Sediment transport in wadi systems

Part 2: Wadi Morphology [email protected]

Mountain front and alluvial plain

Steep upper catchment

Terraced upper catchment – Yemen

Sparse vegetation

Mid catchment wadi channel

Gorge section at mountain front

Outwash fan at mountain front

Unstable low flow channels

Bed sediments sizes reduce downstream

Sediment transport in wadi systems

Quantifying concentrations and size range of transported sediments

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.

10000

Typical regional sediment yield data

yield = 3209 Area -0.21

R 2 = 0.36

Ethiopia Eritrea 1000 100 10 1 10 100 1000

Catchment Area km 2

10000 100000 1000000

Typical wadi bed material sizes

100 80 60 Surface silt deposits % Finer 40 20 Bed material small low flow channel 0.001

0.01

0.1

Size mm 1.0

Wadi bed pit samples 10 100

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

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.

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.

Typical wadi gauging site

Typical pump sampling set up

Modification for higher velocity flows

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

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.

Typical sediment rating relationships for wash load

1000000 100000 10000 1000 100 0.10

1.00

10.00

Discharge m3/s

100.00

1000.00

Examples of simple sediment rating relationships for a Wadi

Xwash = 8738 Q 0.61

Xsus = 252.0 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 m 3 /s )

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.

100 90 80 70 60 50 40 30 20 10 0 0.01

Suspended bed material sizes

Wadi Zabid Wadi Zabid Wadi Laba Wadi Laba 0.1

Suspended Sediment larger than 0.06 mm 1

Size mm

10 100 Bed Material 1000

Sediment sizes transported by a typical wadi

bed load suspended sand silt and clay

Sediment transport in wadi systems

Sediment transport predictors

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.

Simple sediment transport function, bed material load X



h

50

g

2

S g D

50

U



S g u

* 3  1 2 

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

Accuracy of Engulend & Hansen Method

10000 1000 100 10 1 1 10 100 1000

Observed sediment concentration (ppm)

10000

Performance of some sediment transport predictors

Prediction method Engelund and Hansen Brownlie Ackers and White Van Rijn Yang Ranking for accuracy 4 5 1 2 3 Percentage of records within ‘half to twice’ band 56 51 49 48 37