Sediment Transport in Wadi Systems

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Sediment Transport in Wadi Systems

• Part 1 Overview

phil.lawrence_at_sediment.plus.com
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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

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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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Compare with sediment concentrations transported
by wadis
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Typical sediment size range transported in wadis
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High sediment concentrations are transported in
flash floods
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The contribution of high discharges to the total
run off varies widely between wadis.
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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

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Field sediment deposits after one irrigation
Wadi Tuban (Yemen)
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Boundary between irrigated on non irrigated land
Wadi Zabid (Yemen)
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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

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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.

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Traditional spate systems

• Intakes are washed away by large floods,
  preventing the ingress of the coarse sediments
  carried by high wadi flows.

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Traditional Intake Yemen
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Traditional intake Pakistan
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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.

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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.

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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.

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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.

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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.

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Wadi Rima Intake (Yemen)
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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.

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Desilting a canal head reach
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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.

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Farmers check structure
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Silted field outlet
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Silted undersized crossing structure
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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.

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Manually operated gate
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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 1980s

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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, dont 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

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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

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Sediment loads
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Water availability
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Canals and water control structure structures
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Sediment management
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Command issues
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Ownership and O M
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More tomorrow !