HJULSTROM CURVE:

Terms:1. Competence : maximum size of material which a

river is capable of transporting.

2. Capacity : total load actually transported.

3. Mean or critical erosion velocity : approximate velocity needed to pick up and transport, in suspension, particles of various sizes.(minimum velocity for load to be moved)

4. Mean fall or settling velocity : velocity at which particles of a given size become too heavy to be transported and so will fall out of suspension and be deposited. (minimum velocity needed to continue transporting load)

Where is the load from?Sediment in a river comes from a variety of sources.

(i) Outside the river (exogenetic): mass movement, rill and gully erosion and sheet wash.

(ii) From within the rivers channel itself(endogenetic); material from the stream bed and banks.

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The relationship between particle size (competence) and water velocity is shown on a graph called Hjulstrom graph.

Hjulstrom's Diagram plots two curves representing mean/critical erosion velocity curve & mean fall or settling velocity curve.

* The critical velocities tend to be an area rather than a straight line on the graph.

For example, sand can be moved easier than silt and or clay, as fine-grained particles tend to be more cohesive.

High velocities are required to move gravel and cobbles because of their large size.

There are three importance features on Hjulstrom curves.

Non-connected/loose

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Particle eroded

Particles transported

Particles deposited

Mean fall/settling velocity curve

Mean/ critical erosion velocity curve

1. The smallest and largest particles require high velocities to lift them. E.g. particles between 0.1mm and 1 mm require

velocities of around 100mm/sec to be entrained, compared with values of over 500mm/sec to lift clay (0.01mm) and gravel (over 2mm).

Clay resists entrainment due to its cohesion; gravel, due to its weight.

2. High velocities are required for entrainment than for transport

3. When velocity falls below a certain level (settling or fall velocity) those particles are deposited.

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Pick up material