Module 3

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Module 3

• Channel Stability and Bank Protection

John Ratsey john.ratsey_at_ntlworld.com
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Channel Morphology

• A meandering channel can give the wadi the right
  balance between slope, velocity and sediment
  transport ability
• Meanders tend to migrate downstream with time
• Interventions in one location can cause effects
  in another location
• Sand and gravel extraction can upset the
  morphological balance

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Typical meandering channel
Outside of bend will move downstream
Erosion at outside of bends, deposition on inside
of bends
There is a circulation within the overall flow
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Aggradation and Retrogression

• Aggrade rise retrogress fall
• A natural wadi has overall balance between slope
  and sediment transport (in regime) which is an
  overall average of the floods of various sizes
• However, the major floods move much more material
  than the equivalent volume of small floods

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Disturbance of the regime

• Engineering interventions, such as weirs, can
  interfere with the natural movement of sediment
• Sand and gravel extraction can also disturb the
  natural balance Extraction upstream can cause
  lower bed levels downstream

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Example of retrogression
6. Sediment is carried over weir and the
downstream bed profile is restored
5. Eventually wadi has new stable bed profile
3. Sediment is deposited upstream of weir
2. Weir is constructed
4. Sediment is picked up from bed downstream of
weir
1. Original wadi bed profile
Timescale Can be many years or one very big flood
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Example of retrogression
Downstream of Waqar weir, Wadi Siham Bed level
has dropped by several metres. It may be caused
by weir sand / gravel extraction. A further
drop will expose the siphon
Spur is here
Bed is here
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Gabion weir to stabilise bed
Gabion weir constructed to maintain wadi bed
level downstream of concrete weir
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Bank protection

• Spurs encourage sediment deposition
• Spurs may change the flow pattern and cause
  problems elsewhere
• Revetment protects the bank from erosion

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Spur design criteria
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Examples of spurs
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Farmer-built protection
Reinforced with old vehicle chassis
It looks vulnerable to scour
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Brushwood protection
This farmer-built protection is constructed of
brushwood
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Good example of gabion spur
Stepped end reduces impact on high floods but
still protects the bank
Spur keyed into bank to avoid outflanking behind
spur
Apron to protect against scour
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Erosion at head of spur
Damaged apron and scour
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Close up view of gabions
Gabions well-packed and tied together
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Long spur without stepped end
Spur is vulnerable to severe scour at nose since
water has to go around and not over
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Bank protection using vegetation
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Illustration of bank protection
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Example of erosion on bend
Erosion at outside of bend
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Wadi Mawr - 1990
Wadi in 1990
Wadi in 2000
Current bank position
Current bank position
Current bank position
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Wadi Mawr 2000
Current bank position
Current bank position
Area being eroded
Current bank position
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Wadi Mawr 2004
Current bank position
Current bank position
Current bank position
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Effect of spur
Flow deflected to other side of wadi
Area being eroded
Wadi has now breached here
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Revetment

• Revetment is passive protection of slopes
• It does not change the flow pattern
• Revetment is recommended upstream of intakes
  where it will not deflect the flow

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Revetment options
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Slope protected with gabion mattress
Embankment protected by gabions
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Revetment launching apron
Apron constructed below bed level to reduce
abrasion damage and potential bending
Boulders to fill any scour hole
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Protected island
Gravel island covered with gabion mattress to
protect embayment (3 were used). Gaps between
islands allow floods to deposit sediment behind
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Gabions can be flexible
Partial collapse after scour
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but will break
Wires have broken here
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Final thought
Who built this?
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The end of module 3

• Thank you