SURFACE IRRIGATION

SYSTEMES TECHNOLOGY
(CUAE 504)

E. Pandasvika
Faculty of Agricultural Sciences and Technology
Department of Agricultural Engineering
SURFACE IRRIGATION
SYSTEM DESIGNS
DESIGN OF CANALS &
PIPELINES
 DESIGN OF PIPELINES
 In piped surface irrigation systems, water is transported
in closed conduits or pipes in part or all of the
distribution system from the headwork up to the field
inlet.
 The pipes can be all buried, with outlets in the form of
hydrants protruding above ground level on field pipes.
 Or only the conveyance and supply lines can be buried
with field pipes being portable and laid above ground.
 In the latter case, the above ground pipes are made of
aluminium fitted with adjustable gate openings.
 Piped systems for surface irrigation, unlike piped
systems for sprinkler irrigation, do not require a lot of
head at the hydrant outlet.
 DESIGN OF PIPELINES
 The head should only be sufficient to push water through
the irrigation hose that takes the water from the hydrant
to the soil.
 In view of the low head requirements for the systems, it
is possible to employ gravity flow where there is
sufficient head to overcome the frictional losses in pipes.
 In situations where the head is not adequate, small
power pumps would be used with low operational costs.
 Pipes with low-pressure rating are also used for these
systems as they operate at reasonably low pressures.
 If the water level at the headwork is higher than the
water level required at scheme level, the water can be
transported through the pipes by gravity.
 Design of pipelines cont.
 If the water level at the headwork is lower than the water level
required at scheme level, then the water needs to be pumped
through the pipe to arrive at the scheme at the required elevation
necessary to be able to irrigate by gravity from the field inlet onwards.
 For calculation of friction losses, just as in sprinkler design ,
manufacturer friction loss charts or equations like the Hazen-Williams
can be used.
 The Hazen-Williams equation is given below;
 DESIGN OF CANALS
 The canal dimensions and longitudinal slope, whether
for irrigation or drainage, can be calculated through trial
and error with the Manning formula.
 This formula is derived from the continuity equation and
the equation for unsteady flow.
 These equations have been simplified by assuming
steady uniform flow in the canal (this assumes long
canals with constant cross-section and slope).
 The Continuity equation is expressed as:

Q=AxV
Where:
Q = Discharge (m3/sec)
A = Wetted cross-sectional area (m2)
V = Water velocity (m/sec)
 The Manning Formula
The Manning Formula can be expressed
as:
Equation 13
 Canal Parameters.

As and P, and thus R in the Manning formula, can be expressed as d, b and X, where:
d = Water depth (m)
b = Bed width (m)
X = Side slope = horizontal divided by vertical
For a trapezoidal canal, As is the sum of a rectangle and two triangles.
The manning’s formula is applicable to uniform flow. For uniform flow, the flow cross
section is same at all sections along the channel. Amount of water passing every section
is the same – mean velocity is constant.
 Cross-section, perimeter and
hydraulic radius of a canal
 As (Cross sectional area) is sum of a rectangle and the two
identical triangles at the edge and is expressed as:
As = b x d +Xd2 = d (b + Xd)
 P (Wetted Perimeter) is sum of bed width plus the two side
slopes from canal bottom to top of water level. It is
calculated from formula c2 = a2 + b2 and expressed as
P = b + 2(d2+(dx)2)1/2 = b + 2d (1 + X 2) 1/2
 Hydraulic radius R is
R = As/P = d (b + Xd)/b + 2d (1 + X2) 1/2
 When the computed discharge is equal to the required
discharge, the velocity needs to be checked to ensure that
it is acceptable.
 Velocity increases with an increase in gradient.
Recommended maximum gradient is 1:300 or 0.33%. With
high velocities, flow becomes super-critical and becomes
difficult to siphon. The state of flow is checked using the
Froude Number Fr which is given by:
Cross-section, perimeter and
hydraulic radius of a canal
Fr = V/(g x l)1/2
 Where:
V = water velocity (m/sec)
g = gravitational acceleration (9.81m/sec2)
l = hydraulic depth of an open canal, defined as the
wetted cross-sectional area divided by the width
of the free water surface (m)
 Fr = 1 for critical flow
 Fr greater than 1 for super-critical.
 Fr less than 1 for sub-critical flow
 It is important to maintain a Froude number of 1 or less
so that the flow is at or below the critical level.
 Freeboard
 Freeboard (F) is the vertical distance between the top of
the canal bank and the water surface at design discharge.
It gives safety against canal overtopping because of
waves in canals or accidental raising of the water level,
which may be a result of closed gates. The safe
freeboard can be calculated from the following equation:
F = C x h1/2
 Where: C = 0.8 for discharges of up to 0.5 m3/sec,
1.35 for discharges in excess of 80m3/sec
h = water depth (m)
 For lined canals, F ranges from 0.40 m for discharges less than 0.5
m3/sec up to 1.20 m for discharges of 50 m3/sec or more. For very
small lined canals, with discharges of less than 0.5 m3/sec, the
freeboard depths could be reduced to between 0.05-0.30 m.
 Factors affecting canal discharge
Canal gradient
 A canal with steeper gradient but with the same Cross-
section discharges more water than a canal with a
smaller gradient.
Canal roughness
 Influences the amount of water that passes through a
canal. A higher Km (lower n) the higher the ability of the
canal to transport water, hence the smaller the required
cross-sectional area for a given discharge.
Canal shape
 Canals with narrower beds and higher water depths have
a smaller wetted perimeter, and thus a higher discharge,
than canals with larger beds and lower water depths, for
the same cross-sectional area.
 This is due to the fact that the hydraulic radius R (= As/P)
increases if the wetted perimeter decreases, while
keeping the wetted cross-sectional area the same
 Factors affecting canal
discharge
Side slope
 For unlined canals, soil texture determines the side
slope.
Maximum water velocities
 The higher the velocity , the higher the discharge. For
unlined canals, ensure that velocity is non-erosive as
well as self-cleaning.
 Canal Shapes
Although the trapezoidal canal shape is very common,
other canal shapes, including V-shaped, U-shaped,
semicircular shaped and rectangular shaped canals, can
also be designed as shown below
Km and n values for different types of
canal surface (Source: FAO,2002)
 Typical canal side slopes (Source: FAO,2002)

Maximum water velocity ranges for earthen canals on different types of soil
(Source: Peace Corps Information Collection and Exchange, in FAO,2002)

The recommended bed width/water depth (b/d) ratios for earthen trapezoidal
canals •The bed width should be wide enough to allow
easy cleaning.
•A bed width of 0.20-0.25 m is considered to be
the minimum, as this still allows the cleaning of
the canal with small tools such as a shovel.
• Lined trapezoidal canals could have similar b/d
ratios as given in opposite table
Hydraulic parameters for different canal shapes
 Seepage losses in earthen canals
 Unlined earthen canals are the most common means of
conveying irrigation water to irrigated lands.
 Farmers prefer them because they can be built cheaply and
easily and maintained with farm equipment. Unlined canals
are also flexible, as it is easy to change their layout, to
increase their capacity or even to eliminate or rebuild them
the next season.
 However, unlined canals have many disadvantages that
make them less desirable compared to lined canals or
underground pipes. These are:
 They usually lose more water due to seepage, leakage and
spillage
 Rodents can cause leakage
 Frequent cleaning is needed because of weed growth
 Earth ditches can erode and meander, creating problems in
maintaining straight or proper alignments
 Labour costs of maintenance of unlined canals are normally
higher than of lined canals and pipelines
 They provide an ideal environment for the vector of bilharzia
 Seepage losses in earthen
canals cont
 When designing earthen canals, it is important to ensure
that the slope is such that the bed does not erode and that
the water flows at a self-cleaning velocity .
 Relatively flat lands on soils with a high percentage of silt
and clay are the most suitable for canal construction,
because of low infiltration rates.
 In earthen canals, seepage occurs through the canal bed
and sides. In areas where relatively permeable soils are
used to construct canals, high seepage can be expected.
 The higher the seepage losses in the canals the lower the
distribution system (conveyance and field canal)
efficiencies, since much less water than that diverted at
the head works reaches the fields.
 Seepage losses in earthen canals cont.
Seepage is difficult to predict. Two simple ways to estimate seepage losses are:
1. Measurement of inflow into and outflow from the canal at selected points.
The difference between the inflow and outflow measurements will not only
represent seepage losses, but evaporation losses as well.
2. Measurement of the rate of fall of the water level in a canal stretch that has
been closed and where the water is ponding.
•From these losses the estimated evaporation should be subtracted to get the
seepage losses.
• Usually, seepage losses are expressed in m3 of water per m2 of the wetted
surface area of a canal section (P x L) per day. If a field test cannot be carried
out, seepage can be estimated from Table below, which gives average seepage
losses for different types of soil.
Seepage could be localized where a portion of highly permeable material has
been included in the bank or where compaction has been inadequate during
canal construction.
 Canal lining
 Canal lining is generally done in order to reduce seepage
losses and thus increase the irrigation efficiencies.
 It also substantially reduces drainage problems and canal
maintenance as well as water ponding, thus reducing the
occurrence of vector-borne diseases.
 Also, smooth surface linings reduce frictional losses,
thereby increasing the carrying capacity of the canals.
 Material used for canal lining are:
 Clay
 Polyethylene plastic (PE)
 Concrete
 Sand-cement
 Brick
 Asbestos cement (AC)
 The selection of a lining method depends mainly on the
availability of materials, the availability of equipment, the
costs and availability of labour for construction.
 Canal section sizes commonly
used in Zimbabwe
 Dept of Irrigation has adopted a 60o trapezoidal canal
with a depth of 0.3m, a freeboard of 0.05m and bed
widths of 0.25m, 0.30m, 0.375m and 0.5m depending on
gradient and capacity or discharge required.
 Only the bed width is varied
 The total depth of the canals is 0.35m (water depth +
freeboard) is easily reached by construction gangs while
placing concrete.
 It also provides an adequate siphon head and gives
efficient flows within range.
 Narrowest bed width used of 0.25m is easy to clean
 By varying bed width only and not depth, change from
one canal section to another is simplified
Canal capacities for standard DoI canal
sections.
 Calculations of inputs to the
Mannings formula
 In the calculation of canal flow, the following
parameters can be fixed.
 Km or n (canal roughness co-efficient) relates to canal
quality as it affects water velocity in canal and is provided
from tables. Concrete lined canals have a Km value of 55.
 Canal gradient S expressed as a decimal. Most of our
field canals are pegged at 0.002 grade 1:500.
 Side slope X .DoI uses 60 degree trapezoidal canal where
X is 0.58 (Calculated from tan 600 being equal to I/X );
 Water Depth d Most canal sections (for DoI) have a fixed
water depth of 30cm plus 5cm freeboard for easy transition
from one section to the other.
 Bed width b can now be the variable that is used in
the iteration to come up with the required Q discharge.