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 shows the linkage between sediment size and the velocity needed 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.
