How Stream Velocity Controls Competence and Capacity: The Physics of Sediment Transport

In fluvial geomorphology, velocity is the master variable governing a stream's ability to move sediment. It drives both competence (maximum particle size transportable) and capacity (total sediment load transportable per unit time).

Competence: The Size Limit

Competence ∝ Velocity² (approximately). The Hjulström curve shows the relationship:

• Higher velocity → greater lift and drag forces on bed particles.
• Critical erosion velocity for a given grain size defines the threshold.
• Example: 10 cm/s moves fine sand (0.1 mm); 100 cm/s moves coarse gravel (20 mm); 300 cm/s moves small boulders (256 mm).

Competence is a threshold concept: below critical velocity, a particle size doesn't move at all. Above it, it moves readily.

Capacity: The Volume Limit

Capacity ∝ Velocity³ to Velocity⁴ (empirical; Einstein, Engelund-Hansen, Yang equations). Why steeper?

• Higher velocity → more particles in suspension (not just bedload).
• Higher velocity → higher concentration at all sizes.
• Suspended load dominates capacity in most rivers; suspension requires turbulence ∝ velocity².

Capacity is a rate concept: tons/day past a cross-section.

Other Factors (Velocity Isn't Everything)

• Grain size distribution: Mixed sizes → hiding/exposure effects (fine grains sheltered by coarse; coarse exposed on fine matrix).
• Channel slope: Drives velocity (Manning/Chezy); also affects flow depth, shear stress.
• Flow depth: Deeper flow = higher shear stress at same velocity.
• Bedform: Ripples, dunes, plane bed, antidunes — each changes roughness and transport efficiency.
• Sediment supply: Capacity is potential; actual load = min(capacity, supply). Supply-limited vs. transport-limited reaches.
• Cohesion: Clays/silts need higher stress to erode (electrochemical bonds).

Practical Implications

• Floods: Velocity spikes → competence/capacity surge → channel scour, infrastructure damage, sediment pulses.
• Dams: Trap sediment → downstream "hungry water" (high competence, zero supply) erodes channel.
• Restoration: Design velocity for target sediment regime (riffle-pool spacing, bar formation).
• Navigation: Dredging targets capacity reduction (sedimentation rate).

Velocity writes the sediment story. But the full manuscript includes grain size, slope, depth, supply, and time. Read all chapters.