Gagor Force may be applied perpendicular to a surface, such as pressure, or parallel to a surface, such as shear. Air becomes compressed, pressure increases and the riverbank may, in time collapse. Attrition makes the particles of rock smaller. Boundary layers grow thicker with time and distance.
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It was originally published in his doctoral thesis "Studies of the morphological activity of rivers as illustrated by the River Fyris. The graph takes sediment particle size and water velocity into account. Note that the axes are logarithmic. The plot shows several key concepts about the relationships between erosion, transportation, and deposition.
For particle sizes where friction is the dominating force preventing erosion, the curves follow each other closely and the required velocity increases with particle size.
However, for cohesive sediment, mostly clay but also silt , the erosion velocity increases with decreasing grain size , as the cohesive forces are relatively more important when the particles get smaller. The critical velocity for deposition, on the other hand, depends on the settling velocity , and that decreases with decreasing grainsize.
He significantly improved the level of detail in the cohesive part of the diagram, and added lines for different modes of transportation. This curve dates back to early 20th century research on river geomorphology and has no more than a historical value nowadays, although its simplicity is still attractive.
Among the drawbacks of this curve are that it does not take the water depth into account and more importantly, that it does not show that sedimentation is caused by flow velocity deceleration and erosion is caused by flow acceleration.
The dimensionless Shields Diagram is now unanimously accepted for initiation of sediment motion in rivers.
HJULSTROM CURVE PDF
Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example after a flood or during times of drought. It can also occur where wetted perimeter increases relative to the cross sectional area. Flocculation can also occur - where clay particles bond together and hence gain enough mass to sink to the river or sea bed. This is a graph that shows the relationship between the size of sediment and the velocity required to erode lift it , transport it and deposit it. Competence is the maximum size of load that a river can carry, and this is largely determined by velocity. The capacity is slightly different in that this is the total amount of load that is carried.