6.5 River Stability and Bank Protection John Ratsey

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 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 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 bend Erosion at outside of bend

Wadi Mawr Wadi in 1990 Wadi in 2000 Current bank position

Wadi Mawr 2000 Current bank position Area being eroded

Wadi Mawr 2004 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

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