Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 153

then the sediment will be transported in equilibrium and there will be no
deposition or erosion. If the inflowing water carries more sediment than the
equilibrium transport capacity then the extra sediment will be deposited
in the canal until a new equilibrium in the flow conditions is attained.
Under uniform flow conditions, when the inflowing water carries less
sediment ( Q s ) than the equilibrium sediment transport capacity Q se ,two
situations might occur depending on whether there is any sediment motion
on the bottom or not. Motion of sediment is evaluated in terms of the
mobility parameter θ and the critical mobility parameter θ cr .
•

In the first situation ( θ>θ cr ), entrainment of particles and an increas-
ing sediment transport will occur along the canal until adaptation to the
equilibrium sediment transport capacity has been attained. The readjust-
ment of the actual sediment concentration to the equilibrium condition
will take some distance and time. This distance is called the adaptation
length and the time the adaptation time. Initially sediment is picked
up directly downstream of the point where the flow enters the erodible
region. This process will cease after some time when the bottom slope
near the head becomes flatter and less sediment is picked up. Then the
pickup of sediment will move to a point that is further downstream and
the process is continued until the whole canal reach has attained a sed-
iment transport that is in equilibrium with the inflowing sediment load.

• In the second case ( θ<θ cr ), without any motion of sediment along the

bottom ( θ<θ cr ), the actual sediment load ( Q s ) is conveyed without
changes; the sediment transport will continue under non-equilibrium
conditions.
However, the operation of irrigation structures to control and distribute
irrigation water will result in most cases in non-uniform flow and these
flow conditions will consequently change the sediment transport capacity
with distance and in time. A sediment transport that is not in equilib-
rium may occur under both uniform (Figure 5.35) and non-uniform flow
conditions. For gradually varied flow conditions a distinction between
backwater and drawdown effects can be made (see Figure 5.36). In these
flow conditions continuous erosion or deposition takes place even after an
initial adjustment of the sediment concentration to the equilibrium con-
centration. The sediment transport capacity of the water keeps on changing
with distance and time. The flow parameters adapt almost instantly to a
change in conditions and the change in flow pattern is smooth. However,
the adaptation of the sediment concentration requires some time to adjust
to the new conditions. For a backwater there may be a continuous deposi-
tion or first erosion and then continuous deposition or a non-equilibrium
flow depending upon the actual sediment concentration, sediment trans-
port capacity and the type of bed material. For a drawdown condition
there may be first deposition and then erosion, or continuous erosion or
non-equilibrium conditions.