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Sediment transport (Entrainment (Dey and Ali, 2019) (Bed roughness in…
Sediment transport
Entrainment (Dey and Ali, 2019)
The bed sediment entrainment by streamflow remains one of the fascinating problems of fluvial sedimentology.
The sediment entrainment is guided by the mutual interaction between the sediment grains and the streamflow (Giménez‐Curto & Corniero, 2009). The streamflow drives the sediment grains through the fluid forces acting on them and, thereby, the sediment entrainment phenomenon modifies the shape of the physical flow domain through the erosional and depositional processes.
On the other hand, the streamflow is modified by the spatiotemporal changes of the physical flow domain in the presence of sediments.
Rolling, sliding and lifting = bedload transport
The visual observations of bed sediment motion revealed that the sediment grains manifest various kinds of entrainment depending on the flow strength that can be quantified by making use of one of the following measures: flow velocity, flow discharge, bed shear stress and stream power (Graf, 1971)
Among the various techniques to envision the sediment entrainment threshold, the use of bed shear stress is more logical to characterize the entrainment threshold criterion, because the shear stress induced by the flow, exerted on the bed sediments, is more likely to be a measure of streamflow competence (Shields, 1936)
When a sediment grain falls through a fluid, it accelerates due to gravity. he connection between the settling velocity of sediment grains and the entrainment threshold criterion was identified long ago by Rubey (1933).
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Shields
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Previous ways of understanding sediment mobilisation has occurred eg. Hujlstrom (1935) using critical erosion velocity, Gob et al., 2010 using critical specific stream power or Mao et al., 2008 using unit critical discharge.
shear stresses can explain the shapes of river beds, particularly meandering beds (Milan et al., 2001)
Shields criterion can account for the effect of particle shape on the resistance to entrainment (Petit, 1989)
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Importance of sediment
500 million of the worlds population live on deltas, understanding sediments 2mm of under is essential for their livelihood as 85% of deltas experience severe floods, this figure is set to rise in the 21st century due to sea level rise (Syvitski, 2009).
Sediments under 2mm are vectors for pollutants, contaminants and carbons. They can also have metals bonded to the sediments eg. lead and zinc which occurs in NE England. Whilst it has its downsides, it is also fertile so needed for farming.
Kondolf et al., 2014
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sediments are desperately needed by the downstream river system to maintain its morphology and ecology, as well as replenishing vital land at the coast.
Threshold of movement is usefully defined by Shield’s function
– largely a function of sediment size in fully turbulent flows.
However, within-reach scale, sediment entrainment is governed
by balancing gravitational, resistive and drag forces.
Key terms
Competence – represents a measure of the rivers transporting ability – size of the largest particle it can transfer (Barker and ritter, 1975)
Capacity - The maximum amount of sediment of a given size that a stream can transport as bedload. Capacity depends on channel gradient, discharge and the calibre of the load (see Wainwright et al., 2015).
discharge which determines the velocity, the velocity profile effects the shear stresses on the bed which drives competence and capacity within bedload transport.
Fryiers and Brierly, 2013
Entrainment
Frictional resistance influences the efficiency of entrainment. For coarse sediments, clast size is the greatest importance whereas in fine sediments, cohesion controls efficiency.
Hysterisis
Suspended transport is not usually a linear process, as suspended load is controlled more by catchment variation in sediment supply and hydrology than mean peak discharge and transport capacity of the flow.
Clockwise - sediment wave precedes the water wave so sediment concentration peaks prior to discharge.
Anti-clockwise - variation in sediment concentration reflects a peak in discharge prior to a peak in sediment concentration.