Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 120

non-wide canals, the shape will have an important impact on the water
flow and sediment transport. The existence of sidewalls and the varying
water depth on the sides will cause a non-uniform distribution over the
width for both the shear stress and the velocity, and as a consequence, for
the sediment transport.
Generally, the most common shape of an irrigation canal is trapezoidal.
Most of these canals cannot be considered as a wide canal and for this
type of cross section, the imposed boundary condition for the velocity and
the varying water depth on the sides will affect the shear stress and the
velocity and sediment distribution in the lateral direction. In other words,
the variables related to the water flow and sediment transport vary in that
direction, specifically for small values of the B / y ratio.
5.3.2 Initiation of sediment movement
Sediment can be transported in equilibrium or non-equilibrium conditions.
Equilibrium conditions means that for specific hydraulic conditions the
flow conveys a certain amount of sediment without deposition or erosion.
Sediment transport predictors are supposed to describe the transport for
the equilibrium conditions. The sediment transport under non-equilibrium
conditions defines how the flow conveys a certain amount of sedi-
ment, as well as the erosion and deposition that take place at the same
time.
The aim of sediment research is to predict the sediment transport in
relation to a known sediment input. Three modes of sediment motion can
be distinguished: rolling and sliding, saltation and suspension . The modes
of motion are related to the flow conditions and the bed material, especially
the hydrodynamic forces, which are expressed in terms of mean velocity or
bottom shear stress acting on a bed. Firstly, the hydrodynamic forces have
to reach a critical or threshold value for the initiation of motion, before a
small increase of the forces will put the grain or aggregate into motion by
irregular jumps or by rolling of the particles. Secondly, when the hydro-
dynamic forces reach a threshold value for the initiation of suspension
then the sediment particles start to diffuse into the water flow. Based on
the three modes of motion two different types of sediment transport can
be defined:
- bed load;
- suspended load.
Bed-load transport
The transport of bed material particles by a water flow can be in the
form of bed load or suspended load, depending on the size of the bed
material particles and the flow conditions. In natural conditions there
is no sharp division between the bed-load or suspended load transport.