Erosion and deposition in streams
A factor that influences whether sediments will be transported, eroded or deposited is the actual grain size of the sediment, whether clay, silt, sand or gravel. For each of these, it is noteworthy that the stream velocity required to erode each of these (set the grain in motion) is always greater than the stream velocity required to transport the sediment (keep the grain in motion).
As to be suspected, the greater the grain size, the greater stream velocity is required to erode and transport the grain in the water body. However, clay and some fine silt, because of their cohesive forces, resist erosion and therefore require a far greater stream velocity to pick them up from the walls and channel bed of the stream. Clays are also unique in that they will only be deposited if the stream practically stops flowing - clay particles remain in suspension until there is almost no movement in the water and they can settle down through the water column.
The gradient of the channel bed is an important factor in the velocity that the flow can achieve. A stream will flow faster - under the influence of gravity - if the stream bed is steep, and vice versa. Changes in velocity in a stream result in increased erosion or deposition, as is the case with a river flowing down a mountainside onto a flat plain at the foot of the mountain.
The stream will flow rapidly at the top of the course, and upon reaching the plain, will slow down considerably, depositing a large volume of its transported sediment (as the water no longer has the energy to keep the sediment in motion). At the top of the stream's course, the water will flow at its fastest, thereby eroding more rocky material and effectively deepening the upper valley over time.
As mentioned above, the frictional forces caused by the drag of the water against the channel bed and sides slow stream velocity down considerably. By applying this theory to two river channels with different shapes, one would observe that the flatter, shallow and wider river will yield a smaller stream velocity, as a greater surface area of the channel (in cross-section) will be in contact with the water, whereas the narrower, rounder and deeper channel will tend to speed the stream up.
The actual shape of the channel (from aerial view) also plays a role: A river channel with numerous meanders and curves will slow down the stream velocity. In certain cases, artificial steepening of the channel (to increase stream velocity) is necessary. In this process, the channel is straightened (that is, all meanders are effectively cut off and the distance between two points on the stream is decreased) and the gradient increases as a result. This human intervention is sometimes used when the runoff capacity of a river needs to be increased to prevent flooding, or when rivers need to be made more navigable for economic purposes.
Stream velocity increases if the channel bed and sides are relatively smooth, as friction is minimized. The presence of coarse material such as boulders or stones disturbs the ideal laminar flow of the stream and the resulting turbulence slows the entire water column down.
Even sandy sediments can have an effect. If the sand at the bottom of a river is perfectly smooth, velocity will increase, but normally the water creates wavy patterns in the sand below and, in response, these patterns also create a mild drag on the water.
Perfis de velocidade / Velocity profiles
In the diagram above, the areas of maximum stream velocity are shown in dark blue. On the left is the aerial view of the river, whereas the right indicates the cross-sections at each of the points indicated on the aerial view.
The Hjulström curve, named after Filip Hjulström (1902–1982), is a graph used by hydrologists to determine whether a river will erode, transport, or deposit sediment. It was originally published in his doctoral thesis "The River Fyris" in 1935. The graph takes sediment particle size and water velocity into account.
The upper curve shows the critical erosion velocity in cm/s as a function of particle size in mm, while the lower curve shows the deposition velocity as a function of particle size. 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 grain size. The Hjulström curve shows that sand particles of a size around 0.1 mm require the lowest stream velocity to erode.