Transcript Intakes and Diversions - Spate Irrigation Network

Intakes and Diversion Strucures
Designing a Spate System
Lesson 3
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Designing a Spate System
- Diversion structures & intakes;
- Spate canals
- water control & dividing structures;
- Bank protection
- Wadi training structures.
Traditional diversion and water
distribution structures
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divert ephemeral rivers using only local
materials and indigenous skills
relatively high overall water diversion
efficiency
the high labour inputs needed to re-build the
structures
environmental problems resulting from
unsustainable use of trees and brushwood
What we have learnt
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For engineering successful interventions:
– replicate as far as possible traditional
diversion practices
– reflect time commitments and technical
knowledge of the farmers
– facilitate the control of large flood
– replicate water distribution in line with
accepted rules and rights
What we have learnt
ensure a right balance between the
needs of different water uses and users
– improve the effectiveness of the systems
to function with high rates of sediment
transport
– improve the ability to cope with frequent
and large changes to the levels and
alignments
–
rehabilitation and improvement works
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Farmers should be consulted and involved in the
planning, design and execution
Engineers provide a range of technically and
economically viable options
Comprehend the long years of experience that the
farmers have of the systems and Wadi flow,
Assisting farmers in selecting the most appropriate
improvements that replicate and improve upon
traditional schemes
Disappointing performance attributed
to:
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An increased inequity of water distribution
Command and diversion problems due to
high rates of sediment deposition
reduced the WUAs/farmers’ role in diverting
and distributing water
unrealistic assumptions concerning levels
and costs of operation and maintenance
failures to achieve an expected increase in
irrigated area
Understanding each other
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Ensure what you have in mind is what the
farmers envisage
Compromise “perfect” engineering solutions
with levels of risk acceptable to farmers
What we have learnt
ensure a right balance between the
needs of different water uses and users
– improve the effectiveness of the systems
to function with high rates of sediment
transport
– improve the ability to cope with frequent
and large changes to the levels and
alignments
–
What we have learnt
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Do not upset balance of water distribution
and inadvertently favour the better off at the
tops of the systems
Understand water rights and implications
Traditional
Local resources
 Large bund diverting all flow
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Traditional
Diverting only part of flood flow
 Across wide Wadi
 Alongside narrower Wadi to collect only
part of flow
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design of improved spate systems
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The over riding principle is that there is no single
approach
Specific requirements vary widely between and in
some cases within schemes,
Before proposals are finalised, it is essential that
engineers fully understand the way in which the
farmers system has operated
and farmers truly understand and comprehend what
the engineers are proposing for them
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Use local knowledge of floods and how they
judge them to design improved structures
Diversion structures and intakes
Need to be able to divert short duration
flood flows into gravity canal systems
 Ensure that sufficient amounts are
abstracted in the time available
 that the canals systems ensure
command over the fields to be irrigated.
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Diversion structures and intakes
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Have the capacity to convey the water with
minimal sedimentation and scour
Some lands will fall out of command or may
not be economic to irrigate considering the
technically most suitable sites for structures.
Secondary and maybe more problematic
sites should not be chosen just so that small
additional areas can be conveniently added.
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Structures proposed need to be equipped with intakes
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.
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In the flatter areas, often at the downstream
limits of the spate system areas - command
over the irrigated lands can be a problem
Exacerbated over time with the deposition of
silt on the lands,
Intakes must also function to ensure that
sufficient head is maintained over the longer
term
Cope with rising irrigation command levels at
the field
Traditional Intakes
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Advantages:
– Flexibility •
– Appropriate and low cost
– Relatively efficient in water use and
sharing between users
– Restrict diversion of high flows with
high sediment loads
Traditional intakes fall into two main
types
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spur deflection
Wadi
Deflecting Spur
Wadi
To fields
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bund diversion
Bund
A
A
Wadi
A-A
To Fields
2-5 m
5-7m
Common Features
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Located at outside or just downstream from relatively
mild Wadi bends
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Consist of low spurs flows extending at a slight angle out
into the Wadi
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deep water channel is scoured in floods
where lower flows are channelled during flood recessions;
to intercept the low flow channel,
diverting lower flows to an un-gated canal;
Constructed from locally available materials
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damaged by larger flood flows – equitable use of flood flows
can be maintained and re-constructed by farmers without or with
limited significant external support (bulldozers are often available
to farmers);
Common Features
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Recognize force and damage caused by large and very
large flood flows
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designed to breach or break when they occur,
thereby reducing danger to the spate irrigation system;
Are not “greedy” and do not try to extract all of the flow
Designed to “coax” the flows into the intake taking as
much as they dare without endangering the whole
system.
While the different forms of construction result in varying
degrees of durability, mainly due to available labour and
local materials, all are likely to be damaged or
completely swept away by larger floods.
Advantages of traditional intakes:
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Flexibility:
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Location and layout of traditional intakes easily adjusted to suit
the changing Wadi bed condition;
Deflecting spurs can be extended and diversion bunds moved
upstream when sedimentation on the fields or in the canals
starts to take fields out of command, or stream bank erosion
occurs.
Appropriate and low cost:
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traditional intakes are constructed from local materials using
indigenous skills
can be maintained indefinitely by farmers without outside
support, apart from environmental problems resulting from
unsustainable use of trees and brushwood.
Relatively efficient in water use and
sharing between users:
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Many traditional intakes are used along larger Wadis, each
serving their own separate command area
Can achieve high overall diversion efficiency and better water
equality
Large spate floods will destroy intakes located at the heads of
each spate command area as they pass down a Wadi, with those
upstream intakes breaking sooner than those further downstream.
Once the flood peak (or rising limb of the flood hydrograph) has
destroyed traditional diversion bunds at one location, the flow
passes to the next one further downstream and so on,
sharing water between many offtakes and not permitting the
upstream site to “steal” all of the flow.
Lower sites experience less severe floods, exposed for longer
periods, thereby compensating for water not received from other
smaller floods that only reach the upper intakes.
Restrict diversion of high flows with
high sediment loads:
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Failure of deflecting spurs, diversion bunds and
breach sections of main canal at high Wadi
discharges abruptly lower the water level at the
canal intakes,
reduces discharges that are diverted, limiting the
damage to the downstream canals and field systems
Preventing the incursion of high concentrations of
coarse bed material sediments transported in large
floods
Important disadvantage associated
with traditional diversion structures
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enormous input of labour needed to maintain and reconstruct
intakes that are damaged, or washed out by large floods
the continual use of new brushwood and tree material needed
to reinforce the bunds.
The timing of repairs and breaches means that there is often a
shortage of suitable material around the diversion sites
Time is water lost so need to rebuild the bunds as soon as
possible
High recession flood flows often prohibit access to river bed
material
Deposition of silt and wet area around bund means access
even by machinery very difficult
Improved diversion structures
Raised weir;
– Gated scour or under sluice;
– Gated canal head regulator; and
– Guide or divide wall.
 divert the maximum possible amount of
water
 capacities per unit area being 10 to 20
times perennial irrigation schemes
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Improved Intakes
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More durable diversion spurs and division
groynes;
Improved diversion bunds and check weirs;
Controlling the flows admitted to canals;
Basic gated or orifice control intakes; and
Rejection spillways.
Intake Capacity
a limited number of major diversion
structures
 large new canals that connect into and
traverse the existing traditional canal
network
 increase in the inequity between upstream
and downstream users’
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Intake Capacity
conventional economic analyses
 are insufficient to meet the requirements of
the previously commanded areas
 design duties too low
 numerous intakes and canals reveals
consolidation into one system supplied
 An overall Water User Association
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Objective for improvement
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Make it easy for farmers to operate and maintain
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;
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Ensure an appropriate balance between the needs of
different irrigation water uses and other water users
(agriculture, drinking water, downstream users, etc);
Help maintaining the continuity to function with high rates
of sedimentation, with as much as possible being
deposited on the fields and not in the canal beds; and
Help farmers to cope with frequent and sometimes large
changes to intake invert levels and initial reaches of
canal alignments due to significant changes in Wadi
channels following floods.
Diversion Spurs
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Durable spurs constructed on deep foundations and
protected from scour can be successful provided that
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the designer is sufficiently experienced with designing under
spate flow conditions
is familiar with the construction material and techniques
Aware of process of scour under peak flood flows.
Durability of improved forms of traditional intake, in terms of
damage suffered and number of times re-construction is
necessary in a “normal” spate season, presented below.
Durability of traditional and improved gabion
diversion spurs in Eritrea (Adapted from Haile, 1999)
Number
of
times
reType of Material used for construction
is
necessary
diversion. spur
during normal spate season
(Average)
Traditional Wadi bed
material and brushwood
Stone
Gabion
2-4
0-1
Can last for up to 5 years –
need concrete crest
protection
Intake Capacity
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Need to convey large volumes of water by gravity to 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
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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
Disadvantage of single new intake
approach
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Gives upstream users control over diversion of a larger
proportion of the annual flows
in turn leads to an increase in the inequity between
upstream and downstream users’ access to water.
often resulted from the way in which systems are being
operated in response to pressures from powerful local
interests, rather than from inherent technical design
deficiencies
or misunderstanding of traditional water distribution
arrangements.
An equitable distribution of flows would have been
possible in some upgraded systems if larger intake
capacities had been provided.
Disadvantage of single new intake
approach - Wadi Mawr, Yemen
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Complicated system was devised for dividing flows - never used
as the farmers did not understand the Engineers,
Too theoretical – need to be tailored to farmers
education/understanding
All WUAs not properly involved from the start.
Net result that upstream users control all the water that was able
to enter the system,
Not sufficient to meet all needs, downstream users in middle and
end parts of the command area could not receive sufficient water.
Those farmers who could afford it, changed to using groundwater
for irrigation
Those with less resources farmers have now succeeded in
getting donor support to revive the older traditional intakes
Reason for Inappropriate Designs
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Most upgrading and modernisation activities related to intake
and diversion improvements - donor supported.
Used conventional economic analyses to reach what they
consider as cost effective designs
Resulted in a diversion capacity for new intakes less than
the combined capacity of the traditional intakes.
An over riding problem in Wadi Siham in Yemen where all
such intakes are insufficient to meet the requirements of the
previously commanded areas.
Designers did not seem comprehend the traditional means
for sizing intakes or if they did, it was not made clear to
farmers and local authorities that only part of the previously
command and irrigated area would continue to be irrigated
under the new intake system.
Example of Better Approach
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In Wadi Zabid, designs adopted comprise a
number of separate intakes built from
gabions
Based on traditional locations and design
duties (15 l/s/ha up to about 60 l/s/ha ).
The smaller the area, the larger the unit flow
Gated intakes
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Provide a capability to regulate the flow into a canal
Can be considered where improved more durable
diversions such as weirs are used.
Gates should be as wide as possible considering the
intake requirements
Electrical or hydraulic operation preferred to manual due
to the very short time available after the start of flood
flows to open and close the gates.
If manual operation is considered, high gain gears must
be included for fast gate operation.
Vertical lift gates wider than 2m are not suitable for
manual operation especially in spate systems.
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Intake gates should be provided with large trash
diverters/excluders to trap the very large transported
items such as trees, but not restrict flow to the
intakes.
located upstream from the intakes
where possible to guide large debris over the weir
sections or around the diversion bunds or spursto
ensure that no blockage of the intake and loss of
water for the farmers occurs.
Access to clean these racks etc. must be included so
that machines can be used for regular maintenance.
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As a safety feature, and assuming that gates may be left open when
excessive floods occur, the gated opening must be designed to
operate as an orifice as described for enhanced local intakes above.
There is a danger that the breast wall is set to low, thereby reducing
the actual flows that can enter the intake.
Example - Barquqa diversion weir and intake on the Wadi Siham in
Yemen, the breast wall was set so that less than the stated design
flow (5 m3/sec for 3,700 ha) specific flow (l/s/ha) meant that the
served command area had to be reduced to 1,700 ha and an
additional new weir (Dabaishia Weir) and new main canal was built
Essential that intake flows are related to command areas and
downstream main canal capacities.
Hydraulic calculations for free flow and submerged flow are needed
to cross check the elevation of the breast wall and intake size
Operation
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Ease of Rapid opening and closure
Considers local conditions – no power; poor
communication; time of floods; etc
Canals in Spate Schemes
traditional systems are diverted to short,
steep canals
 split flows to reduce flood discharges to
manageable flow rates
 Gates not used; control of flows by
proportional dividers/farmer management
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New/improved canal structures
Check and drop structures;
 Flow splitting structures;
 Field offtakes; and
 In-field structures
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Canals in Spate Schemes
Bed slopes of traditional canals in the original (before modernization) Wadi Zabid system in Yemen.
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
QUESTIONS?