HI. Sediment Transport
Chapter I : Bed Material Suspension and
Transport in Steady Uniform Currents
1
Bed Load Transport
1.1
Introduction
51
Usually, the transport of particles by rolling, sliding and saltating is called the
bed-load transport. For example, Bagnold (1956) defines the bed-load transport
as that in which the successive contacts of the particles with the bed are strictly
limited by the effect of gravity, while the suspended load transport is defined as
that in which the excess weight of the particles is supported wholly by a random
succession of upward impulses imported [and impacted] by turbulent eddies.
Einstein (1950), however, had a somewhat different approach. Einstein defined
the bed-load transport as the transport of sediment particles in a thin layer of 2
particle diameters thick just above the bed by sliding, rolling and, sometimes, by
making jumps with a longitudinal distance of a few particle diameters. The bed
layer is considered as a layer in which the mixing due to the turbulence is so
small that it cannot influence the sediment particles, and therefore suspension of
particles is impossible in the bed-load layer. Further, Einstein assumed that the
average distance traveled by any bed-load particle is a constant distance of 100
particles, independent of the flow conditions, transport rate and the bed
composition. In Einstein's view, saltating particles belong to the suspension
mode of transport, because the jump lengths of saltating particles are
considerably longer than a few grain diameters.
Many formulas to predict the bed-load transport rate are described in the
literature. The earliest equation is that of Du Boys (1879), who assumed that
sediment particles move along the bottom in layers of progressively decreasing
velocities in a vertical downward direction. The first empirical formula was
presented by Meyer-Peter and MOiler (1948). They performed flume experiments
with uniform particles and with particle mixtures. Based on data fitting, a still
frequently used and relatively simple formula has been obtained. Kalinske (1947)
and Einstein (1950) introduced statistical methods to represent the turbulent
behavior of the flow. Kalinske assumed a normal distribution for the
instantaneous fluid velocity at grain level. Einstein gave a detailed but
complicated statistical description of the particle motion in which the exchange
probability of a particle is related to the hydrodynamic lift force and particle
weight. Einstein proposed the d35 as the effective diameter for particle mixtures
and file d65 as the effective grain roughness diameter.
Chapter I : Bed Material Suspension and
Transport in Steady Uniform Currents
1
Bed Load Transport
1.1
Introduction
51
Usually, the transport of particles by rolling, sliding and saltating is called the
bed-load transport. For example, Bagnold (1956) defines the bed-load transport
as that in which the successive contacts of the particles with the bed are strictly
limited by the effect of gravity, while the suspended load transport is defined as
that in which the excess weight of the particles is supported wholly by a random
succession of upward impulses imported [and impacted] by turbulent eddies.
Einstein (1950), however, had a somewhat different approach. Einstein defined
the bed-load transport as the transport of sediment particles in a thin layer of 2
particle diameters thick just above the bed by sliding, rolling and, sometimes, by
making jumps with a longitudinal distance of a few particle diameters. The bed
layer is considered as a layer in which the mixing due to the turbulence is so
small that it cannot influence the sediment particles, and therefore suspension of
particles is impossible in the bed-load layer. Further, Einstein assumed that the
average distance traveled by any bed-load particle is a constant distance of 100
particles, independent of the flow conditions, transport rate and the bed
composition. In Einstein's view, saltating particles belong to the suspension
mode of transport, because the jump lengths of saltating particles are
considerably longer than a few grain diameters.
Many formulas to predict the bed-load transport rate are described in the
literature. The earliest equation is that of Du Boys (1879), who assumed that
sediment particles move along the bottom in layers of progressively decreasing
velocities in a vertical downward direction. The first empirical formula was
presented by Meyer-Peter and MOiler (1948). They performed flume experiments
with uniform particles and with particle mixtures. Based on data fitting, a still
frequently used and relatively simple formula has been obtained. Kalinske (1947)
and Einstein (1950) introduced statistical methods to represent the turbulent
behavior of the flow. Kalinske assumed a normal distribution for the
instantaneous fluid velocity at grain level. Einstein gave a detailed but
complicated statistical description of the particle motion in which the exchange
probability of a particle is related to the hydrodynamic lift force and particle
weight. Einstein proposed the d35 as the effective diameter for particle mixtures
and file d65 as the effective grain roughness diameter.