Sediment Transport
in Wadi Systems
Part 1: Overview
phil.lawrence@sediment.plus.com
Managing sedimentation in spate
irrigation schemes
Part 1 – Overview (Today)
Part 2 – Sediment transport in spate
systems
Part 3 – Sediment Control structures –
features and design methods
Part 4 – Review of Sediment
Management options + exercise
Part 1 Overview of
sedimentation issues in spate
irrigation systems
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Sediment concentrations carried by some
perennial rivers
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6750
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Sediment Concentration ppm
Compare with sediment concentrations
transported by wadis
180000
160000
140000
120000
100000
80000
78000
60000
40000
38000
20000
22041
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1720 1849
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Typical sediment size range
transported in wadis
bed load,
suspended
sand
silt and clay
High sediment concentrations are
transported in flash floods
Water level record - Mishrafah March 1981
4.5
4
3.5
Levels (m)
3
2.5
2
1.5
1
0.5
0
18:00
6:00
18:00
13 March 1981
6:00
18:00
14 March 1981
6:00
18:00
15 March 1981
6:00
The contribution of high discharges to the total run off
varies widely between wadis.
% of annual run off volume in range
60
50
40
W Rima
W Laba
30
20
10
0
<2
2–5
5 – 10
10 – 20
Discharge range m3/s
20 – 50
>50
Fine sediments are transported through
canals to the fields
• A major feature - spate systems build their
own soils
• Field rise rates of 5 or more cms/ year
observed in some systems, older spate
irrigated areas have silt deposits many
metres thick.
• Rising command levels are a feature of
spate irrigation systems
Field sediment deposits after one
irrigation – Wadi Tuban (Yemen)
Boundary between irrigated on non irrigated
land – Wadi Zabid (Yemen)
Summary – features of sediment transport in
wadis
• Very high sediment loads carried by flash floods, but in some wadis
an appreciable proportion of the annual run off occurs at relatively
low discharges in flood recessions and seasonal base flows.
• Total load sediment concentrations exceeding 10 percent by weight
are transported by floods in some wadis.
• Wadi bed materials can range from boulders and cobbles to silts,
but sediment transport is dominated by the finer sediment fractions,
silts clays and fine sand transported in suspension. Large
sediments, coarse sand, gravel, cobbles, and small boulders,
typically represent only 5 percent or less of the annual sediment
load.
• Very large quantities of fine silts delivered the fields
Sediment management in spate systems
• Silt wanted by farmers on the fields (fertility)
• But larger sediments transported at high wadi
discharges flows will, if diverted, settle and block
canals
• Ideally all fine sediments (silts) will be
transported to the fields, while large sediments,
coarse sand and larger, should be excluded
from canals.
Traditional spate systems
• Intakes are washed away by large floods,
preventing the ingress of the coarse
sediments carried by high wadi flows.
Traditional Intake – Yemen
Traditional intake – Pakistan
Sediment management in traditional
systems
• Traditional spate canals are very steep
compared with conventionally designed
perennial canals, flow at high velocities,
and have a very high sediment
transporting capacity for fine sand and
silts.
Sediment management in traditional
systems
• Usually all the flow in a canal is diverted to
at a single point and then flows from field
to field. A high sediment transporting
capacity is maintained right through to the
fields and flows are not headed up at
water control structures.
Sediment management in traditional
systems
• When command starts to be lost intakes
can easily be moved further upstream, by
extending a diversion spur and or moving
the canal intake.
Summary of sediment management features
in traditional systems
•
Diversion spurs are washed away by large floods, preventing diversion of
high concentrations of coarse sediments. (As multiple intakes are used
diversion from floods usually still possible at downstream intakes)
•
Diversion bunds spanning a wadi are only used in lower reaches of wadis
where the coarse sediments have settled on the wadi bed and only fine
sediments are transported.
•
Canals are very steep compared with canals in conventional perennial
irrigation schemes – providing a high sediment transporting capacity.
•
Usually all the flow in a canal is diverted fields at a single point, a high
sediment transporting capacity is maintained from the wadi through to the
fields.
•
When command starts to be lost by rising field levels intakes can easily be
moved further upstream.
Sediment problems in modernised spate
systems
• Permanent diversion structures enable much
larger discharges to be diverted from spate
flows. Diversion from wadi flows carrying very
high sediment loads is possible. Sediment
sluices, canal closures in flood peaks and
sediment exclusion/extraction facilities are used
to reduce the loads of coarser sediment fractions
entering canals.
Wadi Rima Intake (Yemen)
Canal design
• In the first systems that were modernised
lower canal slopes than observed in
traditional systems were provided.
• Limited sediment transporting capacity in
canals = severe canal sedimentation
problems = high/unaffordable maintenance
requirements.
Desilting a canal head reach
Water distribution
• In some early schemes water distribution
systems similar to those used in perennial
schemes were adopted where water is
supplied to numerous field outlets at the
same time. Farmers then head up flows at
undersized outlets, promoting canal
sedimentation.
Farmers check structure
Silted field outlet
Silted undersized crossing structure
Timely operation of manual sluice and intake
gates in spate flows difficult or impossible
• Water levels change very rapidly at flood
peak
• Operators do not how big a flood peak is
going to be
• Multi peak spates common
• Farmers resist ”wasting” water by
operating scour slices etc.
Manually operated gate
Rising command levels
• Adequate provision for rising command
levels must be made when permanent
diversion structures with reasonably long
anticipated design lifetime are adopted.
• In Wadi Zabid (Yemen) the upstream
weirs have been raised twice since the
early 1980’s
Summary sediment management strategy
for modernised schemes
• Limit the diversion of coarser sediments by appropriate
intake sitting and design, use of scour sluices and,
where feasible provide sediment control structures
• Transport fine sediments through canals to the fields,
(steep canals, maintain high flow rates to fields, don’t
head up flows at water control structures.)
• Make provision for the inevitable rise in command levels
• Anticipate and quantify the need for canal de-silting and
plan for it
Exercise – differences between spate
and conventional irrigation practice
• Make a list of, and discuss the key differences
between spate and conventional irrigation,
focussing on:






Sediment loads
Water availability
Canals and water control structure structures
Sediment management
Command issues
“Ownership” and O & M
Sediment loads
Spate irrigation
Perennial / seasonal
irrigation
Sediment concentrations Sediment concentrations
up too and exceeding
rarely exceed 0.2 % by
10% by weight
weight
Water availability
Spate irrigation
Spate floods with highly
variable and unpredictable
timing number and
magnitude. In some cases
supplemented with low
seasonal base flows
Perennial / seasonal
irrigation
Reasonably predictable
seasonal flows allowing
continuous diversion at
controlled flow rates.
Canals and water control structure
structures
Spate irrigation
Steep canals operating a
high flow velocities
Flows supplied to very
limited number of field offtakes (only 1 at a time in
most traditional systems)
Perennial / seasonal
irrigation
Low sediment transporting
capacity provided by
conventional canal design
methods
Water supplied to numerous
outlets simultaneously
Sediment management
Spate irrigation
Many sediment
management options,
particularly those involving
sediment flushing, not
appropriate for spate
schemes
Perennial / seasonal
irrigation
Range of tried and tested
sediment management
options available.
Command issues
Spate irrigation
Command levels in
upstream areas that receive
most water and the largest
sediment loads may be
rising by 5 cms./year or more
Perennial / seasonal
irrigation
Command levels usually
fixed over the design life of
diversion and water control
structures .
“Ownership” and O & M
Spate irrigation
Perennial / seasonal
irrigation
Traditional systems are
farmer constructed operated
and managed. Some spate
irrigation systems are the
largest farmer managed
systems in the world.
Customary water rights and
arrangements and
obligations for O and M well
developed.
Larger systems often agency
designed operated and
maintained, with turnover to
artificially created farmer
groups at some point within
the water distribution
system.
More tomorrow !