Deposition and the Hjulstrom curve

Overview

Hjulstrom curve and three fields on a Hjulstrom diagram.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why does a river deposit its load?

   Model answer

   Because it loses the energy needed to keep carrying it. Velocity falls when the gradient decreases, when the river enters a lake or the sea and its flow spreads out, when discharge falls after a flood, when water spills out of the channel onto a floodplain and becomes shallow, when friction increases against a shallow or rough bed, or where an obstacle slows the flow. As velocity drops below the settling velocity for a given particle size, that material can no longer be held and it falls to the bed, the coarsest and heaviest first.

2. 2

   What is a Hjulstrom curve, and what does each axis show?

   Model answer

   A Hjulstrom curve is a graph relating the velocity of water to the size of particle it will erode, transport or deposit. The horizontal axis shows particle diameter in millimetres, from clay through silt and sand to gravel and boulders, on a logarithmic scale. The vertical axis shows water velocity, usually in centimetres per second, also on a logarithmic scale. Two curves are plotted on it, and they divide the graph into fields of erosion, transport and deposition.

3. 3

   What do the two curves on a Hjulstrom diagram represent?

   Model answer

   The upper curve is the critical erosion velocity, sometimes called the entrainment velocity - the minimum velocity needed to pick a stationary particle of that size up off the bed and set it moving. The lower curve is the mean settling, or fall, velocity - the velocity below which a particle of that size that is already moving can no longer be held and is deposited. The two are not the same, because it takes more energy to start a particle moving than to keep it moving.

4. 4

   Describe the three fields on a Hjulstrom diagram.

   Model answer

   Above the critical erosion velocity curve is the erosion field: at these velocities the river has enough energy both to entrain particles of that size from the bed and to keep carrying them. Between the two curves is the transport field: the river cannot pick up a stationary particle of that size, but material already in motion continues to be carried. Below the settling velocity curve is the deposition field: the river no longer has enough energy to hold particles of that size and they settle out.

5. 5

   At roughly what particle size is the critical erosion velocity at its lowest, and why does that matter?

   Model answer

   The critical erosion velocity curve is J-shaped, reaching its minimum at fine sand of around 0.2 to 0.5 millimetres, where a velocity of only about 20 centimetres per second is needed to entrain the grains. This matters because fine sand is the material most easily picked up by a river, and it is therefore the size fraction most readily eroded from a sandy bed or bank even in gentle flows. Both larger and smaller particles need higher velocities to entrain.

6. 6

   Why does fine clay need such a high velocity to be picked up, when it is the smallest material of all?

   Model answer

   Because clay particles are cohesive. They are so small that electrostatic and electrochemical forces bind them tightly to one another, and clay swells and packs into a dense mass that acts as a single body rather than as separate grains. In addition, the particles are flat and lie flush in a very smooth bed, so they present almost no surface for the current to grip and they sit within the still layer of water at the bed. Overcoming that cohesion takes a surprisingly high velocity, often greater than that needed to move sand or even fine gravel, which is why the erosion curve turns sharply upwards at the fine end.

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