Sediment Size Comparison
Clay: < 1/256mm
Silt: 1/16mm-1/256mm
Sand:1/16mm – 2mm
Gravel: 2mm – 4mm
Pebbles: 4mm – 64mm
Cobbles: 64mm – 256mm
Boulders: > 256mm
The Hjulström Curve illustrates the relationship between sediment size and the velocity required to erode, transport, or deposit. The upper line shows the erosional velocity or critical erosion velocity needed to initiate sediment erosion.
The lower line shows the fall or settling velocity
In between the two there is the transportation of sediments.
A big gap between the critical erosion and deposition line implies that sediments will be transported further, the opposite happens for a small gap where a relative drop in velocity (critical fall velocity) causes sediments to be deposited.
Filip Hjulström’s curve implies that sediments such as sand need low velocities to be dislodged from the bed. This is because sand particles are incohesive; they don’t stick together.
However, clay and silt need higher velocities (the similar velocity as cobbles and boulders) to be dislodged or entrained, something which is odd since clay particles are smaller than sand and pebbles.
The reason is particularly the cohesive nature of clay particles (they stick together). However, once clay particles are dislodged from the bed they are carried further distances in suspension (the big transport gap on the diagram).
The diagram also shows that erosion, i.e. the picking up of sediments, require high velocities than transportation.
Limitations
The Hjulström curve neglects other underlying factors such as vegetation and gradient that determine particle movement. On gentler gradients, particles tend to resist movement than on steeper terrains.
What Other Students Are Asking
What is the Hjulström Curve?
A graph showing how water velocity and sediment size influence whether particles are eroded, transported, or deposited.
Who invented the Hjulström Curve and when?
It was developed by Swedish geographer Filip Hjulström in the 1930s (doctoral thesis published 1935)
How does the Hjulström Curve work?
It plots critical erosion (upper line) and deposition (lower line) velocities against sediment grain size, with a transport zone between the two
Why does clay require higher velocity to erode than sand?
Because clay particles are highly cohesive (they stick together) making them difficult to dislodge.
What sediment size is easiest to erode?
Medium sands (around 0.1 mm) need the lowest water velocity to be eroded
What does the gap between erosion and deposition lines indicate?
The “transport zone”—the wider the gap, the longer sediments remain suspended before deposition
What are the limitations of the Hjulström Curve
It ignores factors like water depth, flow acceleration/deceleration, and spatial complexity; it’s a simplified, historical tool
What is the Hjulström-Sundborg diagram?
A refined version introduced in 1956 by Åke Sundborg, expanding details especially for fine cohesive sediments
