Module 2 (part 1)

Improving Diversion Works

John Ratsey [john.ratsey@ntlworld.com]

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 area

Fields 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)

Vel

oci

ty h

ead

• Allow for the energy head (v²/2g) in the designs

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

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 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

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