Intakes and Diversion Strucures Designing a Spate System

1. Intakes and Diversion StrucuresDesigning a Spate System Lesson 3 IanMcAnderson@aol.com

2. Designing a Spate System • - Diversion structures & intakes; • - Spate canals • - water control & dividing structures; • - Bank protection • - Wadi training structures.

3. Traditional diversion and water distribution structures • divert ephemeral rivers using only local materials and indigenous skills • relatively high overall water diversion efficiency • the high labour inputs needed to re-build the structures • environmental problems resulting from unsustainable use of trees and brushwood

4. What we have learnt • For engineering successful interventions: • replicate as far as possible traditional diversion practices • reflect time commitments and technical knowledge of the farmers • facilitate the control of large flood • replicate water distribution in line with accepted rules and rights

5. What we have learnt • ensure a right balance between the needs of different water uses and users • improve the effectiveness of the systems to function with high rates of sediment transport • improve the ability to cope with frequent and large changes to the levels and alignments

6. rehabilitation and improvement works • Farmers should be consulted and involved in the planning, design and execution • Engineers provide a range of technically and economically viable options • Comprehend the long years of experience that the farmers have of the systems and Wadi flow, • Assisting farmers in selecting the most appropriate improvements that replicate and improve upon traditional schemes

7. Disappointing performance attributed to: • An increased inequity of water distribution • Command and diversion problems due to high rates of sediment deposition • reduced the WUAs/farmers’ role in diverting and distributing water • unrealistic assumptions concerning levels and costs of operation and maintenance • failures to achieve an expected increase in irrigated area

8. Understanding each other • Ensure what you have in mind is what the farmers envisage • Compromise “perfect” engineering solutions with levels of risk acceptable to farmers

11. What we have learnt • ensure a right balance between the needs of different water uses and users • improve the effectiveness of the systems to function with high rates of sediment transport • improve the ability to cope with frequent and large changes to the levels and alignments

12. What we have learnt • Do not upset balance of water distribution and inadvertently favour the better off at the tops of the systems • Understand water rights and implications

15. Traditional • Local resources • Large bund diverting all flow

17. Traditional • Diverting only part of flood flow • Across wide Wadi • Alongside narrower Wadi to collect only part of flow

22. design of improved spate systems • The over riding principle is that there is no single approach • Specific requirements vary widely between and in some cases within schemes, • Before proposals are finalised, it is essential that engineers fully understand the way in which the farmers system has operated • and farmers truly understand and comprehend what the engineers are proposing for them

23. Use local knowledge of floods and how they judge them to design improved structures

25. Diversion structures and intakes • Need to be able to divert short duration flood flows into gravity canal systems • Ensure that sufficient amounts are abstracted in the time available • that the canals systems ensure command over the fields to be irrigated.

26. Diversion structures and intakes • Have the capacity to convey the water with minimal sedimentation and scour • Some lands will fall out of command or may not be economic to irrigate considering the technically most suitable sites for structures. • Secondary and maybe more problematic sites should not be chosen just so that small additional areas can be conveniently added.

27. Structures proposed need to be equipped with intakes that prevent large uncontrolled flows from entering canals • only floods that can be contained within the canal capacities are allowed to pass down the system • downstream damage to channels and field systems is minimised • designed to limit the entry of the very high concentrations of coarse sediments • Structures also need to act as erosion control devices in unstable Wadis, characterized by lateral movements of low/medium flow channels within the wider Wadi cross sections, streambank erosion and head-cutting, etc.

28. In the flatter areas, often at the downstream limits of the spate system areas - command over the irrigated lands can be a problem • Exacerbated over time with the deposition of silt on the lands, • Intakes must also function to ensure that sufficient head is maintained over the longer term • Cope with rising irrigation command levels at the field

29. Traditional Intakes • Advantages: • Flexibility • • Appropriate and low cost • Relatively efficient in water use and sharing between users • Restrict diversion of high flows with high sediment loads

30. Traditional intakes fall into two main types • spur deflection

31. bund diversion

33. Common Features • Located at outside or just downstream from relatively mild Wadi bends • deep water channel is scoured in floods • where lower flows are channelled during flood recessions; • Consist of low spurs flows extending at a slight angle out into the Wadi • to intercept the low flow channel, • diverting lower flows to an un-gated canal; • Constructed from locally available materials • damaged by larger flood flows – equitable use of flood flows • can be maintained and re-constructed by farmers without or with limited significant external support (bulldozers are often available to farmers);

34. Common Features • Recognize force and damage caused by large and very large flood flows • designed to breach or break when they occur, • thereby reducing danger to the spate irrigation system; • Are not “greedy” and do not try to extract all of the flow • Designed to “coax” the flows into the intake taking as much as they dare without endangering the whole system. • While the different forms of construction result in varying degrees of durability, mainly due to available labour and local materials, all are likely to be damaged or completely swept away by larger floods.

35. Advantages of traditional intakes: • Flexibility: • Location and layout of traditional intakes easily adjusted to suit the changing Wadi bed condition; • Deflecting spurs can be extended and diversion bunds moved upstream when sedimentation on the fields or in the canals starts to take fields out of command, or stream bank erosion occurs. • Appropriate and low cost: • traditional intakes are constructed from local materials using indigenous skills • can be maintained indefinitely by farmers without outside support, apart from environmental problems resulting from unsustainable use of trees and brushwood.

36. Relatively efficient in water use and sharing between users: • Many traditional intakes are used along larger Wadis, each serving their own separate command area • Can achieve high overall diversion efficiency and better water equality • Large spate floods will destroy intakes located at the heads of each spate command area as they pass down a Wadi, with those upstream intakes breaking sooner than those further downstream. • Once the flood peak (or rising limb of the flood hydrograph) has destroyed traditional diversion bunds at one location, the flow passes to the next one further downstream and so on, • sharing water between many offtakes and not permitting the upstream site to “steal” all of the flow. • Lower sites experience less severe floods, exposed for longer periods, thereby compensating for water not received from other smaller floods that only reach the upper intakes.

37. Restrict diversion of high flows with high sediment loads: • Failure of deflecting spurs, diversion bunds and breach sections of main canal at high Wadi discharges abruptly lower the water level at the canal intakes, • reduces discharges that are diverted, limiting the damage to the downstream canals and field systems • Preventing the incursion of high concentrations of coarse bed material sediments transported in large floods

38. Important disadvantage associated with traditional diversion structures • enormous input of labour needed to maintain and reconstruct intakes that are damaged, or washed out by large floods • the continual use of new brushwood and tree material needed to reinforce the bunds. • The timing of repairs and breaches means that there is often a shortage of suitable material around the diversion sites • Time is water lost so need to rebuild the bunds as soon as possible • High recession flood flows often prohibit access to river bed material • Deposition of silt and wet area around bund means access even by machinery very difficult

39. Improved diversion structures • Raised weir; • Gated scour or under sluice; • Gated canal head regulator; and • Guide or divide wall. • divert the maximum possible amount of water • capacities per unit area being 10 to 20 times perennial irrigation schemes

40. Improved Intakes • More durable diversion spurs and division groynes; • Improved diversion bunds and check weirs; • Controlling the flows admitted to canals; • Basic gated or orifice control intakes; and • Rejection spillways.

50. Intake Capacity • a limited number of major diversion structures • large new canals that connect into and traverse the existing traditional canal network • increase in the inequity between upstream and downstream users’