AS Theme 1 – Hydrology & Fluvial Geomorphology: River Channel Processes and Landforms

Three fundamental river functions
- Erode the channel
- Transport material
- Create erosional & depositional landforms
Schumm’s basin concept
- Upland zone → erosion/sediment production
- Mid-catchment → transfer/transport
- Lowland/coastal → deposition
- Processes overlap but one normally dominates per zone

Two energy forms
- Potential: weight × elevation of water
- Kinetic: gravitationally driven downslope motion
Simplified energy statement
- “Energy of the river” ∝ Volume of water (discharge) + Velocity
Rules of thumb
- Increased energy ↑ → gradient, width, depth, channel capacity, velocity, erosion/transport efficiency all ↑
- Decreased energy ↓ → gradient & velocity ↓, erosion ceases, deposition begins, channel narrows/shallows

Four transport mechanisms
- Traction: large particles slide/roll along bed
- Saltation: intermittent hopping/bouncing of sand-sized grains
- Suspension: fine silt/clay held within flow
- Solution: dissolved ions (mainly carbonates)
Competence = maximum particle diameter that can be moved
Capacity = total load volume that can be carried
Deposition triggers
- Low discharge after dry periods
- Velocity fall entering lake/sea
- Inside meander (shallow, low-energy)
- Sudden load increase (e.g. landslide)
- Flood overbank flow (velocity drops outside channel)

Upper Course
- Very steep gradient; high velocity (low friction)
- Dominant vertical & headward erosion (abrasion, attrition, hydraulic action, solution)
- Transport mainly large boulders; deposition of oversized clasts
- Landforms: waterfalls, rapids, potholes, inter-locking spurs, gorges
Middle Course
- Gentler gradient; moderate velocity
- Erosion: mainly abrasion/attrition; lateral erosion begins
- Transport: cobbles via traction, sand/silt in suspension; coarser material gradually deposited
- Landforms: rapids, small meanders, incipient floodplain
Lower Course
- Very gentle/flat gradient; low velocity (more friction)
- Erosion reduced; lateral erosion on meander outer bends
- Transport: mixed fine load (pebbles → clay)
- Deposition dominant (sand & gravel)
- Landforms: large meanders, pools & riffles, braided reaches, extensive floodplain

Abrasion (Corrasion)
- Bedload grinds channel bed/banks like sandpaper
- Most effective in high-load, high-velocity streams
Attrition
- Clasts collide together → become smaller, rounder
Hydraulic Action
- Sheer water force + pressure changes in cracks; common near waterfalls/rapids
Solution (Corrosion)
- Chemical weathering & dissolution of soluble rocks (limestone, chalk)

Graph distinguishes velocities needed to
1. Initiate erosion (lift)
2. Maintain transport
3. Allow deposition (settling)
Axes
- x-axis: grain diameter (mm)
- y-axis: flow velocity (cm s⁻¹)
Particle size ranges
- Clay < 0.004 mm
- Silt 0.004–0.06 mm
- Sand 0.06–2 mm
- Gravel 2–15 mm
- Cobbles/Boulders > 15 mm
Key observations
- ~1 mm sand needs lowest velocity to be eroded → lack of cohesion
- Very fine clays need high velocity to erode (cohesion) but almost zero to remain suspended
- Boulders demand highest velocity to erode, yet settle rapidly if velocity falls
- Narrow erosion–deposition band for coarse sediments → small velocity drop causes deposition
Common exam interpretations (labelled points 1–5 in text) included in curve discussion
Limitations
1. Assumes smooth, uniform channels
2. Ignores natural flow variability
3. Poorly represents gravel-bed rivers

Discharge definition: volume of water passing a point per unit time
- Formula Q = A \times V where A = cross-sectional area, V = mean velocity
- Units: m^{3}\,s^{-1} (cumecs)
Velocity: distance travelled per unit time (m s⁻¹)
Controls on velocity
1. Channel shape → Hydraulic Radius R = \frac{A}{P} (area ÷ wetted perimeter)
- Semi-circular cross-section is most efficient
- Stream A (higher R) > velocity than Stream B (low R)
1. Channel roughness
- Coarse banks/bed increase friction → velocity loss
1. Channel slope/gradient
- Steeper slope → more potential energy → higher velocity

Laminar Flow
- Parallel sheets, minimal mixing; rare in natural rivers
Turbulent Flow
- Multidirectional eddies; dominant in most channels; turbulence increases after friction is overcome
Helicoidal Flow
- Corkscrew motion within meanders; laterally transfers load from outer to inner bend (erosion → deposition)

Straight
- Rare, short reaches; often maintained by structural controls
Meandering (single-thread, sinuous)
- Most energy-efficient form; maintained by bank erosion & point-bar deposition
Braided
- Multiple interlacing channels around mid-channel bars
- Characteristic of highly variable discharge & high sediment load (semi-arid, pro-glacial)

Discharge Q: Q = A \times V
Hydraulic Radius R: R = \frac{A}{P}
Representative Hjulstrom values
- 0.22 mm grains lifted at ≈20 cm s⁻¹
- Cobbles eroded at >170 cm s⁻¹
- 0.01 mm particles deposited at ≈0.3 cm s⁻¹
- 250 mm boulders settle at ≈30 cm s⁻¹ (smallest boulders)

River management must respect natural competence/capacity limits to avoid siltation
Urbanisation & deforestation raise suspended load, altering channel equilibrium
Understanding flow patterns (turbulent vs. laminar) informs engineering designs (bridges, levees)