Sediment Dynamics in Alluvial Rivers and Channels

Instructional Objectives
On completion of this lesson, the student
shall be able to learn the following:
1. The mechanics of sediment mo
vement in alluvial rivers
2. Different types of bed fo
rms in alluvial rivers
3. Quantitative assessment
of sediment transport
4. Resistance equations for flow
5. Bed level changes in alluvial channe
ls due to natural and artificial
causes
6. Mathematical modelling
of sediment transport
2.10.0 Introduction
In Lesson 2.9, we looked into the
aspects of sediment generation due to
erosion in the upper catchments of a
river and their transport by the river
towards the sea. On the way, some of
this sediment mi
ght get deposited, if
the stream power is not
sufficient enough. It was noted that it is the shear
stress at the riverbed that causes the
particles near the bed to move provided
the shear is greater than
the critical shear stress
of the particle which is
proportional to the particle size. Henc
e, the same shear generated by a
particular flow may be able to move of
say, sand particles, but unable to
cause movement of gravels. The particles which move due to the average
bed shear stress exceeding the critical
shear stress of the particle display
different ways of movement depending on
the flow condition, sediment size,
fluid and sediment densities, and the channel conditions.
At relatively slow shear stress, the particles roll or slide along the bed. The
particles remain in continuous cont
act wit the bed and the movement is
generally intermittent. Sediment material
transported in this manner is termed
as the
contact load
. On increasing the shear st
ress, some particles loose
contact with the bed for some time, and
hop or bounce from one point to
another in the direction of flow. The sediment particles moving in this manner
fall into the category of
saltation load
.
Contact load and saltation load
together is generally termed as
bed load
, that
is, the sediment load that is
transported on or near the bed.
The further increase in shear stress,
the particles may go in suspension and
remain thus due to the turbulent fluc
tuations and get carried downstream by
stream flow. These sediment
particles are termed as
suspension load
. In
most natural rivers, sediments are
mainly transported as suspended load.
Bed load and suspended load together constitute, what is termed as,
total
load
. A knowledge of the rate of total s
ediment transport for given flow, fluid
and sediment characteristics is necessary
for the study of many alluvial river
processes. Engineers always
need to bear in mind t
he fact that alluvial
streams carry not only water but also se
diment and the stability of a stream is
closely linked with the sediment and transpor
t rate. Alluvial channels must be