13.3 present Erosion and also Deposition

As we debated in thing 6, flowing water is a an extremely important mechanism for both erosion and also deposition. Water circulation in a present is generally related come the stream’s gradient, however it is likewise controlled through the geometry the the present channel. As presented in figure 13.14, water flow velocity is diminished by friction along the stream bed, so that is slowest in ~ the bottom and also edges and fastest near the surface and in the middle. In fact, the velocity just below the surface is typically a little higher than best at the surface because of friction in between the water and also the air. On a curved section of a stream, circulation is more quickly on the outside and slowest on the inside.

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Figure 13.14 The relative velocity of currently flow depending on whether the stream channel is right or bent (left), and with respect to the water depth (right).

Other determinants that impact stream-water velocity are the size of sediments on the present bed — because big particles often tend to slow the flow an ext than small ones — and also the discharge, or volume the water happen a point in a unit that time (e.g., m3/second). During a flood, the water level always rises, so over there is an ext cross-sectional area because that the water to circulation in; however, as lengthy as a river continues to be confined come its channel, the velocity that the water flow likewise increases.

Figure 13.15 shows the nature of sediment transportation in a stream. Huge particles remainder on the bottom — bedload — and also may only be moved during rapid flows under flood conditions. They can be moved by saltation (bouncing) and also by traction (being driven along through the pressure of the flow).

Smaller particles might rest on the bottom some of the time, where they have the right to be moved by saltation and traction, however they can also be held in suspension in the flow water, especially at greater velocities. As you recognize from intuition and also from experience, streams the flow rapid tend come be stormy (flow courses are chaotic and also the water surface appears rough) and also the water may be muddy, while those that flow more slowly have tendency to have laminar flow (straight-line flow and a smooth water surface) and clear water. Turbulent flow is an ext effective than laminar circulation at keeping sediments in suspension.

Stream water also has a dissolved load, which represents (on average) about 15% the the fixed of product transported, and includes ion such as calcium (Ca+2) and also chloride (Cl-) in solution. The solubility that these ions is not impacted by circulation velocity.

Figure 13.15 modes of transport of sediments and dissolved ion (represented by red dots v + and also – signs) in a stream.

The faster the water is flowing, the bigger the particles that can be preserved in suspension and also transported in ~ the flow water. However, together Swedish geographer Filip Hjulström discovered in the 1940s, the relationship in between grain size and also the likelihood that a grain gift eroded, transported, or deposited is no as simple as one can imagine (Figure 13.16). Consider, because that example, a 1 mm serial of sand. If that is relaxing on the bottom, it will remain there till the velocity is high sufficient to erode it, roughly 20 cm/s. However once it is in suspension, that exact same 1 mm fragment will stay in suspension as long as the velocity no drop listed below 10 cm/s. For a 10 mm gravel grain, the velocity is 105 cm/s to it is in eroded native the bed but only 80 cm/s to continue to be in suspension.

Figure 13.16 The Hjulström-Sundborg diagram reflecting the relationships between particle size and also the tendency to it is in eroded, transported, or deposit at different current velocities

On the other hand, a 0.01 mm silt fragment only needs a velocity of 0.1 cm/s to stay in suspension, but requires 60 cm/s to it is in eroded. In various other words, a small silt grain calls for a better velocity to be eroded 보다 a serial of sand the is 100 time larger! for clay-sized particles, the discrepancy is even greater. In a stream, the most easily eroded particles are tiny sand grains in between 0.2 mm and also 0.5 mm. Anything smaller sized or bigger requires a higher water velocity to be eroded and also entrained in the flow. The main reason because that this is that small particles, and especially the tiny grains that clay, have a solid tendency to stick together, and so are daunting to erode from the stream bed.

It is important to be conscious that a stream can both erode and deposit sediments in ~ the same time. At 100 cm/s, for example, silt, sand, and medium gravel will certainly be eroded from the present bed and also transported in suspension, outlet gravel will be hosted in suspension, pebbles will be both transported and deposited, and also cobbles and also boulders will stay stationary ~ above the present bed.

Exercise 13.3 knowledge the Hjulström-Sundborg Diagram

Refer to the Hjulström-Sundborg chart (Figure 13.16) to answer this questions.

1. A good sand grain (0.1 mm) is resting on the bottom of a stream bed.

(a) What present velocity will it take to obtain that sand grain into suspension?

(b) once the fragment is in suspension, the velocity starts come drop. In ~ what velocity will it ultimately come ago to remainder on the stream bed?

2. A present is flow at 10 cm/s (which way it bring away 10 s to go 1 m, and also that’s pretty slow).

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(a) What dimension of particles can be eroded at 10 cm/s?

(b) What is the largest particle that, once currently in suspension, will stay in suspension in ~ 10 cm/s?

A stream frequently reaches its biggest velocity as soon as it is close come flooding over its banks. This is well-known as the bank-full stage, as shown in figure 13.17. As soon as the flooding stream overtops the banks and also occupies the wide area that its overwhelming plain, the water has a much larger area to flow through and the velocity fall significantly. At this point, sediment the was being carried by the high-velocity water is deposited close to the edge of the channel, developing a natural financial institution or levée.

Figure 13.17 The advancement of natural levées throughout flooding the a stream. The sediments that the levée come to be increasingly fine far from the stream channel, and even finer sediments — clay, silt, and also fine sand — space deposited across most the the flood plain.