6.6

Canals and Command Area Structures

(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

Lowland spate areas Slope Mid-land spate areas Steep to flat Very flat alluvial soils Sediment load Mixed – including coarse sediment Short and steep Type of canals Main challenge is to prevent heavy sediment deposition in channels Mainly fine silts Gently guiding water – sometimes trees in channels help to slow and stabilize Main challenge is to prevent erosion of channels

Example: Canals in Spate Schemes in Upstream Area

Canal

Mansury Rayyan Bagr

Maximum capacity (m

40 60 40

3 /s) Average bed slope (m/km)

3.8

3.7

3.7

Gerhazi 50 3.9

Mawi 60 4.8

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

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 be adjusted 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

SOME HINTS

Important to survey channel bed downstream and check for gullies The downsteam apron should be long and deep enough to withstand the upstream formation of gullies

SOME HINTS

Use of geotextiles

Prevents wash-out of fine materials underneath the gabions, which can lead to overturning Use of geotextile underneath gabions

SOME HINTS

Geotextile inside gabion mattrass

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 North channel: -500 ha -low lying area Mochiwal Division Point West Canal: - 3000 ha

Problem in the past

Darabam Zam North channel: -500 ha -low lying area West Canal: - 3000 ha An earthen bund was built at this division point. However it would break quickly and all water would disappear into North Channel, making it impossible to control water here (all channel bunds broken quickly), while leaving no water for West Channel.

Considerable damage in downstream area of North Channel

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

This spectacular impact was due to the selection of this very crucial site as well as a good design.

The site selection was done by experienced farmers.

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

General principle!

Discuss and agree the water distribution structures with the representative and authorized group of water users:    location proportion design

Acknowledgement

This presentation was prepared with thanks to: Tzegai Teklemariam WRRI DI Khan Team Philip Lawrence Ian MacAnderson