Hjulström curve

The Hjulström diagram describes the limits for the stability of clastic sediments with increasing or decreasing water flow rate. Other influencing factors on the stability of the roughness of the river bed, the cohesion between the individual particles at very small grain sizes, the surface forces and with larger particles, the weight force. It is an empirically obtained graph doppellogarithmisches with the average grain size d on the abscissa, and the average speed V on the ordinate. The use of the mean velocity, however, is the major flaw of this model, because it is only a proxy for all on a particle at the river surface forces. The Hjulström chart thus describes the relationship between particle size and flow rate. The diagram is divided into three areas:

• Erosion
• Transport
• Sedimentation

Even if the average flow velocity alone of course does not provide the optimal approach to the explanation flussmorphologischer processes, so the chart reflects still relatively accurately the observed dependencies of erosion, transport and sedimentation of other important control variables.

In principle follows from the diagram:

• The higher the flow velocity v, the greater the transported grain ( d) be ( v is proportional to d)
• The smaller the particle diameter d, the smaller v promote the necessary flow rate to an erosion of the riverbed.
• But: Under a grain diameter of 0.3 mm to take the cohesive forces ( electrostatic forces ) between the ( clay ) particles and the curve rises accordingly. But these particles are first eroded, they stay the longer moving.