6.4 Improving Diversion Works

1. 6.4Improving Diversion Works John Ratsey [john.ratsey@ntlworld.com]

2. Objective for improvement • Make it easy for farmers to operate and maintain reducing the large inputs of labour or other resources to maintain; • Prevent large and uncontrolled flood flows from damaging canals and field systems; • Help maintaining the distribution of water within the system in line with accepted rules and rights, • Providing flexibility to accommodate changing hydrological patterns, future changes in water distribution and cropping pattern • Equality of water shares between upstream and downstream users;

3. Therefore.. • 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.

4. Typical modern diversion structure Sediment basins Main canal Gated canal intake Flow Sluiceway Outlet for flushing basins Concrete weir

5. Shortcoming of these structures: • Area in front of weir silts up >> diversion in the end by traditional structures • Gate operation often problematic >> no operator at the right time or gates getting stuck • Sedimentation ponds not operated and not maintained >> sedimentation problem • Large investment in single off-take – often with long conveyance channel>> creating conflicts and depriving downstream water users

6. Improved modernized structure:key components • A control structure to limit / stop too high flow into canal (orifice; gated intake) • A spillway to reject excess flows back to the river • A sluiceway to remove sediment / maintain low flow channel • Works to divert water to intake and prevent changes in river bed through scour: • bed bar (or bed stabilizer) or weir • gravel embankment – to guide water to intake, but with capacity to breach in case of high floods

7. Typical layout of improved intake

8. 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?

9. 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

10. 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 • Managing for the big floods: gravel or soil embankments will breach to enable passing of big floods

11. 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

12. 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

13. Layer of sediment from one flood Sediment thickness about 5cm from one flood

14. 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

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

16. Use local knowledge of floods and these are judged to design improved structures

17. Typical wadi rating curve

18. Options: bed bar/ weir • We do not necessarily need weirs • Weirs are useful to: • Create head • Divert water • Stabilize the river bed BUT • Create head: this may also be achieved by having the intake further upstream • Stabilize the river bed: bed bars or bed stabilizer will prevent scour of the river bed and will stabilize the river bed as well • Divert water: as in spate river the area in front of the weir is often silted up farmer guide water to the intake by making additional diversion structure from gravel, sand or brushwood anyway THEREFORE • Bedbars with gravel embankments or soil bunds may be more durable and cost effective solutions

19. Bed stabilizer: burried into river bed Provide weepholes to prevent that subsurface flow is blocked and lost to downstream areas

20. In gravelly river bed bars should be made of concrete – in sandy areas gabion bed stabilizers can be considered

21. If going for a weir also consider options without stilling basins but with cascaded downstream toe

22. Options: intake • 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 unless a large command is reached (which is often possible in downstream areas)

23. Intake Capacity • Need to convey large volumes of water by gravity to fields during the short periods when Wadi flows occur. • Hydrological information on Wadi flows are in almost all cases very limited. • The timing, duration and maximum discharge of spate flows are thus unpredictable, • Areas traditionally irrigated varying significantly from year to year. • Water conveyance and distribution systems developed for perennial irrigation are not appropriate for spate systems.

24. Intake Capacity • Much larger capacities are needed (10 to 100 times greater than for perennial systems for a given area) • Flow velocities will be higher than conventional canals as the water is heavily laden with sediment and generally pass through much coarser material that have been deposited over the years. • Traditional intakes and their modern replacements can both fulfil these above functions, although by different means and with large differences in capital and maintenance costs and requirements

25. Gerhazy canal intake - plan Wadi Concrete bed bar Gravel embankment Intake Canal

26. Gerhazi canal intake

27. Intake for Gerhazy canal Farmers use a gravel embankment to close the intake Breastwall with three orifices

28. Space for the big flood Main river bed

29. Bagr canal intake Designed as one orifice but modified to two gates, but blockage by trash is a problem Gravel embankment to divert water

30. Initial layout for Al Mossefiyah Intake Another canal Curved gabion wall Width ~100m Wadi flow Gabion wall Gabion wall to restrict intake opening size Al Mossefiyah canal

31. Suggested layout for Al Mossefiyah Intake Possible bed bar if acceptable to downstream users Gabion wall designed as spillway Intake structure with orifice Gravel embankment to be constructed by farmers Wadi bank

32. Wadi Zabid weir 5 Canal 50% of flow Masonry Dividing Structure Gravel embankments will breach in major flood to pass water downstream Concrete bed stabilizer Canal 50% of flow

33. Divide wall at Zabid weir 5 Weir 5 Gravel embankments Concrete bed bar Masonry divide wall

34. Economics • Building flood resistant structures can be expensive • The benefits, from reduced maintenance work and improved crops, may be moderate • Functional requirements may be changed after 25 or 50 years (higher command levels, even fewer floods)

35. Improvements to modern intakes • Works to improve sediment removal • Eg. skimming weir in front of intake • Removal of divide wall • Works to improve hydraulic performance and flow division • Weir crest raising to restore command • Works to help flow management

36. Skimming weir Skimming weir constructed in front of head regulator. However, water is needed for flushing

37. Sediment in canal Gravel in canal reduced, but not avoided