Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 121

Bagnold (1966) defines bed load as the particles which are in successive
contacts with the bed and the processes are governed by gravity. The bed-
load transport includes the rolling and sliding and the saltation modes
in the bed layer. Bed-load transport processes and the formula to com-
pute the yield have been given by many scientists. The simplest yet fairly
reliable empirical equation is based on experimental data by Meyer-Peter
(Raudkivi, 1990). Einstein (1950) introduced statistical methods to rep-
resent the turbulent behaviour of the flow and gave a bed-load function.
Bagnold's (1966) equation is based on a physical concept and analysis. He
introduced an energy concept and related the sediment transport rate to the
work done by the fluid. Equations by Engelund (1966), Ackers and White
(1973) and Yalin (1977) are mainly based on the Einstein or Bagnold
concepts but deduced using dimensional analysis. Van Rijn (1984a)
solved the equations of motion of an individual bed-load particle and
computed the saltation characteristics and particle velocity as a function
of the flow conditions and the particle diameter for plane bed conditions.

Suspended load transport

Suspended load is that part of the sediment transport that moves with
the water flow without contact with the bottom. It includes the suspension
mode and the wash load, which consist of cohesive and very fine sediments
(smaller than 0.05 mm) and which tend to be suspended by Brownian
motion (Raudkivi, 1990).
Transport of suspended sediment takes place when the bed shear veloc-
ity ( u ∗ ) exceeds the particle fall velocity ( w s ). The particles can be lifted
to a level at which the upward turbulent forces will be comparable to or
higher than the submerged particle weight and as a consequence the con-
tact of the particle with the bed is occasional and random in the suspension
mode. The particle velocity in the longitudinal direction is almost equal
to the fluid velocity. Usually, the behaviour of the suspended sediment
particles is described in terms of sediment concentration, which is the
solid volume per unit fluid volume or the solid mass (kg) per unit fluid
volume. The principle feature that distinguishes the suspended sediment
transport from the bed-load transport is the time taken for the suspension
to adapt to changes in flow conditions (Galappatti, 1983).
According to Bagnold (1966), suspension will occur for a bed shear
velocity ( u ∗ ) equal or larger than the particle fall velocity ( w s ), while van
Rijn (1984b) argues that suspension will start at considerably smaller bed
shear velocities. From his experimental data van Rijn (1984b) proposed
the following conditions for the initiation of suspension:

u ∗ ,cr
w s =

4 D − 1
∗

for 1 < D ∗ ≤ 10 (5.63)