What Size Sediment Needs The Fastest Water Velocity To Be Transported

Equally we discussed in Chapter half dozen, flowing water is a very important machinery for both erosion and deposition. Water flow in a stream is primarily related to the stream's gradient, only it is also controlled by the geometry of the stream channel. Every bit shown in Figure 13.xiv, h2o period velocity is decreased by friction forth the stream bed, then information technology is slowest at the bottom and edges and fastest nearly the surface and in the middle. In fact, the velocity just below the surface is typically a little higher than correct at the surface considering of friction between the water and the air. On a curved section of a stream, menstruation is fastest on the exterior and slowest on the inside.

Figure 13.14 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]

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 h2o depth (right). [SE]

Other factors that affect stream-h2o velocity are the size of sediments on the stream bed — because large particles tend to slow the menstruum more than minor ones — and the discharge, or volume of water passing a point in a unit of measurement of time (eastward.g., m³/second). During a alluvion, the water level always rises, so at that place is more cross-sectional surface area for the water to menses in; however, every bit long every bit a river remains confined to its channel, the velocity of the h2o flow also increases.

Effigy xiii.xv shows the nature of sediment transportation in a stream. Large particles rest on the bottom — bedload — and may just be moved during rapid flows nether flood atmospheric condition. They can be moved by saltation (bouncing) and past traction (being pushed along by the forcefulness of the flow).

Smaller particles may rest 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, especially at higher velocities. As yous know from intuition and from experience, streams that flow fast tend to be turbulent (flow paths are chaotic and the water surface appears crude) and the water may be muddy, while those that flow more slowly tend to have laminar menstruum (straight-line flow and a smooth water surface) and articulate h2o. Turbulent flow is more effective than laminar flow at keeping sediments in break.

Stream water likewise has a dissolved load, which represents (on average) about 15% of the mass of material transported, and includes ions such as calcium (Ca^+two) and chloride (Cl-) in solution. The solubility of these ions is not affected by flow 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 tin can exist 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 beingness eroded, transported, or deposited is non as uncomplicated as one might imagine (Figure 13.xvi). Consider, for example, a 1 mm grain of sand. If it is resting on the lesser, it will remain there until the velocity is high enough to erode it, around 20 cm/south. But once it is in suspension, that same 1 mm particle will remain in intermission as long as the velocity doesn't driblet beneath x cm/s. For a 10 mm gravel grain, the velocity is 105 cm/due south to exist eroded from the bed but but 80 cm/s to remain in break.

Figure 13.16 The Hjulström-Sundborg diagram showing the relationships between particle size and the tendency to be eroded, transported, or deposited at different current velocities

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

On the other paw, a 0.01 mm silt particle only needs a velocity of 0.one cm/s to remain in suspension, but requires 60 cm/s to be eroded. In other words, a tiny silt grain requires a greater velocity to be eroded than a grain of sand that is 100 times larger! For dirt-sized particles, the discrepancy is even greater. In a stream, the near easily eroded particles are small sand grains between 0.2 mm and 0.v mm. Anything smaller or larger requires a higher water velocity to exist eroded and entrained in the flow. The main reason for this is that pocket-size particles, and specially the tiny grains of clay, accept a stiff tendency to stick together, 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/s, for example, silt, sand, and medium gravel will exist eroded from the stream bed and transported in break, coarse gravel volition exist held in suspension, pebbles will be both transported and deposited, and cobbles and boulders volition remain stationary on the stream bed.

Exercises

Practise 13.3 Understanding the Hjulström-Sundborg Diagram

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

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

(a) What stream velocity volition it have to go that sand grain into break?

(b) In one case the particle is in break, the velocity starts to drop. At what velocity will information technology finally come up back to rest on the stream bed?

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

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

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

A stream typically reaches its greatest velocity when it is close to flooding over its banks. This is known every bit the bank-full stage, equally shown in Figure thirteen.17. Equally soon every bit the flooding stream overtops its banks and occupies the wide area of its flood manifestly, the water has a much larger area to menstruation through and the velocity drops significantly. At this point, sediment that was beingness carried by the high-velocity water is deposited nearly the border of the aqueduct, forming a natural bank or levée.

Figure 13.17 The development of natural levées during flooding of a stream. The sediments of the levée become increasingly fine away from the stream channel, and even finer sediments — clay, silt, and fine sand — are deposited across most of the flood plain. [SE]

Effigy xiii.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 well-nigh of the flood plainly. [SE]