1 / 39

6.6 Canals and Command Area Structures

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

min
Télécharger la présentation

6.6 Canals and Command Area Structures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 6.6Canals and Command AreaStructures

  2. (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

  3. Traditional channels in spate areas Main challenge is to prevent heavy sediment deposition in channels Main challenge is to prevent erosion of channels

  4. Example: Canals in Spate Schemes in Upstream Area Bed slopes of traditional canals in the original (before modernization) Wadi Zabid system in Yemen.

  5. 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.

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

  7. 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.

  8. Farmers check structure

  9. Silted field outlet

  10. Silted undersized crossing structure

  11. (2) Spate canal design methods

  12. 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

  13. 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.

  14. (3) Command area structures • Check and drop structures; • Flow splitting structures; • Field offtakes; and • In-field structures (see also module 4)

  15. 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)

  16. Model 1: Flow divider Distribution can be adjusted with brushwood Flow approaches

  17. 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

  18. Model 2: Flow distribution structure

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

  20. Use of geotextiles SOME HINTS Prevents wash-out of fine materials underneath the gabions, which can lead to overturning Use of geotextile underneath gabions

  21. Geotextile inside gabion mattrass SOME HINTS

  22. CASE OF LOWLAND COMMAND FLOW DIVISION MOCHIWAL, DI KHAN, PAKISTAN

  23. 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.

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

  25. Case: Mochiwal Flow Division Darabam Zam • North channel: • 500 ha • low lying area Mochiwal Division Point West Canal: - 3000 ha

  26. Problem in the past Darabam Zam • North channel: • 500 ha • low lying area 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. West Canal: - 3000 ha

  27. Considerable damage in downstream area of North Channel

  28. Solution Solution: 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

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

  30. Cost: • USD 2000 • Benefit • 3500 ha • USD 20,000 a year!

  31. 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.

  32. General principle! Discuss and agree the water distribution structures with the representative and authorized group of water users: • location • proportion • design

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

More Related