1. Sediment transport in wadi systems
Part 2: Wadi Morphology

2. Mountain front and alluvial plain

3. Steep upper catchment

4. Terraced upper catchment – Yemen

5. Sparse vegetation

6. Mid catchment wadi channel

7. Gorge section at mountain front

8. Outwash fan at mountain front

9. Unstable low flow channels

10. Bed sediments sizes reduce downstream

11. Sediment transport in wadi systems
Quantifying concentrations and size range of transported sediments

12. Sediment supply
Very little data is available on the sediment loads transported in wadis.
Some information may be obtained through comparison with neighbouring catchments where data is available, or from regional sediment yield data.
Measurement programmes are sometimes included in the design of major projects. A short focussed measurement programme can provide the essential data needed for the design of sediment control structures.

13. Typical regional sediment yield data

14. Typical wadi bed material sizes

15. Sediment transport -Some definitions
Total
Sediment
Transport
Total Bed Material Load (sands, gravels, etc)
Wash
Load (silts, clays, etc)
Bed Load (rolling, bouncing, dune migration)
Suspended Bed Material Load (originates from bed)
Wash Load
Bed Load
Suspended Load

16. The components of a wadi sediment load
Wash load. Transporting capacity in a wadi virtually unlimited, concentrations are supply controlled, wash load concentrations thus have to be measured.
Suspended bed material load, sand sizes carried in suspension, sediment sizes and concentrations are related to local hydraulic conditions. Can be measured or approximately predicted using sediment transport equations.
Bed load large sediments moving by sliding and bouncing close to the wadi bed. Very difficult to measure, usually predicted using bed load equations.

17. Sediment measurements
Carried out at gauging sites at the same time as discharge measurements.
Conventional sediment sampling methods not suitable for flood flows in wadis – very high flow velocities and large quantities of trash make it impossible to suspend samplers in the flow.
Valuable data on wash load concentrations can be obtained without too many difficulties from surface dip samples.
Information on suspended sand load, needed to aid the design of sediment control structures, can be obtained by pumping from fixed nozzles.

18. Typical wadi gauging site

19. Typical pump sampling set up

20. Modification for higher velocity flows

21. Fixed sampling nozzles, more robust versions have been used in wadis

22. Sediment rating relationships
Unless automatic sampling equipment is used a gauging team has to camp at the gauging site over the “flood” season to ensure that samples are collected during significant flood events.
Sediment concentration data is then used to develop correlations with wadi discharge.
Separate relationships are developed for wash load and suspended bed material load.
Bed load equations are used with site data describing the sediment sizes and hydraulic conditions to estimate bed load.

23. Typical sediment rating relationships for wash load

24. Examples of simple sediment rating relationships for a Wadi
Xwash = 8738 Q 0.61
Xsus = Q 1.061
(Where X is the sediment concentration in parts per million, correlations shown were developed for wadi Laba in Eritrea from pump sampling data in flows ranging between 1 and 80 m3/s )

25. Sediment sizes in transport
Bed material load sediment sizes are needed to assist the design of sediment control structures.
The sediment sizes moving as bed load and suspended bed material load can be estimated using a bed material size grading curve and sediment transport relationships.
Suspended bed material sizes can also be derived from sediment samples if measurements have been made.

26. Suspended bed material sizes

27. Sediment sizes transported by a typical wadi

28. Sediment transport in wadi systems
Sediment transport predictors

29. Sediment transport predictors
Many predictors available, with a range of complexity and data requirements.
All are “calibrated” with the aid of field data or include empirical coefficients.
Predictive ability is not very good, which should be remembered when computational sedimentation models are used.

30. Simple sediment transport function, bed material load

31. Comparison of predicted sediment concentrations with those observed in rivers and canals

32. Performance of some sediment transport predictors