Module 2 (part 1) Improving Diversion Works John Ratsey [[email protected]]
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Transcript of Module 2 (part 1) Improving Diversion Works John Ratsey [[email protected]]
![Page 2: Module 2 (part 1) Improving Diversion Works John Ratsey [john.ratsey@ntlworld.com]](https://reader036.fdocuments.in/reader036/viewer/2022062315/56649e975503460f94b9a7a6/html5/thumbnails/2.jpg)
Traditional diversions
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Modern diversion structure
Concrete weir
Gated canal intake
Sluiceway
Sediment basins
Outlet for flushing basins
Main canal
Flow
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Optimum layout of intake
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Key components
A control structure to limit / stop flow into canal
• A spillway to reject excess flows back to the wadi
• A sluiceway to remove sediment / maintain low flow channel
• Works to divert water to intake (bed bar / gravel embankment / weir)
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Typical layout of improved intake
Main flow
>4/5 x WLine of possiblebed bar or buried gabionexcavated low mattressesflow channel or head of obarbund built by farmers A side spillweir
B possible sluice
<1/5 x WC B
ACanal
Offtake COrifice orGated Orifice
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Questions for farmers
• 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?
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Design Options• 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
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Design considerations
• Best location to be determined by comparison of 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
• Gravel embankments will breach to enable passing of big floods
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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
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Al Zoran Intake
Intake is here
Canal is also in outside of bend
and needs protection
The original intake was probably further
downstream and has moved upstream to maintain command
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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
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Layer of sediment from one flood
Sediment thickness about 5cm from one
flood
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Rates of field level rise
• Scheme Annual rise rate, mm/year • Wadi Laba Eritrea Upstream fields 8–32 • (Measured 1998/99) Middle fields 6–18 • Downstream fields 5–9 • Wadi Laba Eritrea (Long term estimate) 30 • Eastern Sudan 139 • Baluchistan mountain systems > 50 • Wadi Zabid Upstream fields 20–50
• 30mm per year x 30 years is 90cm
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Edge of Zabid irrigation area
Fields are several metres above the
desert
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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
Velocity head
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5 3 3.5
Velocity (m/s)
Vel
oci
ty h
ead
• Allow for the energy head (v²/2g) in the designs
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Typical wadi rating curve
Typical Wadi Rating Curve
193.50
194.00
194.50
195.00
195.50
196.00
196.50
0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00 1600.00
Flow (m³/s)
Lev
el (
m)
Mean annual flood
1 in 5 year flood
1 in 10 yearl flood
1 in 20 year flood
1 in 50 year flood
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Options
• 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
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Wadi Zabid weir 5
Canal 50% of flow
Canal 50% of flow
Concrete bed bar
Masonry wall
Gravel embankments will breach in major flood to
pass water downstream
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Divide wall at Zabid weir 5
Concrete bed bar
Masonry divide wall
Weir 5
Gravel embankments
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Gerhazy canal intake - plan
IntakeCanal
Wadi
Gravel embankment
Concrete bed bar
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Gerhazi canal intake
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Intake for Gerhazy canal
Farmers use a gravel
embankment to close the intake
Breastwall with three orifices
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Space for the big flood
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Bagr canal intake
Designed as one orifice but modified to two gates, but
blockage by trash is a problem
Gravel embankment to
divert water