River Processes and Landforms

River channels are dynamic geomorphological systems shaped by the interaction of water flow, sediment transport, and underlying geology. For AQA A-Level Geography, a thorough understanding of the quantitative relationships governing erosion, transport, and deposition — alongside the resulting landforms — is essential for achieving top marks.

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The Hjulström Curve

The Hjulström curve (1935) is a graph showing the relationship between stream velocity and sediment particle size for erosion, transport, and deposition.

Key Relationships

• Erosion velocity: The minimum velocity needed to entrain (pick up) a particle from the bed.
• Transport velocity: The velocity needed to keep a particle in motion once entrained (always lower than the erosion velocity for the same particle size).
• Deposition: Occurs when velocity falls below the transport threshold.

Critical Observations

Particle Size	Behaviour
Clay and silt (< 0.06 mm)	Require disproportionately high velocities to erode because cohesive forces bind fine particles together. Once eroded, they remain in suspension at very low velocities.
Sand (0.06–2 mm)	Easiest to erode — the curve dips to its lowest point around 0.5 mm (medium sand). Requires only ~20 cm/s to entrain.
Gravel and cobbles (> 2 mm)	Require progressively higher velocities to erode due to their mass. Transport velocity is only slightly less than erosion velocity.
Boulders (> 256 mm)	Only moved during extreme flood events with velocities exceeding 1–2 m/s.

Exam Significance: The Hjulström curve explains why river channels often have sandy beds (sand is easily eroded and transported) and why clay-rich estuaries exist (clay is only deposited at near-zero velocities).

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Competence and Capacity

These two measures describe a river's ability to transport sediment:

• Competence: The maximum particle size (diameter) that a river can transport at a given velocity. Competence increases with the sixth power of velocity — doubling velocity increases competence by 2⁶ = 64 times. This explains why floods can move boulders that are immovable under normal flow conditions.

• Capacity: The total volume or mass of sediment that a river can carry. Capacity depends on both velocity and discharge, and typically increases downstream as discharge increases.

Key Distinction: A river may have high capacity (carrying large total sediment loads) but low competence (only moving fine particles), or vice versa during a localised, high-velocity flood.

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Channel Efficiency and Hydraulic Radius

Hydraulic Radius (R)

The hydraulic radius measures channel efficiency:

R = A / P

Where:

• A = cross-sectional area of flow (m²)
• P = wetted perimeter (m) — the length of the channel bed and banks in contact with water

A larger hydraulic radius indicates a more efficient channel — less energy is lost to friction with the bed and banks. Semi-circular channels are theoretically most efficient, but natural channels approximate a wide, shallow parabolic shape.

Downstream Changes

Variable	Downstream Trend	Explanation
Discharge	Increases	Tributary inputs add water
Channel width	Increases	Bank erosion accommodates greater discharge
Channel depth	Increases	Bed erosion deepens the channel
Hydraulic radius	Increases	Greater depth relative to wetted perimeter
Velocity	Generally increases	Despite lower gradient, increased efficiency more than compensates
Sediment size	Decreases	Attrition reduces particle size; finer sediment carried further
Channel roughness	Decreases	Finer bed material and fewer large obstructions

Common Misconception: Students often assume velocity decreases downstream because gradient decreases. In fact, average velocity typically increases downstream because the larger, deeper channel has a higher hydraulic radius, reducing proportional friction losses.

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Reynolds Number

The Reynolds number (Re) distinguishes between laminar and turbulent flow:

Re = (V × d × ρ) / μ

Where:

• V = velocity (m/s)

• d = depth (m)

• ρ = fluid density (kg/m³)

• μ = dynamic viscosity (Pa·s)

• Re < 500: Laminar flow — smooth, parallel layers of water. Extremely rare in natural rivers; found only in thin films of water on smooth surfaces.

• Re > 2,000: Turbulent flow — chaotic, with eddies and vortices. This is the normal condition in rivers and is essential for sediment erosion and transport.

Why It Matters: Turbulent flow creates the lifting and dragging forces needed to entrain sediment. Without turbulence, rivers would have minimal erosive power.

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