Sediment transport in wadi
systems
Part 2: Wadi Morphology
phil.lawrence@sediment.plus.com
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.
Typical regional sediment yield data
10000
yield = 3209 Area-0.21
R2 = 0.36
Yield t/km2/y
1000
Ethiopia
Eritrea
100
10
1
10
100
1000
Catchment Area km
10000
2
100000
1000000
Typical wadi bed material sizes
100
80
60
B ed m aterial sm all low flow
channel
S urface silt
deposits
% F iner
40
20
W adi bed pit
sam ples
0.001
0.01
0.1
1.0
S ize m m
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
Concentration ppm
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 m3/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.
Suspended bed material sizes
Wadi Zabid
Wadi Zabid
Wadi Laba
Wadi Laba
100
90
80
70
Suspended Sediment
larger than 0.06 mm
% Finer
60
50
40
Bed Material
30
20
10
0
0.01
0.1
1
10
Size mm
100
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
50
X 
h
g
Sg
2
D 50
U
S
u*
3
 1
2
g
Comparison of predicted sediment
concentrations with those observed in rivers
and canals
Accuracy of Engulend & Hansen Method
Predicted sediment concentration (ppm)
10000
1000
100
10
1
1
10
100
1000
Observed sediment concentration (ppm)
10000
Performance of some sediment transport
predictors
P rediction m ethod
Engelund and H ansen
B rown lie
Ackers and W h ite
V an R ijn
Yan g
R ankin g for
accuracy
P ercentage of records w ith in
‘half to tw ice’ band
1
2
3
4
5
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