module 2 (part 1)
DESCRIPTION
Module 2 (part 1). Improving Diversion Works. John Ratsey [[email protected]]. Traditional diversions. Modern diversion structure. Sediment basins. Main canal. Gated canal intake. Flow. Sluiceway. Outlet for flushing basins. Concrete weir. Optimum layout of intake. - PowerPoint PPT PresentationTRANSCRIPT
Traditional diversions
Modern diversion structure
Concrete weir
Gated canal intake
Sluiceway
Sediment basins
Outlet for flushing basins
Main canal
Flow
Optimum layout of intake
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)
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
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?
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
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
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
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
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
• 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
Edge of Zabid irrigation areaFields are several metres above the
desert
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)
Velo
city
hea
d• Allow for the
energy head (v²/2g) in the designs
Typical wadi rating curveTypical 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)
Leve
l (m
)
Mean annual flood
1 in 5 year flood
1 in 10 yearl flood
1 in 20 year flood
1 in 50 year flood
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
Wadi Zabid weir 5Canal 50% of
flow
Canal 50% of flow
Concrete bed bar
Masonry wall
Gravel embankments will breach in major flood to
pass water downstream
Divide wall at Zabid weir 5
Concrete bed bar
Masonry divide wall
Weir 5
Gravel embankments
Gerhazy canal intake - plan
IntakeCanal
Wadi
Gravel embankment
Concrete bed bar
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
Bagr canal intake
Designed as one orifice but modified to two gates, but
blockage by trash is a problem
Gravel embankment to
divert water