C2 Fluvial Landforms

Meander flows

• Thalweg (where water swings side to side, approaching different banks alternatively)

• Helicoidal flow (current from outer to inner bank)

Meander features

• Point bar and River cliff

  • Point bar: deposition of sediment at inner convex bank

  • River cliff: erosional undercutting of bank at outer concave bank

• Pools and Riffles

  • Pools: deeper

  • Riffles: shallower

  • A response to sinuous movement that occurs naturally in flowing water in a river channel

Meander formation

1. In a almost straight channel, thalweg takes on a sinuous course which results in alternating erosion and deposition on either bank

2. Asymmetric shoals create zones of accelerated and decelerated velocity

3. Zones of acceleration engage in erosion, forming pools

4. Zones of deceleration engage in deposition, forming riffles

5. As helicoidal flow flows into meandering channel, centrifugal forces cause water to pile up against outer bank and erode, forming river cliffs

6. When water moves towards convex bank, frictional drag slows river velocity and deposition occurs, forming a slip off slope

7. As erosion continues on concave and deposition occurs on convex, concave banks retreat and convex banks advance —> leading to increase in sinuosity ratio

Braided channel features

• Anabranches

• Mid channel bars (sometimes vegetated)

Braided channel formation

1. Wet szn: high discharge erodes and transports a large amount of coarse bedload

   1. Conditions: presence of unconsolidated river banks that allow for easy lateral erosion, large supply of coarse bedload

2. Lateral erosion: lateral erosion made possible by high dishcarge leads to development of wide and shallow channel

3. Dry szn: lower discharge leads to deposition of bedload that the river was carrying, low hydraulic radius of river facilitates deposition process

   1. Condition: fluctuating discharge

4. Deposition of coarse bedload occurs first which leads to disruption of water flow that leads to more deposition of sediment around it. When these deposits protrude above water surface, they form mid channel bars

5. Some bars may be vegetated over time and are more stable as vegetation helps to bind deposit together

Delta features

1. Delta Plain

2. Delta Front

3. Prodelta

Delta plain

• Where river meets ocean

• Low flat area that includes distributary creeks

• Coarser materials deposited here

Delta front

• Subaqueous area where sediment-laden river meets saline seawater and sediment is actively deposited

• Steeply sloping portion of delta where deltaic deposits slope downwards from sea level to sea floor

• Characterised by fine sediments, from bigger to smallest seaward

Prodelta

• Lies seaward of delta front as a layer of clay and silt

• Sediment is layered with silt, rock fragments and thin shell beds

Formation of River-dominated deltas

1. As the river approaches its mouth, sediment load the river carries from fluvial erosion is deposited at the mouth

2. Slowing velocity of river and build up of sediment allows river to break from its single channel

3. Forms deltaic lobe that include a distributary network of smaller channels that branch off from mainstream river

Formation of Wave-dominated deltas

1. Waves redistribute the sediment and the shape of the delta is remodelled by the direction of the wave approach

2. Riverborne sediment is pushed back to give delta a straighter appearance, leading to development of wide beaches with smooth shorelines

3. Has few distributaries, often only a single channel

4. Arcuate: triangle shaped

5. Cuspate: pointed

Formation of Tide-dominated deltas

1. Channels become aligned with direction of tidal currents, hence have wide delta fronts

2. In a strong tidal zone, riverborne sediments moves back and forth over a length of channel, forming funnel-shaped river mouths resembling estuaries

Factors influencing Deltas

1. Dominant fluvial / coastal processes

2. Human activities (dams, groundwater extraction, sand mining)