Module 2 (part 1)

1. Module 2 (part 1) Improving Diversion Works John Ratsey [john.ratsey@ntlworld.com]

2. Traditional diversions

3. Modern diversion structure Sediment basins Main canal Gated canal intake Flow Sluiceway Outlet for flushing basins Concrete weir

4. Optimum layout of intake

5. Key components A control structure to limit / stop flow into canal • A spillway to reject excess flows back to the wadi • A sluiceway to remove sediment / maintain low flow channel • Works to divert water to intake (bed bar / gravel embankment / weir)

6. Typical layout of improved intake

7. 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?

8. 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

9. 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 • Gravel embankments will breach to enable passing of big floods

10. 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

11. Al Zoran Intake The original intake was probably further downstream and has moved upstream to maintain command Canal is also in outside of bend and needs protection Intake is here

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. Edge of Zabid irrigation area Fields are several metres above the desert

16. 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

17. Typical wadi rating curve

18. Options • 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

19. Wadi Zabid weir 5 Canal 50% of flow Masonry wall Gravel embankments will breach in major flood to pass water downstream Concrete bed bar Canal 50% of flow

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

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

22. Gerhazi canal intake

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

24. Space for the big flood

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