Module 1 : Understanding the Wadis John Ratsey [[email protected]]
6.5 River Stability and Bank Protection John Ratsey [[email protected]]
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Transcript of 6.5 River Stability and Bank Protection John Ratsey [[email protected]]
6.5River Stability and Bank Protection
John Ratsey [[email protected]]
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
Typical meandering channel
There is a circulation within the overall flow
Erosion at outside of bends, deposition on
inside of bends
Outside of bend will move
downstream
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
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
Example of retrogression
1. Original wadi bed
profile
2. Weir is constructed
3. Sediment is deposited upstream of
weir
4. Sediment is picked up from
bed downstream of weir
5. Eventually wadi has new
stable bed profile
6. Sediment is carried over weir
and the downstream bed profile is restored
Timescale: Can be many years or one very big flood
OLD BED LEVELNEW BED LEVEL
RIVER MORPHOLOGY GONE WRONG:AFTER THE CUTTING OF A RIVERAINE FOREST THE YANDOFERO RIVER CHANGE ITS DOWNSTREAM COURSE AND BED LEVELS DROPPED 1-3 METERS
YANDOFERO RIVER, KONSO, ETHIOPIA
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
Gabion weir to stabilise bed
Gabion weir constructed to
maintain wadi bed level downstream of
concrete weir
Alternative option: bed stabilizer
Bank protection
Spurs encourage sediment deposition Spurs may change the flow pattern and cause
problems elsewhere Revetment protects the bank from erosion
Spur design criteria
Examples of spurs
Farmer-built protection
Reinforced with old vehicle
chassis
It looks vulnerable to scour
Brushwood protection
This farmer-built protection is
constructed of brushwood
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
Erosion at head of spur
Damaged apron and
scour
Close up view of gabions
Gabions well-packed
and tied together
Long spur without stepped end
Spur is vulnerable to severe scour at nose since water has to go around and not over
Bank protection using vegetation
Illustration of bank protection
Example of erosion on bendErosion
at outside of bend
Wadi Mawr - 1990
Wadi in 1990
Wadi in 2000
Current bank
position
Current bank
position
Current bank
position
Wadi Mawr 2000
Current bank
position
Current bank
position
Current bank
position
Area being
eroded
Wadi Mawr 2004
Current bank
position
Current bank
position
Current bank
position
Effect of spur
Flow deflected to other side of
wadi
Wadi has now
breached here
Area being
eroded
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
MINOR REVETMENT OPTIONS
Revetment Description Advantages Disadvantages1:2 slope
0.3m thickgabion mattress 1 Gabion Mattress 300mm thick wire basket 1 Flexible, can settle with bank 1 Wires can break
2.5m containing 150 - 200mm dia 2 Can form lauching apron 2 Limited longevity3m (5 - 10kg) stones 3 Farmers can supply stone
on filter fabric
1.5m
Filter Fabric 2 Rock Rip Rap 500mm thick layer 1 Flexible, can settle with bank 1 Can be dislodged under high
containing 250 - 300mm dia 2 Can form lauching apron velocity
(20 - 60kg) stones 3 Requires machines for 2 Too heavy to be supplied by Scour on filter fabric placement farmers
1:2 slope0.3m thick 3 Cemented 300mm thick layer 1 Good abrasion resistance 1 Not flexible, can crack and settlepitching Pitching containing 250mm dia (20kg) 2 Requires good compaction of
stones bedded in mortar and 3 Difficult to be supplied by farmers2.5m mortared joints 4 Needs toe to prevent undermining
4 Plain Concrete 300mm thick layer 1 High longevity 1 Not flexible, can crack and settleGood compaction 1.5m on compacted sub grade 2 Good compressive strength 2 Requires good compaction of
3 Difficult to be supplied by farmers4 Needs toe to prevent undermining
Scour
Revetment options
Slope protected with gabion mattress
Embankment protected by
gabions
Revetment + launching apron
Boulders to fill any
scour hole
Apron constructed below bed level to reduce abrasion
damage and potential bending
Protected island
Gravel island covered with gabion mattress to protect embayment (3 were used). Gaps between islands allow floods to deposit sediment
behind
Gabions can be flexible
Partial collapse
after scour
but will break
Wires have broken here