The Hjulström diagram describes the stability of clastic sediment deposits or other particle accumulations (e.g. regolith or soil ) with regard to the flow velocity of water . It is a double logarithmic empirically obtained diagram with the mean grain size d on the abscissa and the mean speed v on the ordinate . The Hjulström diagram shows the relationship between grain size and flow velocity. It is named after the Swedish geographer Filip Hjulström (1902–1982).
Structure and statement
The diagram is divided into three fields by two curves, the so-called limit speeds or critical speeds :
Basically it follows from the diagram:
- The flow velocity v , which is necessary for the erosion of a grain or particle, is always greater than the flow velocity which is necessary to prevent this particle from being deposited ( to keep it in suspension ).
- The higher the flow velocity, the larger the transported grain ( d ) can be ( v is directly proportional to d )
- The smaller the grain diameter d , the lower the flow velocity v necessary to cause erosion. But: below a grain diameter of 0.3 mm, the cohesion between the particles increases and with it the flow speed required for erosion. As soon as such particles are eroded, however, they remain in suspension all the longer. The "Transport" field is therefore the largest for the smallest grain sizes.
It should be noted that the mean flow velocity and mean grain size are only substitute quantities for all forces acting on a particle. Important factors influencing the stability of particle accumulations are the roughness of the surface, the cohesion between the individual particles and the weight of an individual particle. However, all of these factors are at least partially taken into account by Hulström's approach, because the surface roughness and the weight force correlate positively with the grain size, the cohesion correlate negatively with the grain size. This means that even if it cannot explain all exogenous processes involving liquid water on its own, the diagram nonetheless reflects relatively precisely the observed dependencies of erosion, transport and sedimentation on flow velocity and grain size.
The “Zasada diagram” , like the Hjulström diagram, illustrates the relationship between mean grain size and average flow velocity with regard to the processes of erosion, transport and sedimentation, but depending on gravitation . It was developed to compare the transport and sedimentation conditions on Earth and Mars . The mathematical approach on which the diagram is based can in principle be extrapolated to all other terrestrial planets, but it has not yet been proven that surface-forming processes caused by liquid water occurred or even recently occurred on other objects in the solar system.
The Zasada diagram shows that the erosion and sedimentation processes that shape the landscape are strongly influenced by the gravity of a planet: Similar structures on Earth and Mars are due to different flow speeds and flow rates during the deposition. This allows conclusions to be drawn about the geological history of Mars: Because the flow velocity on Mars may be lower in order to create the same structures as on Earth, and because the flow velocity is generally directly proportional to the flow rate, there is a possibility that Mars in In its past it had less water than its current river valleys and delta structures initially suggest.
The Shields diagram describes the transition zone between calm and movement of sand and gravel in a river bed. In the original Shields diagram, the critical flow intensity is plotted against the sedimentological Reynolds number.
In the modified Shields diagram , the critical flow intensity is plotted against the dimensionless grain size.
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