6.6Canals and Command AreaStructures

(1) Canals in Spate Schemes

traditional systems are diverted to short, steep canals in the upstream areas (with considerable slope)

In downstream area (with low slope and less heavy sediment content) water is guided gently

split flows to reduce flood discharges to manageable flow rates

gates not used; control of flows by proportional dividers and by small earthen canal bunds

Traditional channels in spate areas

Mid-land

spate areas

Lowland spate areas

Slope Steep to flat Very flat alluvial soils

Sediment load Mixed – including coarse sediment

Mainly fine silts

Type of canals Short and steep Gently guiding water – sometimes trees in channels help to slow and stabilizeMain challenge is

to prevent heavy sediment

deposition in channels

Main challenge is to prevent erosion of channels

Bed slopes of traditional canals in the original (before modernization) Wadi Zabid system in Yemen.

CanalMaximum 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

Example: Canals in Spate Schemes in Upstream Area

Canal design

In the first systems that were modernised lower canal slopes than observed in traditional systems were provided.

Limited sediment transporting capacity in canals = severe canal sedimentation problems = high/unaffordable maintenance requirements.

Desilting a canal head reach – immense work if one gets it wrong

Water distribution

In some early schemes water distribution systems similar to those used in perennial schemes were adopted where water is supplied to numerous field outlets at the same time. Farmers then head up flows at undersized outlets, promoting canal sedimentation.

Farmers check structure

Silted field outlet

Silted undersized crossing structure

(2) Spate canal design methods

Spate canal design methods

No scouring – no silting” criteria – not for spate “Regime” design methods mostly for canals carrying low

sediment loads but Simons and Albertson method include equations for canals with sand beds and cohesive banks, carrying “heavy” sediment loads – have been used in spate systems

Rational methods provide the most logical method of designing canals to achieve a specified sediment transporting capacity. Chang, 1985 method provides predictions of slopes and bed widths that are similar to that observed in many spate systems

SHARC package for canal design

Use canal surveys to aid design in modernised

schemes

Canal designs in modernised schemes are best based on the slopes and cross sections of (stable) existing canals. Design of enlarged, extended or new canals can then be derived using the Chang equation, with a judicious choice of input parameters to provide a good match with the slopes and cross sections observed in existing canals.

(3) Command area structures

Check and drop structures; Flow splitting structures; Field offtakes; and In-field structures (see also module 4)

Gabion Distribution Structures

Advantages: Stabilize the channel bed Proportional distribution of the flow

Disadvantages Downstream scour and gullying may

undermine the structure Gabions may be difficult to repair (gabion

mats not easily available)

Model 1: Flow divider

Flow approaches

Distribution can beadjusted with brushwood

Flow divider

Advantages Easy to adjust flow distribution Not sensitive to gullying

Disadvantages Only works where soil is hard and stony –

otherwise it creates scour and erosion of banks

Model 2: Flow distribution structure

The downsteam apron shouldbe long and deep enough to withstandthe upstream formation of gullies

Important to survey channel beddownstream and check for gullies

SOME HINTS

Use of geotextiles

Use of geotextile underneath gabions

SOME HINTS

Prevents wash-outof fine materialsunderneath the gabions, which canlead to overturning

Geotextile inside gabion mattrassSOME HINTS

CASE OF LOWLAND COMMAND FLOW DIVISION

MOCHIWAL, DI KHAN, PAKISTAN

In lowland spate irrigation a main challenge is to spread water gently over a command area, thus maintaining manageable velocities in the flood channels and avoid the floodwater going to low-lying

areas quickly.

The spreading of these large quantities of flood water can often be achieved with simple and low cost structures

Case: Mochiwal Flow Division

Darabam Zam

Mochiwal Division Point

North channel:-500 ha-low lying area

West Canal:- 3000 ha

Problem in the past

Darabam Zam

An earthen bund was built at this division point. However it would breakquickly and all water would disappearinto North Channel, making it impossibleto control water here (all channel bundsbroken quickly), while leaving no water forWest Channel.

North channel:-500 ha-low lying area

West Canal:- 3000 ha

Considerable damage in downstream area of North Channel

SolutionSolution: Flow Division Structure

North Channel

West Channel

The flow division structure allows both channels to be irrigated with flood water at the same time, letting in a manageable flow into

North Channel as well as West Channel

Flow division structure:-Three gated gates, one open -Initially use of stoplogs but replaced with gates and hoisting gear

Cost:-USD 2000Benefit-3500 ha-USD 20,000 a year!

This spectacular impact wasdue to the selection of this verycrucial site as well as a good design.

The site selection was done byexperienced farmers.

Farmer contributed to cost of structure and are maintaining the site.

Discuss and agree the water distribution structures with the representativeand authorized group of water users:

location proportion design

General principle!

Acknowledgement

This presentation was prepared with

thanks to:

Tzegai Teklemariam

WRRI DI Khan Team

Philip Lawrence

Ian MacAnderson