2.14 Meandering Channels Flashcards
How and why do meandering channels form in the tropics?
What is sinuosity?
Measure of how ‘straight’ a stream’s path is
What sinuosity index and above would be considered a meandering channel?
(Above or equal to) 1.5
What is the sinuosity index equation?
Sinuousity index = Channel length / Straight line distance
What are the key characteristics of meandering streams (form)?
- Single-threaded
- Sinuous channel form (sinuosity index >1.5)
- Pools and riffles occur regularly, spaced 5-7 times the channel width apart
What are the flow characteristics of meandering streams?
- Regular discharge, high energy (often found in humid tropics with heavy rainfall)
- Dominated by thalweg (line of fastest flow)
What are the bank and load characteristics of meandering streams?
- Banks are made of cohesive materials (clay, silts)
- Mixed load (primarily clay, silts, some gravels)
What are the cross-sectional characteristics of a meandering river?
- Straight sections
- Meander bends
- Thalweg
- Erosion and Deposition:
– Outer bend (river cliff)
– Inner bend (slip-off slope/point bar)
What are the general conditions that favour the formation of meanders?
- Gentle gradient
- Mixed sediment load
- Erodible but cohesive banks
- Space to migrate
- Stable discharge
- Pools and riffles
Why do gentle gradients contribute to the formation of meanders?
Allows for slower flow and increased sinuosity
Elaborate on ‘space to migrate’ when discussing the factors behind formation of meanders.
Meanders require open areas like alluvial (clay, silt, sand) floodplains, away from physical barriers like hills or mountains.
Elaborate on ‘stable discharge’ when discussing factors behind formation of meanders.
Consistent flow with minimal variations
What are the characteristics of pools?
- Deeper,
- Faster-flowing areas
- Contains fine sediment
- Found in curved sections
What are the characteristic of riffles?
- Shallow and wider
- Contains coarse materials (e.g. gravel)
- Found in straighter sections
- Often visible during low flow
Describe and explain Stage 1 of Keller’s model of the formation of meandering channels.
Description: Straight channel with asymmetrical shoals of deposition on the river bed.
Explanation:
– Friction with the channel bed and banks cause turbulence
– Thalweg (fastest-moving water) follows a sinuous path
– Sediment is deposited in slower-flow areas alongside the thalweg, forming shoals that extend downstream
** Note that at Stage 1, it is SHOALS, not POINT BARS
Describe and explain Stage 2 of Keller’s model of the formation of meandering channels.
Description: Straight channel with asymmetrical shoals; incipient pools and riffles are formed.
Explanation:
– Incipient riffles form at points of inflection where energy is low, creating sediment bars.
– These riffles increase channel roughness, leading to more sediment build-up.
– The steeper gradient accelerates flow, scouring the riverbed and forming incipient pools downstream.
– This process triggers erosion, curving of the channel, and the initial development of meanders.
Describe and explain Stage 4 of Keller’s model of the formation of meandering channels.
Description: The channel becomes more sinuous, and pools and riffles are clearly established, spaced 5-7 times the channel width
Explanation: Alternating patterns of erosion and deposition create the sequence of pools and riffles.
** Energy expenditure at pools and riffles is linked, but on a separate flashcard.
** Role of helicoidal flow of water is linked, but on a separate flashcard.
Describe energy expenditure at the RIFFLES during Stage 4 of Keller’s model of the formation of meandering channels.
(-) High energy loss due to increased channel roughness and shallow waters (more friction) –> flow is slowed
(+) To balance energy, coarse sediment is deposited, steepening the gradient.
(+) Straight –> less friction
Describe energy expenditure at the POOLS during Stage 4 of Keller’s model of the formation of meandering channels.
(+) Thalweg is deflected downstream (deeper –> more friction) from the riffle to the outer bank where the pool is located. As the river moves from riffles (steeper) to pools (gentler), the flow accelerates. The available energy builds up due to a reduction of friction in the pools.
(-) Energy is used for pool deepening.
(-) Impact on banks due to pool curvature
Describe the role of helicoidal flow of water in meandering channels. (Stage 4 of Keller’s model)
- As water enters a curved channel, centrifugal forces push it against the outer bank, causing erosion and undercutting of the alluvial banks.
- The elevated water surface creates a hydraulic gradient (wetted perimeter) toward the inner bank, generating helicoidal flow (secondary flow).
- This flow slows near the inner bank, depositing sediment and forming point bars (evolved from earlier asymmetrical shoals).
- As the channel migrates laterally, erosion on the outer bank is balanced by deposition on the inner bank.
What is helicoidal flow?
A corkscrew-like water movement which helps shape meanders.
