6.5
River Stability and Bank Protection
John Ratsey [john.ratsey@ntlworld.com]
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
Erosion at outside of
bends, deposition on
inside of bends
There is a
circulation within
the overall flow
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
6. Sediment is
carried over weir
and the
downstream bed
profile is restored
4. Sediment is
picked up from
bed downstream
of weir
2. Weir is
constructed
5. Eventually
wadi has new
stable bed
profile
3.
Sediment is
deposited
upstream of
weir
1. Original
wadi bed
profile
Timescale: Can be many years or one very big flood
YANDOFERO RIVER, KONSO, ETHIOPIA
OLD BED LEVEL
NEW 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
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 bend
Erosion
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
Area
being
eroded
Current
bank
position
Wadi Mawr 2004
Current
bank
position
Current
bank
position
Current
bank
position
Effect of spur
Flow
deflected to
other side of
wadi
Area
being
eroded
Wadi has
now
breached
here
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
Revetment options
MINOR REVETMENT OPTIONS
Revetment
2.5m
Description
Advantages
Disadvantages
1 Gabion Mattress 300mm thick wire basket
containing 150 - 200mm dia
(5 - 10kg) stones
on filter fabric
1 Flexible, can settle with bank 1 Wires can break
2 Can form lauching apron
2 Limited longevity
3 Farmers can supply stone
2 Rock Rip Rap
500mm thick layer
containing 250 - 300mm dia
(20 - 60kg) stones
on filter fabric
1 Flexible, can settle with bank 1 Can be dislodged under high
velocity
2 Can form lauching apron
3 Requires machines for
2 Too heavy to be supplied by
placement
farmers
3 Cemented
Pitching
300mm thick layer
containing 250mm dia (20kg)
stones bedded in mortar and
mortared joints
1 Good abrasion resistance
1
2
3
4
Not flexible, can crack and settle
Requires good compaction of
Difficult to be supplied by farmers
Needs toe to prevent undermining
4 Plain Concrete
300mm thick layer
on compacted sub grade
1 High longevity
2 Good compressive strength
1
2
3
4
Not flexible, can crack and settle
Requires good compaction of
Difficult to be supplied by farmers
Needs toe to prevent undermining
1.5m
2.5m
1.5m
Slope protected with gabion mattress
Embankment
protected by
gabions
Revetment + launching apron
Apron constructed
below bed level to
reduce abrasion
damage and
potential bending
Boulders to
fill any
scour hole
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