Hjulstrøm curve

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The Hjulstrøm curve is a graph used by hydrologists to determine whether a river will erode, transport, or deposit sediment. The graph takes sediment size and channel velocity into account.

The x-axis shows the size of the particles in mm. The y-axis shows the velocity of the river in cm/s. The tree lines on the diagram show when different sized particles will be deposited, transported or eroded.

The curve uses a double logarithmic scale. It shows several key ideas about the relationships between erosion, transportation and deposition. The Hjulstrøm curve shows that particles of a size around 1mm require the least energy to erode, as they are sands that do not coagulate. Particles smaller than these fine sands are often clays which require a higher velocity to produce the energy required to split the small clay particles which have coagulated. Larger particles such as pebbles are eroded at higher velocities and very large objects such as boulders require the highest velocities to erode. When the velocity drops below this velocity called the line of critical velocity, particles will be deposited or transported, instead of being eroded, depending on the river's energy.