feet and the
velocity 14 or 15 feet per second. The downstream slope of the weir may
be about 1 in 15, and the upstream slope 1 in 6. Where the river bed is
of boulders the velocity may be still higher. The faces of the weir are
usually of hammer-dressed stone. A lock for the passage of rafts is
added if necessary.
Unless the banks of the river are high, it is necessary to construct
embankments to prevent the river water, when headed up by the weir
during the floods, from spilling over the country with possible damage
to the canal. If the river has side channels they have to be closed. The
stream may also have to be trained, by means of guide banks or spurs, so
as to remain in one channel and flow past the canal head and not form
shoals against it. Where the river is unstable, it may shift its course
so as to strike the weir obliquely and this may cause excessive heading
up at one side of the weir. In such cases it is usual to divide the weir
into bays or sections, each about 500 feet long, by “divide walls”
running at right angles to the weir.
The free-board or height of the masonry walls and tops of embankments
above H.F. Level is about 5 feet.
The span of each opening in the under-sluices is generally 20 to 35
feet. The piers may be 5 feet thick. It is usual to make each alternate
pier project upstream further than the others so that long logs coming
down the river during floods, broadside on, may be swung round and not
be caught and held against the piers.
[Illustration: FIG. 6.]
[Illustration: FIG. 6A.]
Figs. 6 and 6A show the headworks of the Upper Chenab Canal now under
construction (CHAPTER IV.) The site is in a low flat plain, but no
better site could be found. The weir consists of 8 bays of 500 feet
each. The crest is 10 feet above the river bed and the falling shutters
6 feet high. The slopes are 1 in 6 and 1 in 15. The bulk of the work is
rubble masonry in lime. The lower layer upstream of the crest is of
puddle; upstream of the second line of wells it is rubble masonry in
half sand and half lime; upstream of the lower line of wells it is of
dry stone and there is an intermediate layer of rubble masonry in lime
with the stones laid flat. Below the crest there is a wall of masonry 9
feet thick and on the crest there are two strips of ashlar between which
the shutters lie when down. The extreme upstream and downstream portions
of the bed protection are of dry stone and 4 feet thick while next to
the weir are concrete blocks 2 feet thick resting on dry stone. The
width of the crest is 14 feet, of the weir 140 feet, of the protection
70 feet upstream and 110 feet downstream. The guide banks have tops 40
feet wide and 18 and 14 feet above the crest of the weir in the upstream
and downstream lengths respectively, the side slopes being 2 to 1 and
the water slope being covered, up to H.W. level, by dry stone pitching 4
feet thick. The left guide bank runs upstream for 3,250 feet from the
centre line of the canal and the right 2000 feet from the line of crest
shutters. The under-sluices have 8 bays of 35 feet each and the canal
head regulator 36 openings of 6·5 feet each, the large openings shown in
the figure being sub-divided. The crest of the weir is no less than 10
feet above the river bed and the shutters add 6 feet to this. The floor
of the under-sluices is 4 feet higher than the river bed. There is thus
ample allowance for a possible rise in the river bed.
2. =The Contour Map.=--The contour map, besides showing the contours of
the country to be irrigated and of a strip of country, even if not to be
irrigated, which will be traversed by the main line, should show all its
main features, namely:--streams, drainages, railways, roads,
embankments, reservoirs, towns, villages, habitations, and the
boundaries of woods and cultivated lands. It should also show the
highest water levels in all streams or existing canals. A map showing as
many as possible of the above features should be obtained and lines of
levels run for the contours. In doing this, the points where the lines
of levels cut or pass near to any of the above features or boundary
lines, should be noted. It may be necessary to correct inaccuracies in
the plan or to supply defects in it. The greater the trouble taken to do
this the less will be the trouble experienced later on.
The heights of the contour lines will, in very flat country, have
eventually to be only 1 foot apart. This will necessitate running lines
of levels half-a-mile apart at the most, and preferably 2000 feet apart,
the pegs in each line being about 500 feet apart. In less flat country
the heights of the contour lines can be further apart than 1 foot.
Whatever distance apart is decided on for them, the survey should be
done once for all. On one of the Indian canals in flat country, the
lines of levels were at first taken 5 miles apart, the branches roughly
aligned and then further surveys made. This led to great expense and
delay and the procedure has not been repeated.
In making a contour survey, a base line, as centrally situated and as
long as possible, should be laid down, with side lines parallel to it
near the boundaries of the tract. The cross lines at half-mile or other
intervals should then be laid down. Some of them may run out beyond the
side lines. Circuits of levels should be run along the base line, the
side lines and the two extreme cross lines and be carefully checked. The
remaining cross lines should then be levelled. All the levels having
been shown on the map the contours should be sketched in. The scale of
the map for a large project may be two inches to a mile. If it is likely
that the survey will have to be extended, it will be easier to do this
by prolonging the base line and running more cross lines, than by
prolonging each of the cross lines already surveyed. This can be borne
in mind when selecting the base line.
3. =Alignments and Discharges.=--On the contour map the proposed
alignments of the canal, branches, distributaries, and escapes,
determined after careful consideration of all matters affecting them,
are shown. The tracts to be irrigated by each branch and each
distributary are now marked off, the “irrigation boundaries” following
approximately the valleys and lines of drainage. Any large tracts of
land which cannot be irrigated are of course shown and are excluded.
Forests or other lands which are not to be irrigated should be similarly
dealt with, otherwise confusion is likely to arise later. The commanded
area dependent on each distributary is now ascertained from the map. A
certain percentage being deducted for scattered unculturable areas the
culturable commanded areas are obtained. The proportion to be irrigated
(in India in the kharif) having previously been decided, the number of
acres to be actually irrigated by each distributary is arrived at.
The next step is to ascertain the discharges.[9] A general duty for the
whole canal having been estimated by considering the actual figures for
other canals the full supply of the canal at its head is arrived at.
(CHAPTER I, Art. 6). In Northern India it will be the kharif duty and
kharif full supply. Since some water is lost by absorption in the
channels, the duty of the water on a branch is higher than that of the
whole canal based on its head discharge, and the duty on a distributary
is higher still. In designing a canal, an attempt has to be made to
estimate the losses of water in the main canal and branches, so that the
duties of the branches and distributaries may be estimated and the
channels designed accordingly. On the Western Jumna Canal the figures
were estimated to be as follows:--
Kharif. Rabi.
Average discharge at canal head (c. ft. per sec.) 3536 2755
Duty based on the discharge (acres) 98 138
Estimated loss of water in canal and branches (c. ft. per
sec.) 400 300
Average discharge at distributary head (c. ft. per sec.) 3136 2455
Duty based on the discharge (acres) 111 154
[9] In this Article and in the rest of this Chapter it is assumed that
the canal is a Northern Indian one. Any modifications necessary to
suit canals in other countries will readily suggest themselves.
The question of duty is one which if not carefully considered, may cause
some confusion. A canal and branches, having been designed with certain
assumed duties and with discharges based on certain values of N in
Kutter’s co-efficient, have, let it be supposed, been constructed to a
greater or less extent. When the time comes for constructing the
distributaries, the engineers concerned may have different ideas, based
on later experience, as regards the probable duty and the most suitable
value of N. If they design the distributaries with a higher duty and a
lower value of N, it is obvious that they can provide more
distributaries than at first designed, or can increase their lengths. In
either case they would provide for an increased commanded area. If they
do not do this, they ought to adhere to the values at first proposed,
thus making the channels larger than, according to their ideas, would be
necessary. These larger channels will be able to do more irrigation, by
an increase, not in the commanded area, but in the proportion of it
which is irrigated. Any other course would result in the canal carrying
more water than could presumably be used by the distributaries. Again,
the question how the assumed duty was arrived at may need consideration.
It may have been arrived at by taking the duty figures of some existing
canal, based on discharge figures which were the result, not of observed
but of calculated discharges, and if the calculations were based on a
value of N which experience has proved to be wrong, a correction is
obviously needed. Many mistakes of the kinds indicated above have been
made, not perhaps in the case of a project which has been recently got
up and is then quickly carried out in its entirety, but in one which is
carried out slowly or after a long period has elapsed or in one which
consists of extensions of an existing system. So great, however, is the
elasticity of a channel--by which is meant its capacity for adapting
itself to varied discharges, a small change in the depth of water
causing a great change in the discharge--and so considerable has been
the uncertainty as to the real duty to be expected, that any mistakes
made have not usually resulted in any serious trouble.
[Illustration: BIFURCATION AT TAIL OF CANAL.
The Distributaries have Gates and Winches.
_To face p. 41._]
It has been stated (CHAPTER I, Art. 2) that it is not desirable to let
one channel tail into another. In old canals a distributary used
sometimes, after running parallel to a canal, to be brought back towards
it and tail into it. The advantage of this was that the distributary had
not to be made very small towards the tail and that, if the demand
abruptly ceased, the distributary was not likely to breach. The
principle was, however, essentially bad. The lower part of the
distributary was obviously too near the canal and not centrally
situated as regards the irrigated strip. The portion at the extreme tail
was superfluous. Again, whatever volume of water was carried through the
distributary and back into the canal, was needlessly detached instead of
being kept in bulk. Moreover the duty of water on such a distributary
cannot be ascertained without a tail gauge and the observation of
discharges at the tail. There are similar objections to one distributary
tailing into another. Each should be separate and distinct.
A major distributary is one whose discharge is more than 40 c. ft. per
second. It may be as much as 250 c. ft. per second. A branch, as soon as
it reaches a point where its discharge becomes only 250 c. ft. per
second should be considered as a major distributary. A minor
distributary is one whose discharge is from 8 to 40 c. ft. per second. A
minor distributary is nearly always a branch of a major distributary.
There are instances of “direct minors,” i.e., minors taking off from
canals or branches. Such a minor, unless its discharge is a large
fraction of that of the canal which supplies it--and this can seldom be
the case--is objectionable because the petty native official who has to
see to the regulation of supplies can manipulate the supply easily and
without detection, and the number of persons irrigating from it being
small, he can make private arrangements with them. On the Sidhnai Canal
there are some half-dozen distributaries each of which had one or two
minors which took off close to the head of the distributary. The people
who irrigated from the minors managed to get the heads shifted and taken
off direct from the canal, on the ground that, the water level in the
canal being higher than in the distributary, there would be better
command and less silt deposit. The irrigation on all these minors ran up
to a figure far in excess of what had been intended, to the detriment of
lands further down the canal. The minor heads have all been
retransferred to the distributaries, the difficulty as to command being
got over, as it should have been at first, by constructing weirs in the
distributaries. The fall in the water surface at the distributary head,
i.e., the difference between the water level in the canal and that in
the distributary downstream of its head but upstream of the weir, is
quite trifling or even inappreciable.
In some of the older Indian canals it was the custom to place the heads
of distributaries, not just above a fall but several hundred feet above
it, the idea being that the distributary then received less silt. This
practice has now been discontinued. There is no valid reason for
following it.
The question whether, when a channel crosses a road on the skew, a skew
bridge should be constructed or curves introduced into the road or
channel, is one which requires some consideration. As far as possible
the lines of channels should be fixed so as to cross important[10] roads
on the square or with a small angle of skew. In the case of main canals
or branches, the introduction of special curves is generally out of the
question, but if the road is not straight something can be done by
shifting the line one way or the other. In the case of “major”
distributaries, curves can to some extent be introduced. In the case of
“minor” distributaries it is often possible to curve the channel, with a
radius of say 500 feet, so that it will cross the road at right angles.
There is very little objection to a skew bridge if the angle of skew is
not great. The angle of crossing having been made as near to 90° as
possible, the bridge can be made skew though not necessarily so much
askew as the road. Slight curves can be introduced into the road. When
the road is made askew, a bridge on the square involves at least three
considerable curves (Fig. 7) and the taking up of extra land. It also
causes, in perpetuity most likely, a more or less inconvenient and
unsightly arrangement and one which, in most countries, would not be
tolerated. When the angle of skew is not great, it is best to introduce
no curve at all into the road. In the case of a “village” road, which
may be more or less undefined and liable to be shifted, the difficulty
about land may not be great, but even in this case the angle of crossing
should, if possible, be kept near to 90°, especially in the case of
minors, and where curves have to be introduced into the road they should
be suitable ones. Abrupt angles are not only unsightly but are unfair to
the cart drivers. The crossings of village roads by the minors of a
certain great modern canal have been stigmatised as “hideous.” Indian
canals can afford to do work properly.
[10] In India “district” and “provincial” roads.
[Illustration: FIG. 7.]
4. =Remarks on Distributaries.=--Before a canal system can be properly
designed, it is necessary to determine certain points in connection with
the working of the distributaries. A distributary is intended to
irrigate a certain kharif area. Its average kharif supply is determined
from the assumed kharif duty. It generally runs full in the kharif but
not always. In a very dry tract such as the Montgomery district of the
Punjab, the demand is so great and so steady that a distributary
practically runs full through the greater part of the kharif. In such a
case the canal or branch must be so designed that it can keep all
distributaries full at the same time. Its F.S. discharge will be the sum
of all the F.S. discharges of the distributaries plus the losses of
water by absorption.
But in other cases, especially if the rainfall is considerable, a
distributary does not require its full supply, either all through the
kharif or for long at a time. An estimate must then be made of what it
will require. It may be estimated that its requirements will be met if,
during the period of greatest demand, it is closed for two days out of a
fortnight and receives full supply for the remaining twelve days. In
this case, since the various distributaries need not all be closed on
the same days, the canal or branch can be so designed that it will carry
a full supply equal (after deducting losses) to ⁶⁄₇ths of the aggregate
full supplies of the distributaries. In other cases the fraction may be
³⁄₄ths. It is likely to be lower the greater the rainfall of the
district. Even in the case when the distributaries run full through
nearly the whole of the kharif, there will be periods when they only run
with about ³⁄₄ths full supply. If full supply were run at such times,
many of the outlets would discharge more water than was required, the
cultivators would partly close them, and breaches in the banks of the
distributary might result. Thus the water level of a distributary must
always be so arranged that it will have a good “command” when it is
running with about three-fourths of the full supply discharge. The water
level with ³⁄₄ths full supply is generally ·5 to ·75 feet below the full
supply level but it should be calculated in each case. Generally it will
be correct to make the water level, when ³⁄₄ full supply is run, about 1
foot above the high ground traversed by the distributary, excluding any
exceptionally high portions of small area. A more exact method is given
in Art. 9. The greater the proportion of the culturable area which is to
be irrigated, the less should be the area of any high land which is
excluded. The F.S. levels of the distributaries at their off-takes must
be settled in accordance with the foregoing remarks, and these F.S.
levels must be entered on the plan. Neglect to thus fix the F.S. levels
of distributaries before designing the canals has frequently led to
trouble.
The head needed at a bifurcation in order to get the supply into a
branch or distributary is always small unless the velocity is high. For
a velocity of 3 feet per second the head required is only about ·16 ft.,
for 2 ft. per second ·1 ft.
On an Inundation Canal which has no weir across the river, the mean
supply downstream of the regulator (which is built a few miles down the
canal lest it should be damaged by the shifting of the river) is, as has
been mentioned, about half the full supply. The command in such canals
is not generally very good. A distributary can often obtain only mean
supply and it should be designed so as to command the country when it is
carrying mean supply. A detailed description of Inundation Canals in
Northern India, is given in _Punjab Rivers and Works_.
Let M, F, m, f, be the mean and full supply discharges at the heads of a
canal and of an average distributary on it and let the number of
distributaries be n. It has been seen (Chap. I. Art. 6.) that M = ·8F
about. Let k be the proportion of the supply lost by absorption in canal
and branches. Then n m = (1 - k) M = ·8 (1 - k) F. If the distributaries
all run with full supplies--at the time of greatest demand--for 4 days
out of 5, then,
nf = 1·25 (1 - k)F
f 1·25
- = ---- = 1·56
m ·8
Since k depends on the wetted area, it is not likely to be so great for
F as for M, but the above gives a general idea of the ratio of the full
kharif discharge to the mean kharif discharge. On a large canal the
circumstances of the distributaries will not all be similar. Some will
run full for a greater proportion of their time than others. They can be
divided into groups and the ratio of the full to the mean supply
calculated for each group. The mean supply is, as above stated, obtained
from the area to be irrigated, and the duty as estimated at the
distributary head.
At one time a system was introduced of making distributaries of large
size with the idea of running them for short periods. One reason given
for abandoning this arrangement, was that there was a tendency to run
such a distributary for too long. This reason is not very intelligible.
It would be applicable to any distributary which was not intended to be
run without cessation. The result would be that some other distributary
would be kept short of water and this would imply extremely bad
management. The chief reason against such a distributary is the greater
cost of its construction. It would effect a saving of water. The ratio
of the discharge to the wetted area would be high, though this would be
to some extent neutralized by the greater frequency of closures, since,
when water is admitted to a dry channel, the absorption is at first
great. There would also be some difficulty in the distribution of the
water because of the short period for which it would remain open. It
will be seen (Chapter III. Art. 5), that it is desirable to open and
close always at the same hour of the day. An ordinary distributary might
run for 11 days out of 14. One of double the size could not conveniently
be run for 5¹⁄₂ days. A distributary can always be enlarged if
necessary, but if made too large it is extremely difficult to make it
smaller.
It was also, at one time, usual to make minors, when there were several
on a distributary, of large capacities so that they ran in turns. The
preceding remarks apply to this case. The system has been abandoned.
5. =Design of Canal and Branches.=--The apportioning of discharges to
the various channels having been effected as described in Art. 2, the
designing of the canal and branches is proceeded with. Rough
longitudinal sections of all the lines are prepared by means of the
contour map, the ground levels being shown at intervals of one foot--or
whatever the vertical distance between the contours may be--and the
horizontal distances obtained from the map by scaling.
On these longitudinal sections the lines proposed for the bed and F.S.
levels are shown reach by reach and also the mean velocities and
discharges.
The laws of silting and scouring and the principles on which channels
should be designed are fully gone into in _River and Canal Engineering_.
It is there explained that, for a channel of depth D, there is a certain
critical velocity, V₀, which just prevents the deposit of the silt,
consisting of heavy clay and fine sand, found in Indian rivers--this
silt enters the canal in such immense quantities that the canal silt
clearances would be impossible if much of it was deposited in the
channels--that sand of grades heavier than [·1] may deposit in the head
of a canal and well nigh threaten its existence, that the clear water
entering the canal in winter may pick up and carry on some of the sand
but that proper steps for preventing the deposit in the canal can be
taken at the headworks. This last question has been referred to in Art.
1. The following additional rules for designing canals in Northern India
are chiefly taken from those given by Kennedy in the explanatory notes
to his Hydraulic Diagrams, which are in use in the Irrigation Branch in
Northern India.
(1) Near the hills where the bed is of shingle the velocity may exceed
V₀. A few other soils will stand 1·1 V₀.
(2) In ordinary channels any excess over V₀ will give much trouble
lower down.
(3) In the first four or five miles of a distributary, V₀ should be
allowed and gradually be reduced to ·85 V₀ at the tail, the gradient
being reduced if convenient, while a minor or branch distributary
should have less than V₀ at its off-take and still less at the tail.
The sand is drawn off by the outlets and in the lower part of a
distributary it is often non-existent.
(4) If there is efficient silt trapping at the head of the canal any
figures arrived at by the preceding rules should be multiplied by ·9.
(5) In the case of a canal having its head far from the hills, the
sand is finer and any figures arrived at as above may be multiplied
by, perhaps, about ·75, but further experience is needed to decide
this.
(6) If the soil is very poor, especially if the depth of water is more
than 6 or 7 feet, the velocity should be less than V₀--say ·9 V₀--so
as not to cause falling in of the banks. Depths of more than 9 or 9·5
feet should, as far as possible, be avoided for the same reason.
(7) At a bifurcation, one branch channel may have no raised sill, and,
owing to its smaller depth, it may draw off no surface water and get
an undue share of rolling sand. Its velocity should be greater than V₀
and that of the other branch be less than V₀.
(8) At such a bifurcation it may be necessary, during times of low
supply, to head up the water in the main channel and some silt may
temporarily be deposited in it. When the heading up ceases, the silt
is scoured away but it mostly goes into the branch whose bed level is
the lower. It is best to design such bifurcations so that the sill
levels of the two branches are equal and, if possible, so that their
bed levels are equal.[11] Otherwise the channel which is likely to get
most silt should have the steeper gradient.
(9) Any existing well established régime should not be tampered with.
[11] Appendix A in _River and Canal Engineering_ deals with some
instances of fallacies in questions concerning flow in open streams.
An extract from it describing a remarkable divide wall recently
constructed at the head of the Gagera branch, Lower Chenab Canal, is
given in Appendix A of this book.
Experience shows that in designing Irrigation Channels in the plains of
India in accordance with Kennedy’s figures, the maximum ratio of bed
width to depth of water is as follows:--
Discharge, c. ft. per second 10 25 100 200 500 1,000
Ratio 3·5 4 4·5 5 6 6
The actual gradients of the canals generally range from about 1 in 8,000
for a main canal to 1 in 2,000 for the tail of a distributary, but near
the head of a canal where the bed is of boulders and shingle, the
gradient may be as steep as 1 in 1,000.[12] The velocity in this last
case may be 5 feet per second but generally it is not more than 3 or 4
feet per second in canals and branches, and 1 to 2 feet per second in
distributaries.
[12] On the Upper Jhelum Canal, 1 in 970.
In designing the channels, N, in Kutter’s co-efficient, may be taken as
·0225 or ·020, according to judgment. For new and smooth channels ·020
is generally correct. A channel generally becomes rougher by use but
sometimes it becomes smoother. Cases have occurred in which N has been
found to be ·016. This question is discussed in _Hydraulics_, Chap. VI.
The bed width of a canal is reduced, where a distributary takes off, in
such a way that when the canal and distributary are both running full,
the depth of water in the canal continues to be uniform and the flow to
be uniform. When the distributary is closed there is heading up in the
canal upstream of the off-take, but not enough to make any appreciable
difference unless the capacity of the distributary is a large fraction
of that of the canal and even then no harm is likely to result.
The preceding rules and principles being taken into consideration, the
channels are designed. The bed levels, gradients and depths are so
arranged as to give the velocities suited to the soil and to maintain
the proper relation of depth to velocity. The bed width is arranged so
as to give the proper discharge. The full supply level of the canal and
branches has also to be so arranged that it shall be higher, at each
distributary off-take, than the full supply level of the distributary.
It is desirable to be able to give a distributary its full supply even
when the canal is low. Generally the slope of the country along any line
is greater than would be suitable for the bed, and “falls” are
introduced. The off-take of a distributary is generally just above a
fall and there is generally an ample margin between its F.S. level and
that of the canal. The discharge of the canal during the greater part of
the rabi may be only about half the full supply. This discharge should
be estimated and the water level corresponding to it calculated and
shown on the longitudinal section. If possible the levels should be so
arranged that even with its least supply the water level in the canal
will enable full supply to be given to a distributary. If this cannot
otherwise be managed it may be necessary to construct a regulator in the
canal below the head of the distributary so that, during low supplies,
the water can be headed up. It has been stated in _River and Canal
Engineering_, Chapter IV., that such heading up, if temporary, is not at
all likely to cause silt deposit in the canal. The designing of the
distributaries is not proceeded with at this stage.
Since no irrigation is usually done directly from the canal and
branches, they are designed without any particular connection between
the level of the water and that of the country traversed. Dangerously
high embankments are of course avoided as far as possible. The bed is
designed at such a level that the excavation and embankment at any place
will be, as nearly as possible, equal. Land in India is cheap. When the
excavation exceeds the embankment the balance is made into a spoil bank.
When the excavation is less than the embankment the balance is got from
borrow-pits.
The side slopes of channels in excavation are generally 1 to 1, in
embankment 1¹⁄₂ to 1. The sides of channels of small or moderate size
usually become about ¹⁄₂ to 1, or even vertical, by the deposit of silt
on the slopes. This reduction of area is allowed for in the design i.e.
the bed width is so designed that the channel will carry the required
discharge, not with the side slopes as executed, but when they have
become ¹⁄₂ to 1. In large canals however the sides do not always silt up
but rather tend to fall in. When this is expected to occur the allowance
above described is not made. Berms are left so that if any part of the
sides fall in, the bank will not also fall in. The berms also allow of
the channel being widened if that ever becomes necessary. Type sections
are given in Figs. 8 and 9.
6. =Banks and Roads.=--Figs. 8 and 9 show the banks and spoil.
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
The scale is 6 feet to an inch. The depth of water, in this particular
case, is 7 feet, and the bank, excluding the small raised bank, 2 feet
above the water. The inside edge of the bank, where the small raised
bank is shown, is kept parallel to the canal for a considerable
distance. Its position is got by drawing a line, shown dotted, at,
generally, 1¹⁄₂ to 1. The embanked part of the slope is actually made at
1¹⁄₂ to 1, but the excavation is at 1 to 1, so that a berm is left. The
width of this berm of course varies as the depth of digging varies. If
there is likely to be much falling in of the sides the berm can be made
wider, the dotted line starting, not from the edge of the bed, but from
a point further in. On an inundation canal in sandy soil the berm may be
20 feet wide. In figure 8, the inside slope above the berm is supposed
to have silted up to a slope of 1 to 1. In cases where it is expected
the whole inside slope will silt to ¹⁄₂ to 1 the dotted line, to give
the edge of the bank, can be shifted towards the channel so that the
berm at the ground level when the channel is excavated will be very
small for the minimum depth of digging. There is no need for the inner
edge of the bank to run parallel to the canal for great distances. Its
position can be shifted whenever suitable and the width of the berm at
ground level varied. This prevents the occupation of a needlessly great
width of land. It used at one time to be not unusual to make a bank with
a berm on the land side, similar to that formed by the spoil in Fig. 8,
but at about the level of full supply in the canal. The principle is not
a good one. Salient angles are liable to be worn away. If earth has to
be added to a bank to strengthen it, the whole can be widened or the
rear slope flattened. The roadway is shown 18 feet wide, which is nearly
the maximum. For the drainage of rain water it has a transverse slope,
away from the canal, of about 1 in 50. The small raised bank on the
canal side is to give safety to wheeled vehicles. It is provided on the
patrol bank[13] on main lines and places where there is much traffic or
where there is plenty of width of bank to spare. When the ground level
is, for a considerable distance, above the proper bank level--which is
at a fixed height above the F.S. Level--so that the road and its
side-drain have to be cut out, much earthwork can be saved by allowing
them to be at a higher level and, in the case at least of the non-patrol
road, giving the road a reduced width.
[13] A canal has an unmetalled driving road--called the “patrol road”
or “inspection road” on one bank. This road is reserved for the use of
officials. Otherwise, it would soon be cut up and worn away, and the
cost of repairs would be excessive. The patrol road should be on that
bank which is, in the morning (the time when inspections are usually
made) in the shade of trees planted on the landward side. Trees are
not usually planted near the water edge as they are sometimes blown
down. In Northern India the canals generally flow in a southerly
direction, so that the left bank is best for the patrol bank. On the
other bank there is a bridle road which is open to the public. Near a
rest house--unless there is a bridge actually at the place--the patrol
road should be on the same bank as the rest house. It can if necessary
cross at the first bridge. Frequently there is also on one or both
sides of the canal a “boundary road,” which is open to the public,
along the toe of the outer slope. Along a distributary there may be a
boundary road on one side. It is generally the only road which can
take wheeled traffic, and in this case it should be reserved for
officials unless money is provided to keep it always in repair.
Officials have to be on tour for weeks or months at a time, and in all
weathers. Their baggage carts also have to precede and follow them.
Anything which facilitates their touring about and seeing things for
themselves is, in India, most desirable. At a watercourse crossing the
boundary road along a distributary should be taken by a curved incline
up on to the bank and down again. Thus not only is the cost of a
culvert saved, but any touring official who is driving obtains a view
of the channel which he cannot get from the boundary road.
[Illustration: FIG. 10.]
In shallow digging, the plan of setting back the banks (Fig. 10) and
letting silt deposit as shown by the dotted lines, is one which should
be followed much oftener than it is. It not only gives eventually a very
strong bank, but it enables the borrow pits, from which the earth for
the banks is got, to be dug inside the banks. Outside borrow pits,
besides being a source of expense, owing to compensation having to be
paid to those in whose land they are dug, cause great areas of hollows
which are not only unsightly, but are often full of stagnant water and
are thus a fruitful source of mosquitoes and malaria. Insufficient
attention has hitherto been paid to this matter.
In designing each reach of a canal or branch, type cross sections should
be drawn out for several different depths of digging, _e.g._, one for
very shallow digging, _i.e._, where the bed is little, if at all, above
the ground level, one for deep digging where the ground is higher than
the water level, and one for the “balancing depth,” where the area of
the channel excavation is equal to the earth required for the banks. In
calculating the earthwork the sectional area of the digging or of the
embankment is taken, whichever is the greater.
The proper width and height of bank for any channel depends partly on
the maximum depth of water in the channel, and partly on the discharge.
Given a depth of water of say 8 feet, a breach will obviously be more
disastrous with a great volume of water than with a small volume. The
following statement gives some figures suitable to the rather light and
friable soils of Northern India, but the question is largely one of
judgment. Generally a low and rather wide bank is preferable to a higher
and narrower one. If a road, with or without the small raised bank next
the canal, is required, special widths can, of course, be arranged for.
A 14-foot bank is required for a driving road.
------------+--------------+---------------+---------------
Top Width of|Height of Bank| Greatest | Greatest
Bank. | above F. S. | Admissible | Admissible
| | Discharge. |Depth of Water.
------------+--------------+---------------+---------------
Feet. | Feet. |C. ft. per sec.| Feet.
20 | 2 | 12,000 | 12
18 | 2 | 8,000 | 12
16 | 2 | 5,000 | 11
14 | 2 | 3,000 | 10
16 | 1·5 | 2,000 | 9
14 | 1·5 | 1,500 | 9
12 | 1·5 | 1,200 | 8
10 | 1·5 | 1,000 | 7
9 | 1·5 | 700 | 6
8 | 1·5 | 500 | 5·5
7 | 1·5 | 400 | 5
6 | 1 | 300 | 4·5
5 | 1 | 200 | 4
4 | 1 | 100 | 3·5
3 | 1 | 50 | 3
------------+--------------+---------------+---------------
The spoil in Fig. 8 is shown at a different level from the bank proper,
as it should be to give a neat straight edge to the bank. The width of
the spoil may vary every chain. In Fig. 9 the spoil is raised to avoid
taking up too much land. The spoil presents the best appearance when its
height is kept uniform for as long a length as possible, the width
varying according to necessity, When the height has to be altered, the
change should be made by means of a short ramp. When the spoil is higher
than the road, gaps in it are left at intervals so that rain water can
pass away. When the spoil is heavy for a very short length it can, in
order to avoid a short and unsightly heap, which would result from the
adoption of the section shown in Fig. 9, be placed as in Fig. 8, some of
it being led askew.
The small channel shown outside the bank in Fig. 8 is a watercourse for
enabling trees to be grown. It has, of course, to be graded, and it may
be in cutting or in embankment. If any silt clearances of the canal are
likely to be necessary, the watercourse must be set back to allow room
for the spoil. Such spoil, if sandy, is to a large extent washed down or
blown away and does not accumulate to anything like the extent that
would be expected.[14] Moreover the spoil can extend onto the
watercourse when the trees have grown big, and no longer need watering.
Outside the watercourse is shown the boundary road and the land boundary
pillar. The small channel in Fig. 9 is a drain for rain water. It can be
used as a plantation watercourse if the water is lifted.
[14] This fact has been quoted (_The Pioneer Mail_, “Silt,” 8th March,
1913) as showing that the silt supposed to be cleared is not really
cleared. This may be the case to some extent, but shortage of spoil is
little proof of it.
Where there is no spoil, some extra land, perhaps 20 feet on either
bank, is usually taken up for getting earth from for repairs.
7. =Trial Lines.=--The proposed lines of channel, determined as
explained in Art. 5 should next be laid down on the ground. A line
should consist of a number of straight portions. The curves should not
be put in. Trial pits should be dug at intervals. Some defects in the
line may at once become apparent because the contour map, owing chiefly
to the lines of levels having been taken a considerable distance apart,
is not perfect. A line may pass through a patch of very high or very low
ground or too near to some building or other object with which it is
desirable not to interfere. Alteration may be desirable at a drainage
crossing or at the off-take of a branch. The lines should be corrected
where necessary. Sometimes the corrections may be very considerable.
Allowance can be made for the alterations which will occur when the
curves are laid out. Where there is doubt as to which line is the best,
trial pits may be dug to obtain further information regarding the soil.
The line should now be levelled, careful checks being made, a
longitudinal section of it prepared and the proposed bed, bank and F.S.
level shown. The ground levels ascertained by levelling the line, are
certain to disagree, to some extent, with the contour lines. The latter
were got only by inference from the levels of points in the survey
lines, and they should be corrected in accordance with the fresh levels
now available. If the line does not seem to be the best that can be got,
a fresh line can be marked on the plan and the above procedure repeated.
8. =Final Line and Estimate.=--As soon as the best line seems to have
been found, a large scale plan of the country along its course should be
made by taking bearings or off-sets from points in it to the various
objects and noting where the line cuts them. On this plan will be shown
the exact alignment, the curves being put in and the straight portions
slightly shifted where necessary so that the line may pass at a proper
distance from any buildings or other objects. But before this procedure
is carried out, or while it is being carried out, the estimate for the
work can be prepared from the longitudinal section already taken. Such a
section is of course amply sufficient for a “project estimate,” in which
only approximate figures are given, and it is quite near enough for any
estimate. In the case of small works which have often to be executed
with great promptitude, lamentable delays have occurred owing to the
engineer deferring the preparation of his estimate till he had got the
line exactly fixed. Moreover there is a chance of the labour being
thrown away in case the sanctioning authority directs any change in the
alignment to be made.
In the case of a large scheme, a project estimate is prepared. In this
the earthwork and the area of land to be occupied are calculated pretty
accurately. Designs and estimates are also prepared for the headworks
and for the chief regulators. For works of which there are to be many of
one type--bridges, falls, distributary heads and small drainage
syphons--the cost is arrived at from lump sum figures, one drawing of
each kind being submitted as a type. The distributaries are
approximately estimated at mileage rates. In the case of a small scheme
everything is estimated in detail except perhaps the distributaries or
some of them.
[Illustration: CANAL WITH BRIDGE AND DISTRIBUTARY HEAD.
The Head has Gates and Winches.
_To face p. 61._]
9. =Design of a Distributary.=--A distributary is a canal in miniature
and, like a canal, it may have branches. It has masonry bridges, falls
and drainage syphons. It has, as already mentioned, a masonry regulator
at its head. At the off-take of any branch or distributary there is a
regulator in the head of the branch. If the branch takes off a large
proportion of the water there is a double regulator. A distributary
gives off watercourses as a canal gives off distributaries. The
watercourses belong to the people and not to Government and they are
cleared and maintained by the people. Each watercourse has a masonry
head known as an “outlet” (Fig. 11). The outlet is the point where
the water passes from the hands of Government officials to those of the
cultivators. The outlet is of masonry and its opening is not adjustable
but is fixed in such a way that its discharge, when the distributary is
full, bears, as nearly as can be arranged, the same ratio to the F.S.
discharge of the distributary as the area intended to be irrigated by
the watercourse bears to that intended to be irrigated by the
distributary.
[Illustration: FIG. 11.]
The floor of the outlet is level with the bed of the distributary. It
thus draws off rolling sand which might otherwise accumulate in the
distributary. Small outlets are made of earthenware pipes, about ·4 feet
in diameter, laid in concrete. Two pipes, or three, may be laid side by
side. If more than three would be required, a masonry opening is
adopted. The discharge through an outlet, is generally 2 to 5 c. feet
per second per square foot of outlet area, and the head ·1 to ·5 feet.
For the tract of country allotted to any distributary, a contour map is
prepared on a fairly large scale, say 4 inches to a mile. On the map
the line is laid down and a rough longitudinal section, showing the
ground level, is prepared as in the case of a canal.
It has already been stated (Art. 4) that a distributary is so designed
that its water level, when three-fourths of the full supply is run,
shall be well above the level of most of the ground along its course. In
other words it should have a good command. A good rule is to allow a
fall of ·5 feet from the level of the water in the distributary to that
in the watercourse, a slope of 1 in 4,000 for the water flowing along
the watercourse, and a fall of ·3 feet for the water at the tail of the
watercourse to the level of the ground. This last level is, like the
other ground levels, taken from the contour map. This procedure, in
short, consists in making the water level of the watercourse at its head
govern that of the distributary, just as the water level in the
distributary at its head was made to govern that in the canal.
The enlarged contour map of the distributary area shows, among other
things, the boundaries of the lands belonging to each village. Generally
a watercourse supplies water to only one village. When, however, a
village is far from the distributary, its watercourse has to pass for a
long distance through other villages and it would be wasteful of water
to have two separate watercourses. In such cases one watercourse may
serve two villages or more. When a village is near to the distributary
and its land extends for a long distance parallel to the distributary,
it may have several watercourses for itself alone. A watercourse can
generally be most conveniently dug along the boundary line of two
villages, or there may be some other line which the people particularly
desire.[15] Subject to, or modified by, these considerations a
watercourse is designed to run on high ground like a distributary.
[15] They also frequently wish the “chak”--the area irrigated by a
watercourse--so arranged that two men who are “enemies” shall not be
included in the same “chak.” This condition can be complied with only
up to a certain point. Arrangements may be modified but not in such a
way as to upset the proper rules.
[Illustration: CONTOUR MAP (PART) AND LINE OF DISTRIBUTARY.
The scale is 1 inch to 2 miles. The contour lines at 1 foot intervals
are shown dotted, the roads by double lines. The line of the
distributary, in order to follow the ridge of the country, would have
gone more to the left of the plan near the village. The shifting of the
line to the right brings it nearer to the centre of the irrigated
tract--supposed to be the whole area shown--and enables a single bridge
to be built at the bifurcation of the two roads. Suitable lines for main
watercourses are shown in thin firm lines. It is assumed that the
command is sufficient to enable the watercourses to run off at the
considerable angles shown.
_To face page 63_]
The great object is to reduce the total length of channels, _i.e._,
minors and watercourses. No watercourse can be allowed to run alongside
of or near to another. It may run alongside a canal or distributary when
really necessary to gain command but not otherwise. The longer the
watercourse the larger the chak. The discharge of an outlet may be
anything up to 4 or 5 c. feet per second. This limits the size of a
chak. If a chak is too big it can be split up or a minor can be
designed. Very small chaks are to be avoided, but it is difficult to fix
a minimum size. The irrigation boundary of the distributary, as fixed in
the project, is shown on the map but in practice it will not be exactly
followed. For various reasons the boundaries of a chak may run somewhat
outside it or stop short of it.
Where a distributary gives off a minor and there is a double regulator,
watercourses should, as far as possible, be taken off from one or other
of the branch channels and not from upstream of the double regulator.
Otherwise, irregularities are likely to occur, both of the regulators
being partially closed at the same time--a thing which is never
necessary in legitimate distribution of the supply--in order to head up
the water and increase the discharges of the outlets.
A watercourse nearly always gives off branches and generally a system of
turns is arranged by the farmers among themselves, each branch in turn
taking the whole discharge of the watercourse for a day or part of a
day, the other branches being closed by small dams of earth. To irrigate
a field alongside the watercourse a gap is cut in its bank. For fields
further away, smaller channels run off from the watercourses at numerous
points. Several gaps and several field channels may be in flow at one
time, and there is a dam in the watercourse below the lowest one.
Occasionally, on an old canal, one watercourse crosses another, the
lands irrigated being at different levels, but such crossings do not
often occur in systems of watercourses laid out according to modern
methods. They are, however, quite legitimate.
The lines of the main watercourses are sketched on the map, their
irrigation boundaries shown on it, and F.S. discharges allotted to them
according to the areas which are to be dependent on them. In order that
this may conveniently be done the “full supply duty” or “full supply
factor” for the distributary is obtained. It bears the same ratio to the
ordinary duty that the mean supply bears to the full supply. The total
of the F.S. discharges of all the watercourses should, with an allowance
for loss by absorption in the distributary, be the same as the F.S.
discharge of the distributary. If the results are very discrepant it
shows that the sizes of the outlets need revision. Possibly they may all
be too large.
In “colonization” schemes where a canal is constructed to irrigate waste
lands--which are the property of Government and which are divided into
square blocks and given out to colonists--Government has complete
control of the watercourse system, and can arrange it exactly as
desired, but in other cases landowners often strenuously oppose the
passage of watercourses through their lands. Compulsory procedure
according to legal methods is tedious, but the practical rule is not to
let anyone have water until any watercourses which are to pass through
his land have been not only agreed to but constructed.
In ordinary cases Government possesses no power as to the precise line
on which a watercourse is dug. It fixes the site of the outlet and
assigns certain land to it, and sketches out the line of the
watercourse. If the people choose to alter the line they can do so, but
great alterations in the main watercourses are not generally feasible.
The positions of the outlets[16] having been settled after discussion
with the cultivators, a table is prepared showing the chainage of the
outlets, the probable head or difference between the F.S. level of the
distributary and of the watercourse, and the F.S. discharge. From this
the sizes of the outlets are calculated and shown in another column. If
the length of the outlet barrel is not more than 5 or 6 times the
diameter--in the case of a barrel whose cross section is not round or
square, the mean diameter--the discharge can be calculated as for a
“short tube,” but if longer the formula for flow in pipes should be
used, allowance being, of course, made for the head lost at the
entrance. The outlets generally consist at first of wooden “shoots” or
long tubes, rectangular in cross section. This is because, after they
have been tested by a year or two years’ working, the sizes nearly
always require adjustment and the cultivators often wish to have the
site shifted.
[16] The positions can be slightly altered by the Engineers for any
sufficient reason.
[Illustration: FIG. 11.]
The uncertainty as to the proper size of an outlet is due to several
causes. If the command is very good there may be a clear fall from the
outlet into the watercourse. In this case the discharge depends only on
the depth of water in the distributary, and is known pretty accurately.
But ordinarily the outlet is submerged, and its discharge depends on the
difference between the water levels in the distributary and in the
watercourse. The latter level is not fixed. The cultivators can lower
it, to an extent which depends chiefly on the distance of the fields
from the distributary, by deepening or widening the watercourse. In this
way the discharge of the watercourse is increased except when a dam is
temporarily made in it for the purpose of irrigating any comparatively
high land. This uncertainty as to the discharge can in some cases be got
over by building a cistern (Fig. 11). This has the same effect as
raising the level of the barrel, the real outlet being no longer
submerged, and the discharge depending on the depth of the crest of the
overfall below the water in the distributary. But such cisterns add
greatly to the cost of an outlet, and they can only be adopted when
there is good command. A great cause of uncertainty as to the proper
size of an outlet is the variability of the duty of the water on the
watercourse. The soil may be clayey or sandy, the watercourse may be
short or long, the crops grown may be ordinary ones or may be chiefly
rice, which requires three or four times as much water as most other
crops, and the cultivators may be careful or the opposite. Again, the
people may, if the outlet gives a plentiful supply, often keep it
closed, but there is no record of such closures nor would the people
admit that they occur. These causes may all operate in one direction--on
a whole distributary this cannot happen to the same extent--and thus
enormous differences in duty may occur. There is no way of arriving at
the proper size for an outlet except trial. Observations of the
discharges of the outlets are of very limited use. The discharge may
vary according to the particular fields being irrigated. Observations of
discharges will be useful in cases where the people complain, or when
the discharge is obviously much greater or much less than intended and
will in such cases enable temporary adjustments to be made, but by
placing a dam in a watercourse and turning the water on to a high field
near its head the people can make it appear that the discharge is only a
fraction of what it should be.
On any distributary there are generally some watercourses which have a
poor command, the head at the outlet being, say, ·1 ft. or even less.
Probably the irrigation is a good deal less than it should be. In such
cases the rules may be set aside and a liberal size of outlet given. The
size may be 2 or 3 times the calculated size. There is no harm in this.
The irrigation cannot increase much. Similar cases frequently occur on
inundation canals especially near the heads of canals or distributaries.
The construction of masonry outlets on a distributary is not usually a
final settlement of the matter. Further adjustments become necessary.
This matter will be dealt with in CHAPTER III.
On the older canals little or insufficient attention was given to the
question of the sizes of outlets. The sizes were far too great and, as
long as all the outlets in a distributary remained open, water could not
reach the tail. The distributary used to be divided into two or three
reaches and the outlets in the upstream reaches used to be closed
periodically. The closures had to be effected through the agency of
native subordinates and the system gave rise to corruption on a colossal
scale. The tail villages never obtained anything like their proper share
of water. The upper villages were over-watered and the soil was often
water-logged and damaged. Moreover, even if all concerned had the best
intentions, it was impossible to stop all leakage in the closed outlets,
except by making earthen dams in the watercourses, and great waste of
water resulted from this.
The water level of the distributary with ³⁄₄ full supply, designed so as
to be at least ·5 ft. above the water level in the watercourse heads--or
to be 1 foot above high ground if this simpler plan is adopted--is drawn
on the rough longitudinal section and also the line of F.S., falls being
introduced where desirable and the gradients, F.S. depths of water and
widths of channels being arranged, just as in the case of a canal, so as
to give the required discharges, velocities suited to the soil and a
suitable ratio of depth to velocity. The bed width of a distributary
decreases in whole numbers of feet. The decrease occurs at outlets but
not at every outlet. As the channel becomes smaller its velocity becomes
less and this necessitates, according to the laws of silting and scour,
a reduced depth of water. The height and width of the banks in the tail
portion of a distributary should be made rather greater than
elsewhere--regard being had to the depth and volume of the water--so
that breaches may not occur when the demand abruptly slackens. The
longitudinal section of a distributary should have horizontal lines for
showing the following:
1. Datum |5. Draw-off | 9. Bank width |13. Depth of
| | | digging
2. Bed gradient |6. F.S. discharge|10. Height of |14. Bed level
| | bank |
3. Village |7. Velocity |11. F.S. depth |15. Ground
| | | level[17]
4. Land width |8. V₀ |12. Bed width |16. Chainage[18]
-----------------+-----------------+-----------------+----------------
[17] Called “Natural Surface” in India.
[18] Called “Reduced Distance” in India.
A specimen of a longitudinal section is shown in Fig. 12. It shows only
a few of the above items. In practice all would be shown, large sheets
of paper being used with all the lines and titles printed on them.
When a distributary is constructed the side slopes are made 1 to 1 in
excavation and 1¹⁄₂ to 1 in embankment. The sides usually silt up till
they are ¹⁄₂ to 1 or even vertical. The silting up to ¹⁄₂ to 1 is, as in
the case of a canal, allowed for in the designing. The berms are left so
that, if any part of the side falls in, the bank will not also fall in.
They also allow of widening of the channel. The remarks made in Art. 6
regarding the design of banks, apply to distributaries, especially large
ones.
On a distributary there is seldom much spoil. Where there is no spoil, a
strip of land, outside the bank and 10 feet wide, can be taken up on
either bank from which to obtain earth for repairs. On a minor the width
of the strip is sometimes only 5 feet.
[Illustration: FIG. 12.]