2.2.1 fluvial processes

What are fluvial processes?

Fluvial processes = stream or river-related processes.

Three main processes:

• Erosion

• Transportation

• Deposition

What determines river energy?

Mainly:

• Discharge = volume of water flowing through a channel at a given point, measured in m³/s (cumecs)

• Velocity = speed of river flow

Generally:

• Discharge ↑ → river energy ↑

• Velocity ↑ → river energy ↑

• Velocity doubles → river energy increases 4×

River energy is used to overcome friction, erode the channel and transport sediment.

What is river discharge and how is it calculated?

Q = A × V

Q = discharge
A = cross-sectional area
V = mean velocity

What is the difference between laminar and turbulent flow?

Laminar flow:

• Water travels in parallel paths

• Uniform velocity

• No mixing

• Rare in natural rivers

Turbulent flow:

• Chaotic flow with velocity fluctuations + eddies

• Dominant in natural rivers

• Produces upward motion that supports sediment

About 95% of river energy under normal conditions may be spent overcoming friction associated with turbulence.

What are calibre, competence and capacity?

Calibre = size of load actually carried.

Competence = largest individual particle the river can transport.

• Velocity doubles → competence 64×

Capacity = total amount of material the river can transport.

• Velocity doubles → capacity 8×

Remember:

• Calibre = SIZE

• Competence = MAX SIZE

• Capacity = AMOUNT

What are the 3 main processes of fluvial erosion?

• Corrasion

• Corrosion

• Cavitation

How does corrasion occur?

Corrasion/abrasion = coarse/angular river load grinds, scrapes and scours the bed and banks.

Most effective when:

• Coarse bedload is abundant

• Turbulence is high

Can cause:

• Vertical erosion

• Lateral erosion

Potholes:
Rock fragments trapped in hollows → turbulent eddies rotate them → abrasion enlarges/deepens hollow → pothole forms.

How does corrosion occur?

Corrosion = solvent action of water, where minerals dissolve and are carried away as solute load.

Especially important for:

• Limestone

• Rock salt

• Gypsum

Depends mainly on:

• Chemical weathering

• Mineral/rock susceptibility

Unlike most fluvial erosion, corrosion is largely independent of river discharge and velocity.

How does cavitation occur?

Turbulent flow traps air bubbles.

Air bubbles:

• Compressed into cracks

• Pressure builds

• Bubbles burst/collapse

• Shockwaves produced

• Repeated shockwaves weaken rock → rock splits apart

Common in plunge pools at waterfalls.

Main effect:

• Lateral erosion more significant than downcutting

What is vertical erosion/downcutting and when does it increase?

Vertical erosion = erosion that deepens the river channel.

Typically dominant in the upper course due to:

• Steep gradient

• High velocity/energy

• Coarse bedload → corrasion/potholing

Rate of vertical erosion increases due to:

• Uplift of land

• Fall in sea level

→ river gradient relative to base level steepens
→ river energy ↑
→ increased downcutting

How does lateral erosion occur?

Lateral erosion = erosion that widens the river channel/valley.

At meanders:

Outer/concave bank:

• Velocity ↑

• Erosion ↑

• Undercutting → bank collapse

Inner/convex bank:

• Velocity ↓

• Deposition ↑

→ river migrates laterally → valley widens

What is headward erosion?

Occurs especially at waterfalls.

Sharp gradient → velocity ↑
→ vertical erosion enlarges plunge pool
→ waterfall face undermined
→ overlying rock collapses
→ waterfall retreats upstream

= headward erosion

What is fluvial transportation and what are its 4 processes?

Transportation = downstream movement of previously eroded sediment.

Four processes:

• Traction

• Saltation

• Suspension

• Solution

How do traction and saltation transport sediment?

Traction:

• Large/coarse particles

• Roll + slide along river bed

• Important where energy is sufficient to move coarse bedload

Saltation:

• Usually sand-sized particles

• Move in jumps/bounces

• Hydraulic force lifts particle → travels downstream → gravity returns it to bed

How do suspension and solution transport sediment?

Suspension:

• Fine particles, e.g. silt + clay

• Held up by turbulent upward currents

• Usually forms the largest proportion of river load

• Turbulence ↓ → coarser suspended particles deposited first

Solution:

• Dissolved minerals/ions + organic substances

• Mainly produced by chemical weathering

• Largely independent of river velocity

How does sediment change downstream?

Amount of sediment generally ↑ downstream due to:

• Tributaries

• Channel erosion

• Valley-side inputs

Individual particles generally become:

• Smaller

• Rounder

due to:

• Attrition

• Corrasion

When does fluvial deposition occur?

Deposition occurs when the river is no longer competent to carry its sediment.

Caused by:

• Velocity/energy ↓

• Discharge ↓

• Gradient ↓

• Friction ↑

• River reaches mouth

• Flow slows at inner/convex meander bank

• River becomes overloaded with sediment

Generally:
largest/coarsest particles deposited first → finer particles deposited later

Where does fluvial deposition commonly occur?

River/channel bed:

• Flood ends → velocity ↓ → sediment deposited

Floodplain:

• River overflows → water spreads → velocity ↓ → deposition

Inner/convex meander bank:

• Velocity ↓ → deposition → point bars

How do lateral and vertical accretion form floodplains?

Lateral accretion:

• Outer/concave bank → erosion

• Inner/convex bank → deposition/point bar

• Meander migrates sideways

• → floodplain develops laterally

Vertical accretion:

• Floodwater spreads across floodplain

• Velocity ↓

• Suspended sediment deposited

• Repeated floods → floodplain builds vertically

How do natural levees form?

During floods:

• River overtops banks

• Water leaves channel → velocity/competence ↓ rapidly

• Coarser sediment deposited first near channel

• Finer sediment transported further away

Repeated flooding + deposition:
→ banks build upwards
→ natural levees form

What does the Hjulström Curve show?

Relationship between:

• Particle size

• River velocity

• Erosion

• Transportation

• Deposition

Key thresholds:

• Erosion velocity = minimum velocity needed to pick up/erode particle

• Transportation velocity = minimum velocity needed to keep it moving

• Settling velocity = velocity below which it is deposited

Why are both very large and very fine particles difficult to erode on the Hjulström Curve?

Large particles:

• Heavy → high velocity needed to move them

Very fine clay/silt:

• Strong cohesion between particles

• High velocity needed to detach them

Once clay/silt is entrained, it can remain suspended at very low velocities

What are the main natural factors influencing fluvial processes?

• Climate

• Vegetation

• Sediment characteristics

How do sediment characteristics differ between humid and arid tropical rivers?

Arid tropics:

• Physical weathering dominant

• → generally coarser sediment

• Loose/unconsolidated material may be rapidly transported during floods

Humid tropics:

• Chemical weathering dominant

• → more fine silt/clay

• Perennial rivers can carry large sediment loads

What is a river long profile and how does it change downstream?

Long profile = change in river gradient from source to mouth.

Generally:
steep source → progressively gentler gradient → mouth

Upper course:

• Steep gradient

• Vertical erosion dominant

• Coarse load

Middle course:

• Gradient ↓

• Lateral erosion increasingly important

• Meanders develop

Lower course:

• Gentle gradient

• Deposition increasingly important

• Finer sediment

How do dams affect fluvial processes?

Upstream reservoir:

• Velocity ↓

• Erosion ↓

• Sediment deposited/trapped

Downstream:

• Sediment supply ↓

• River becomes sediment-starved

• River may erode bed/banks to obtain sediment

• → river incision/vertical erosion ↑