6.4
Improving Diversion Works
John Ratsey [john.ratsey@ntlworld.com]
Objective for improvement
 Make it easy for farmers to operate and maintain
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reducing the large inputs of labour or other resources
to maintain;
Prevent large and uncontrolled flood flows from
damaging canals and field systems;
Help maintaining the distribution of water within the
system in line with accepted rules and rights,
Providing flexibility to accommodate changing
hydrological patterns, future changes in water
distribution and cropping pattern
Equality of water shares between upstream and
downstream users;
Therefore..
 Structures proposed need to be equipped with intakes
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that prevent large uncontrolled flows from entering canals
Only floods that can be contained within the canal
capacities are allowed to pass down the system
Downstream damage to channels and field systems is
minimised
Designed to limit the entry of the very high concentrations
of coarse sediments
Structures also need to act as erosion control devices in
unstable Wadis, characterized by lateral movements of
low/medium flow channels within the wider Wadi cross
sections, streambank erosion and head-cutting, etc.
Typical modern diversion structure
Main canal
Sediment
basins
Gated canal
intake
Flow
Sluiceway
Outlet for
flushing
basins
Concrete
weir
Shortcoming of these structures:
 Area in front of weir silts up >> diversion in the end by
traditional structures
 Gate operation often problematic >> no operator at
the right time or gates getting stuck
 Sedimentation ponds not operated and not
maintained >> sedimentation problem
 Large investment in single off-take – often with long
conveyance channel>> creating conflicts and
depriving downstream water users
Improved modernized structure:
key components
 A control structure to limit / stop too high flow into
canal (orifice; gated intake)
 A spillway to reject excess flows back to the river
 A sluiceway to remove sediment / maintain low flow
channel
 Works to divert water to intake and prevent changes
in river bed through scour:
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bed bar (or bed stabilizer) or weir
gravel embankment – to guide water to intake, but with
capacity to breach in case of high floods
Typical layout of improved intake
Main flow
>4/5 x W
Line of possible
bed bar or
excavated low
flow channel or
bund built by farmers
buried gabion
mattresses
head of obar
A
side spillweir
B
possible sluice
<1/5 x W
C
B
A
Canal
Offtake
Orifice or
Gated Orifice
C
Questions for farmers
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Do they want protection against floods
damaging their canal system?
Do they want to be able to stop the flow of
water into their canals?
Do they want to divert the whole of the wadi
flow, or only a part of the flow?
What are their priorities?
Design Options
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Ungated head regulator for existing offtake
Ungated head regulator for existing offtake
with spillway
Gated head regulator for existing offtake
Gated head regulator for existing offtake with
spillway
Optional sluiceway for ejecting sediment and
maintaining low flow channel
Design considerations
 Best location to be determined by comparison of
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wadi level, field levels (with allowance for rising)
and canal slope
Intake structure to occupy less than 20% of wadi
width
Orifice head regulator limits maximum flow into
the canal
Side spillway enables rejection of excess flow
approaching canal intake
Managing for the big floods: gravel or soil
embankments will breach to enable passing of
big floods
Location selection
 Intakes are best located at the outside of a
bend where the low flow channel will be
 However, outsides of bends are most
vulnerable to erosion
Command and field level rise
 The water level at the intake needs to be
enough to supply water to the first fields
during a small flood
 Otherwise the first farmers on the canal will
block the flow to other farmers until they have
irrigated their land
 The design has to consider the likely rise in
field levels within the life of the structure
Layer of sediment from one flood
Sediment
thickness about
5cm from one
flood
Rates of field level rise
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Scheme
Wadi Laba Eritrea
(Measured 1998/99)
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30mm per year x 30 years is 90cm
Wadi Laba Eritrea
Eastern Sudan
Baluchistan
Wadi Zabid
Annual rise rate, mm/year
Upstream fields
8–32
Middle fields
6–18
Downstream fields
5–9
(Long term estimate)
30
139
mountain systems
> 50
Upstream fields
20–50
Design for the big flood
 Before designing anything to be built in the wadis, try
to visualise the big floods
 Make provision for the floods to pass with the
minimum of damage
 Allow for the
0.5
0.4
Velocity head
energy head
(v²/2g) in the
designs
Velocity head
0.3
0.2
0.1
0
0
0.5
1
1.5
2
Velocity (m/s)
2.5
3
3.5
Use local knowledge
of floods and these
are judged to design
improved structures
Typical wadi rating curve
Typical Wadi Rating Curve
196.50
196.00
Mean annual
flood
Level (m)
195.50
1 in 50 year
flood
195.00
1 in 10 yearl
flood
1 in 20 year
flood
194.50
1 in 5 year
flood
194.00
193.50
0.00
200.00
400.00
600.00
800.00
Flow (m³/s)
1000.00
1200.00
1400.00
1600.00
Options: bed bar/ weir
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We do not necessarily need weirs
Weirs are useful to:
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Create head
Divert water
Stabilize the river bed
BUT
 Create head: this may also be achieved by having the intake further
upstream
 Stabilize the river bed: bed bars or bed stabilizer will prevent scour of the
river bed and will stabilize the river bed as well
 Divert water: as in spate river the area in front of the weir is often silted up
farmer guide water to the intake by making additional diversion structure
from gravel, sand or brushwood anyway
THEREFORE
 Bedbars with gravel embankments or soil bunds may be more durable and
cost effective solutions
Bed stabilizer: burried into river bed
Provide
weepholes
to prevent
that
subsurface
flow is
blocked and
lost to
downstream
areas
In gravelly river bed bars should be made of
concrete – in sandy areas gabion bed
stabilizers can be considered
If going for a weir also
consider options
without stilling basins
but with cascaded
downstream toe
Options: intake
 Gates to only be provided if required by the
farmers. A large orifice may be sufficient
 Sluiceways are more appropriate in the
upstream part of wadis where more water is
available for flushing
 Strength / cost of structure will decrease
upstream to downstream, but the benefits will
also decrease unless a large command is
reached (which is often possible in downstream
areas)
Intake Capacity
 Need to convey large volumes of water by gravity to
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fields during the short periods when Wadi flows occur.
Hydrological information on Wadi flows are in almost all
cases very limited.
The timing, duration and maximum discharge of spate
flows are thus unpredictable,
Areas traditionally irrigated varying significantly from year
to year.
Water conveyance and distribution systems developed
for perennial irrigation are not appropriate for spate
systems.
Intake Capacity
 Much larger capacities are needed (10 to 100 times
greater than for perennial systems for a given area)
 Flow velocities will be higher than conventional canals
as the water is heavily laden with sediment and
generally pass through much coarser material that have
been deposited over the years.
 Traditional intakes and their modern replacements can
both fulfil these above functions, although by different
means and with large differences in capital and
maintenance costs and requirements
Gerhazy canal intake - plan
Wadi
Concrete
bed bar
Canal
Intake
Gravel
embankment
Gerhazi canal intake
Intake for Gerhazy canal
Farmers use a
gravel
embankment to
close the intake
Breastwall with
three orifices
Space for the big flood
Main river bed
Bagr canal intake
Designed as one
orifice but modified
to two gates, but
blockage by trash is
a problem
Gravel
embankment to
divert water
Initial layout for Al Mossefiyah Intake
Another canal
Curved
gabion wall
Width ~100m
Gabion wall
Wadi
flow
Gabion wall to
restrict intake
opening size
Al Mossefiyah
canal
Suggested layout for Al Mossefiyah Intake
Possible bed bar
if acceptable to
downstream
users
Intake
structure
with orifice
Gabion wall
designed as
spillway
Wadi bank
Gravel
embankment to
be constructed by
farmers
Wadi Zabid weir 5
Canal 50% of
flow
Canal 50% of
flow
Masonry
Dividing
Structure
Gravel
embankments
will breach in
major flood to
pass water
downstream
Concrete bed
stabilizer
Divide wall at Zabid weir 5
Weir 5
Gravel
embankments
Concrete
bed bar
Masonry
divide wall
Economics
 Building flood resistant structures can be
expensive
 The benefits, from reduced maintenance work
and improved crops, may be moderate
 Functional requirements may be changed
after 25 or 50 years (higher command levels,
even fewer floods)
Improvements to modern intakes
 Works to improve sediment removal
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Eg. skimming weir in front of intake
 Removal of divide wall
 Works to improve hydraulic performance and
flow division
 Weir crest raising to restore command
 Works to help flow management
Skimming weir
Skimming weir constructed in
front of head regulator.
However, water is needed for
flushing
Sediment in canal
Gravel in canal
reduced, but
not avoided