The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the
stream bed. Bed load is complementary to
suspended load and
wash load.
Bed load moves by rolling, sliding, and/or
saltating (hopping).
Generally, bed load downstream will be smaller and more rounded than bed load upstream (a process known as
downstream fining). This is due in part to
attrition
Attrition may refer to
*Attrition warfare, the military strategy of wearing down the enemy by continual losses in personnel and material
**War of Attrition, fought between Egypt and Israel from 1968 to 1970
**War of attrition (game), a model of agg ...
and
abrasion which results from the stones colliding with each other and against the river channel, thus removing the rough texture (
rounding
Rounding means replacing a number with an approximate value that has a shorter, simpler, or more explicit representation. For example, replacing $ with $, the fraction 312/937 with 1/3, or the expression with .
Rounding is often done to ob ...
) and reducing the size of the particles. However,
selective transport of sediments also plays a role in relation to downstream fining: smaller-than average particles are more easily
entrained than larger-than average particles, since the shear stress required to entrain a grain is linearly proportional to the diameter of the grain. However, the degree of size selectivity is restricted by the hiding effect described by Parker and Klingeman (1982), wherein larger particles protrude from the bed whereas small particles are shielded and hidden by larger particles, with the result that nearly all grain sizes become entrained at nearly the same shear stress.
Experimental observations suggest that a uniform free-surface flow over a cohesion-less plane bed is unable to entrain sediments below a critical value
of the ratio between measures of
hydrodynamic
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) a ...
(destabilizing) and
gravitational (stabilizing)
forces acting on sediment particles, the so-called
Shields
A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of ...
stress
. This quantity reads as:
:
,
where
is the
friction velocity, s is the relative particle density, d is an effective particle diameter which is entrained by the flow, and g is gravity. Meyer-Peter-Müller
formula for the bed load capacity under equilibrium and uniform flow conditions states that the magnitude of the bed load flux
for unit width is proportional to the excess of shear stress with respect to a critical one
. Specifically,
is a
monotonically increasing nonlinear function of the excess Shields stress
, typically expressed in the form of a power law.
References
Geomorphology
Sedimentology
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