6.6
Canals and Command Area
Structures
(1) Canals in Spate Schemes
 traditional systems are diverted to short, steep
canals in the upstream areas (with considerable
slope)
 In downstream area (with low slope and less
heavy sediment content) water is guided gently
 split flows to reduce flood discharges to
manageable flow rates
 gates not used; control of flows by proportional
dividers and by small earthen canal bunds
Traditional channels in spate areas
Mid-land
spate areas
Lowland spate areas
Slope
Steep to flat
Very flat alluvial soils
Sediment load
Mixed – including
coarse sediment
Mainly fine silts
Type of canals
Short and steep
Gently guiding water –
sometimes trees in
channels help to slow
and stabilize
Main challenge is to
prevent heavy
sediment
deposition in
channels
Main challenge
is to prevent
erosion of
channels
Example: Canals in Spate
Schemes in Upstream Area
Canal
Maximum
capacity (m3/s)
Average bed slope
(m/km)
Mansury
40
3.8
Rayyan
60
3.7
Bagr
40
3.7
Gerhazi
50
3.9
Mawi
60
4.8
Bed slopes of traditional canals in the original (before modernization) Wadi Zabid system in Yemen.
Canal design
 In the first systems that were modernised
lower canal slopes than observed in
traditional systems were provided.
 Limited sediment transporting capacity in
canals = severe canal sedimentation
problems = high/unaffordable maintenance
requirements.
Desilting a canal head reach – immense work if one
gets it wrong
Water distribution
 In some early schemes water distribution
systems similar to those used in perennial
schemes were adopted where water is
supplied to numerous field outlets at the
same time. Farmers then head up flows at
undersized outlets, promoting canal
sedimentation.
Farmers check structure
Silted field outlet
Silted undersized crossing structure
(2) Spate canal design methods
Spate canal design methods
No scouring – no silting” criteria – not for spate
“Regime” design methods mostly for canals carrying low
sediment loads but Simons and Albertson method include
equations for canals with sand beds and cohesive banks,
carrying “heavy” sediment loads – have been used in spate
systems
 Rational methods provide the most logical method of designing
canals to achieve a specified sediment transporting capacity.
Chang, 1985 method provides predictions of slopes and bed
widths that are similar to that observed in many spate systems
 SHARC package for canal design


Use canal surveys to aid design in modernised
schemes
 Canal designs in modernised schemes are best
based on the slopes and cross sections of (stable)
existing canals. Design of enlarged, extended or new
canals can then be derived using the Chang
equation, with a judicious choice of input parameters
to provide a good match with the slopes and cross
sections observed in existing canals.
(3) Command area structures
 Check and drop structures;
 Flow splitting structures;
 Field offtakes; and
 In-field structures (see also module 4)
Gabion Distribution Structures
 Advantages:
 Stabilize the channel bed
 Proportional distribution of the flow
 Disadvantages
 Downstream scour and gullying may
undermine the structure
 Gabions may be difficult to repair (gabion
mats not easily available)
Model 1: Flow divider
Distribution can be
adjusted with brushwood
Flow approaches
Flow divider
 Advantages
 Easy to adjust flow distribution
 Not sensitive to gullying
 Disadvantages
 Only works where soil is hard and stony –
otherwise it creates scour and erosion of banks
Model 2: Flow distribution structure
SOME HINTS
Important to survey channel bed
downstream and check for gullies
The downsteam apron should
be long and deep enough to withstand
the upstream formation of gullies
SOME HINTS
Prevents
wash-out
of fine
materials
underneath
the gabions,
which can
lead to
overturning
Use of geotextiles
Use of geotextile underneath gabions
SOME HINTS
Geotextile inside gabion mattrass
CASE OF LOWLAND COMMAND FLOW DIVISION
MOCHIWAL, DI KHAN,
PAKISTAN
In lowland spate irrigation a main
challenge is to spread water gently
over a command area, thus
maintaining manageable velocities
in the flood channels and avoid the
floodwater going to low-lying
areas quickly.
The spreading of these large quantities
of flood water can often be achieved
with simple and low cost structures
Case: Mochiwal Flow Division
Darabam Zam
North channel:
-500 ha
-low lying area
Mochiwal Division Point
West Canal:
- 3000 ha
Problem in the past
Darabam Zam
West Canal:
- 3000 ha
North channel:
-500 ha
-low lying area
An earthen bund was built at this
division point. However it would break
quickly and all water would disappear
into North Channel, making it impossible
to control water here (all channel bunds
broken quickly), while leaving no water for
West Channel.
Considerable damage in
downstream area of North Channel
Solution: Flow Division Structure
Solution
North Channel
West Channel
The flow division structure allows both channels to be irrigated with
flood water at the same time, letting in a manageable flow into
North Channel as well as West Channel
Flow division structure:
-Three gated gates, one open
-Initially use of stoplogs but replaced with gates and hoisting gear
Cost:
-USD 2000
Benefit
-3500 ha
-USD 20,000 a year!
This spectacular impact was
due to the selection of this very
crucial site as well as a good
design.
The site selection was done by
experienced farmers.
Farmer contributed to cost of
structure and are maintaining
the site.
General principle!
Discuss and agree
the water
distribution
structures with the
representative
and authorized
group of water
users:
 location
 proportion
 design
Acknowledgement
This presentation was prepared with
thanks to:
Tzegai Teklemariam
WRRI DI Khan Team
Philip Lawrence
Ian MacAnderson