How Does Water Velocity Affect the Size of Particle That Running Water Can Transport?

Affiliate 13 Streams and Floods

xiii.iii Stream Erosion and Deposition

As nosotros discussed in Affiliate half-dozen, flowing water is a very important mechanism for both erosion and deposition. Water flow in a stream is primarily related to the stream'southward gradient, but it is also controlled past the geometry of the stream channel. As shown in Figure 13.14, water flow velocity is decreased by friction along the stream bed, so it is slowest at the bottom and edges and fastest almost the surface and in the center. In fact, the velocity just below the surface is typically a little college than right at the surface because of friction betwixt the water and the air. On a curved section of a stream, flow is fastest on the exterior and slowest on the inside.

Figure thirteen.14 The relative velocity of stream flow depending on whether the stream channel is straight or curved (left), and with respect to the water depth (right). [SE]

Other factors that touch on stream-h2o velocity are the size of sediments on the stream bed — because big particles tend to slow the flow more than small ones — and the discharge, or book of water passing a signal in a unit of measurement of time (eastward.g., g³/2d). During a flood, the water level always rises, so at that place is more than cross-sectional area for the water to period in; however, every bit long as a river remains bars to its channel, the velocity of the water catamenia too increases.

Figure 13.15 shows the nature of sediment transportation in a stream. Big particles residual on the bottom — bedload — and may simply exist moved during rapid flows under flood conditions. They can be moved by saltation (bouncing) and by traction (existence pushed along by the force of the flow).

Smaller particles may residue on the bottom some of the time, where they can be moved by saltation and traction, but they can also be held in suspension in the flowing h2o, peculiarly at higher velocities. As y'all know from intuition and from feel, streams that flow fast tend to be turbulent (flow paths are cluttered and the water surface appears rough) and the h2o may be muddy, while those that menstruum more than slowly tend to take laminar menstruation (straight-line menses and a smoothen h2o surface) and clear water. Turbulent period is more effective than laminar flow at keeping sediments in intermission.

Stream water as well has a dissolved load, which represents (on average) near 15% of the mass of cloth transported, and includes ions such as calcium (Ca⁺²) and chloride (Cl-) in solution. The solubility of these ions is not affected by menstruum velocity.

Figure 13.15 Modes of transportation of sediments and dissolved ions (represented by red dots with + and – signs) in a stream. [SE]

The faster the water is flowing, the larger the particles that can be kept in suspension and transported inside the flowing water. However, as Swedish geographer Filip Hjulström discovered in the 1940s, the human relationship between grain size and the likelihood of a grain being eroded, transported, or deposited is non as simple equally one might imagine (Figure 13.16). Consider, for case, a i mm grain of sand. If information technology is resting on the bottom, it will remain there until the velocity is high enough to erode it, around 20 cm/s. But once it is in interruption, that aforementioned i mm particle will remain in suspension equally long as the velocity doesn't driblet below 10 cm/s. For a ten mm gravel grain, the velocity is 105 cm/southward to be eroded from the bed merely only 80 cm/south to remain in interruption.

Figure thirteen.xvi The Hjulström-Sundborg diagram showing the relationships between particle size and the tendency to be eroded, transported, or deposited at different electric current velocities

On the other hand, a 0.01 mm silt particle only needs a velocity of 0.1 cm/southward to remain in suspension, but requires 60 cm/s to exist eroded. In other words, a tiny silt grain requires a greater velocity to exist eroded than a grain of sand that is 100 times larger! For clay-sized particles, the discrepancy is even greater. In a stream, the most hands eroded particles are modest sand grains betwixt 0.2 mm and 0.v mm. Anything smaller or larger requires a higher water velocity to be eroded and entrained in the period. The primary reason for this is that small particles, and peculiarly the tiny grains of clay, take a stiff tendency to stick together, and so are difficult to erode from the stream bed.

It is important to be aware that a stream can both erode and deposit sediments at the same time. At 100 cm/due south, for example, silt, sand, and medium gravel will exist eroded from the stream bed and transported in suspension, coarse gravel will exist held in suspension, pebbles volition be both transported and deposited, and cobbles and boulders volition remain stationary on the stream bed.

Practice thirteen.three Understanding the Hjulström-Sundborg Diagram

Refer to the Hjulström-Sundborg diagram (Figure 13.16) to reply these questions.

one. A fine sand grain (0.i mm) is resting on the bottom of a stream bed.

(a) What stream velocity will it take to get that sand grain into suspension?

(b) Once the particle is in suspension, the velocity starts to drop. At what velocity will information technology finally come up back to residue on the stream bed?

2. A stream is flowing at 10 cm/s (which means it takes 10 s to go 1 1000, and that's pretty slow).

(a) What size of particles can exist eroded at 10 cm/s?

(b) What is the largest particle that, once already in intermission, will remain in pause at ten cm/southward?

A stream typically reaches its greatest velocity when it is close to flooding over its banks. This is known as the depository financial institution-full stage, as shown in Figure xiii.17. As soon as the flooding stream overtops its banks and occupies the wide area of its overflowing plain, the water has a much larger area to flow through and the velocity drops significantly. At this point, sediment that was beingness carried by the high-velocity h2o is deposited near the edge of the aqueduct, forming a natural depository financial institution or levée.

Effigy thirteen.17 The development of natural levées during flooding of a stream. The sediments of the levée become increasingly fine away from the stream aqueduct, and even finer sediments — clay, silt, and fine sand — are deposited across nearly of the flood plain. [SE]

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Source: https://opentextbc.ca/geology/chapter/13-3-stream-erosion-and-deposition/

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