Lecture Topic: River Transportation
Istiakur Rahman
Assistant Professor
Department of Civil and Environmental Engineering (CEE)
Islamic University of Technology (IUT)
Activities of Running Water: The water that flows along the
river does the following works:
Transportation of the debris + Erosion of the river channel
+Deposition of the sediments
River Transportation: The capacity of a river to transport
particles that it picks up directly from its own channel or that
are provided to it by slope wash, tributaries, or mass
movement is referred to as river transportation.
When a river erodes, the eroded material becomes the river’s
load and the river will then transport this load through its
course until it deposits the load.
There are a few different ways that a river will transport
load depending on how much energy the river has and how
big the load is.
The largest of particles such as boulders are transported by
traction. These particles are rolled along the bed of the river,
eroding the bed and the particles in the process, because the
river doesn’t have enough energy to move these large
particles in any other way.
Slightlysmaller particles, such as pebbles and gravel, are
transported by saltation. This is where the load bounces
along the bed of the river because the river has enough
energy to lift the particles off the bed but the particles are
too heavy to travel by suspension.
Fine particles like clay and silt are transported in suspension,
they are suspended in the water. Most of a river’s load is
transported by suspension.
Solution is a special method of transportation. This is where
particles are dissolved into the water so only rocks that are
soluble, such as limestone or chalk, can be transported in
solution.
Dissolved load: These are the soluble materials and are
carried in solution in the form of ions.
Whether a particle will go in solution depends on its chemical
properties. When the sediment discharge is high, even the
soluble materials may not go in solution as the water is
already saturated. Again if the particle size is too large it
may not dissolve.
The amount of dissolved load depends upon:
(1) Climate (heat), (2) Season (temperature) and (3) Geologic
Setting (presence of sand, pebbles, etc.)
Suspended load: This is the load carried mechanically as
sediment. These are the particles of solid matter that are
swept along in the turbulent current of the stream and
remain in suspension.
The amount of this load depends upon:
(1) intensity of turbulence of water and (2) terminal velocity
of each individual particle
Bed load: These are solid particles, which move along the
riverbed. Bed load mainly consists of gravel and sand
Riverscan only carry so much load depending on their
energy. The maximum volume of load that a river can carry
at a specific point in its course is called the river’s capacity.
The biggest sized particle that a river could carry at a
specific point is called the river’s competence.
For each grain size there is a specific velocity at which the
grains start to move, called entrainment velocity. However the
grains will continue to be transported even if the velocity falls
below the entrainment velocity due to the reduced (or
removed) friction between the grains and the river bed.
Eventually the velocity will fall low enough for the grains to be
deposited. This is shown by the Hjulstrom curve
A graded stream is a system in equilibrium, one in which there
exist a balance of the erosional and depositional processes. It is a
stream that is transporting just that amount of sediment it is
capable of, for its velocity and discharge. While a graded stream
might be both picking up new sediment by erosion and depositing
sediment, there is no net erosion or net deposition. While it might
be argued that a graded stream is a concept, and that a truely
graded stream does not exist, it is a concept that is certainly
approached in nature, and that one that is powerful in predicting
stream behavior. Indeed accepting the premise that all streams
are either graded or are striving toward that condition, will
provide you with a way to anticipate results of external stimuli on
a stream.
Consider what factors influence a streams capacity to
transport sediment (and pick them up by erosion) and its
deposition of sediment. Obviously stream velocity is one
important factor. If something is done to affect a stream, you
should be able to reason how that might affect the stream
velocity (since you know what determines the velocity
(gradient, discharge, channel characteristics)) and therefore
make a logical conclusion regarding how the stream will
adjust in order to restore a graded condition (e.g. will it tend
to erode and pick up more sediment, or will it begin to
deposit sediment?)
Riversare linear systems which show a gradient of characters
along their length. Ideally the longitudinal profile of a river is
concave with a steep upper portion near the source, giving
way to reaches of progressively less gradient as the mouth is
approached. Other features of the river are associated with
this progression. The bed material becomes finer the
shallower the gradient, and because of the increasing amount
of water carried by the river channel this usually becomes
wider from source to mouth.
