6.5River 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

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