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Hjulstrom Curve Notes


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Explaination on hjulstrom curves

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Hjulstrom Curve Notes

  1. 1. Sizes of particles: boulders & cobbles (more than 15 mm), gravel 2 - 15 mm, sand 0.06 - 2 mm, silt 0.004 - 0.06 mm & clay < 0.004 mm Hjulstrom curve: Terms: 1. Capacity: largest amount (weight) of debris that the river can carry. 2. Competence: diameter of the largest particles (size) that can be carried. 3. Critical erosion velocity: lowest velocity at which grain of a given size can be moved. 4. Settling/fall velocity: velocity falls below certain level particle (fall velocity) will be deposited. 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. Interpretation of the Hjulstrom curve/graph.  The x-axis shows the size of the particles in mm.  The y-axis shows the velocity of the river in cm/s.  Hjulstrom graph shows relationship between stream velocity and ability to transport materials of varying sizes. (i.e. finest particles; clay & silt to largest boulders)  There are two curves lines drawn in a Hjulstrom curve; critical erosion velocity and settling or fall velocity. 1
  2. 2. 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. Sand (between 0.06mm and 2 mm) is relatively easily eroded and moved. (at velocity 20cm/secs, sand will be eroded) Particles smaller than these fine sands are often clays (smaller than 0.06mm) which require a higher velocity to produce the energy required to split the small clay particles which have coagulated. (stick together cohesively) Larger particles such as pebbles are eroded at higher velocities and very large objects such as boulders require the highest velocities to erode. (300cm/secs, most eroded) 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. As the velocity of the river decreases again, particles are not deposited but are transported by the water. Boulders are deposited quickly as the velocity decreases, and after that as the size of particle decreases, the longer it is transported. For example, particles of 1mm are deposited when the velocity is at 10cm/sec, however smaller particles of 0.1mm are deposited when the velocity is at 1cm/sec. Clay particles stay afloat in water even when there is no velocity at all as they are very small. 2
  3. 3. Erosion of clay particles needs a velocity similar A river with high to that for pebbles because of their cohesive discharge and properties. velocity increases its capacity & Pebbles, cobblers, competence boulders have allowed narrow gap between increase amount of energy erosion of river needed for erosion channel. and deposition, i.e. only small drop in velocity can caused Velocity deposition of larger Needed to keep particles. particles in suspension Is less than to erode them Velocity needed to transport clay particles is almost zero (clay particles stay afloat in water i.e. in suspension, even when there is no velocity at all as they are very small) 3