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RIVER TRANSPORTATION
Activities of Running Water:
The water that flows along the river does the following works:
a. It transports the debris
b. It erodes the river channels deeper into the bed
c. It deposits sediments at various points along the valley or delivers them to
lakes or oceans
River Transportation:
River transportation is the ability of the river to carry along the particles that a stream
picks up directly from its own channel or that is supplied to it by slope wash,
tributaries or mass movement.
Factors affecting the transportation power of river:
a. Size of the particles to be carried
b. Volume of total load
c. Velocity of river
Some Important Definitions:
1. Load:
The amount of material that a river carries at any time is called its load. Though it
is a volume, it is expressed in tons.
2. Capacity:
The total amount of material a river is capable of carrying under any given set
conditions is called the capacity of the river.
Capacity of a river varies approximately with the third power of the velocity if a
fair proportion of all grain sizes are available ( cv3 ), with a higher power if all the
materials are fine grained and with a lower power if the material is coarse. The
capacity is a function of discharge and velocity.
3. Competence:
The maximum size of particle that a river can carry is called its competence. The
competence of a river is a statement of its ability to move materials in terms of
material size.
Competence is a function of velocity only. The diameter of a particle that a river
can move varies approximately with the square of the stream velocity. ( dv2 ).
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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Classification of Load:
In rivers, load may be carried in solution up to a certain extent, or it may be carried
mechanically as sediments.
a. 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.)
b. 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.
c. Bed Load:
These are solid particles, which move along the riverbed. Bed load mainly
consists of gravel and sand.
Movement of Bed Load:
Particles in the bed load move in three ways: By Saltation, by Rolling and by Sliding:
a. By Saltation:
A particle moving by saltation jumps from one point of the stream to another.
First, it is picked up by a current of turbulent water and flung upwards and if it
is too heavy to remain in suspension, it drops to the stream floor again at
some spot downstream.
b. By Rolling and Sliding:
Some particles are too large and heavy to be picked up by the water
current. But they may be pushed along the streambed and depending on
their shape, they move forward either by rolling or by sliding.
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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Channel Changes in Flood:
We think the flood as the change in the height of water surface and subsequent
inundation but apart from that, change occurs in the stream bed which we can not
see because of the turbidity of water flowing through the stream.
Figure: Stage-Discharge behavior in unsteady flow condition
The figure above represents the unsteady behavior of stage-discharge relationship
at a location of the stream. The maximum stage and the maximum discharge occur
at different times. The changes of the channel bed form that take place during the
flood need clear understanding of this unsteady behavioral pattern.
River channel changes in configuration with this rising and falling stage.
At first the bed may be built up by large amounts of bed load supplied in the
stream during the first phase of heavy runoff. [Observe the elevated bed level as
a result of it]. This stage disappears soon.
The bed is actively deepened by scour as the stream stage rises. This is quite
interesting to mark the lowest elevation of the bed at the highest stage.
In the next stage which can be identified by the falling stage of the stage-
discharge curve, the stage starts to fall and the bed is built back up by the
deposition of the bed load.
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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When the flood crest has passed after the decrease in the discharge, the capacity
of the stream transport load also declines. Some of the particles, that are in motion,
come to rest on the bed in the form of sand and gravel bars. First the largest
boulders and cobblestones will cease (stop) rolling, then the pebbles and gravels
and then the sand. When restored to low stage, the water may become quite clear
with only a few grains of sand rolling along the bed where the current is still fast
enough.
Concept of Equilibrium and Graded Stream:
As an idealized concept, an ‘Equilibrium Condition’ means the supply of load to a
stream from its drainage basin is such that it exactly matches the capacity of stream
to transport. In this type of stream, the hydraulic factors are changing continuously
to bring about a state of equilibrium. This sort of stream is called a ‘Graded Stream’.
A graded stream is one in which, the slope is delicately adjusted over a period of
years to provide, along with the available discharge and prevailing channel
characteristics, just the velocity required for the transportation of the load supplied
to it by the drainage basin.
Factors Controlling Equilibrium of a Stream:
a. Stream Discharge, Q:
It is usually measured in ft3/sec or m3/sec. It is dependent mainly on climate.
The relationship Q = A*Vm is important. Where, A = area of the section
perpendicular to the direction of flow, and Vm = mean velocity of the section.
b. Sediment Discharge and Size of Sediment:
Climate, weathering and all processes delivering sediment to the stream
determine the quantity of sediment discharge (tons/day) and the size of
sediment. The size of grains and the structure of the original rock material are
also important criterion.
c. Slope:
The slope adjusts automatically to provide the velocity necessary for
transporting the amount and size of the material being delivered to the
stream. If the slope is too low for transporting the sediment load, deposition
occurs until the slope is sufficient for transporting. If the slope is so steep that it
provides a velocity greater than that necessary to transport the load, erosion
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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reduces the slope. This new slope will provide the velocity needed to
transport the sediment load.
Channel slope is important in determining stream velocity and so sediment
transport. Generally, as the channel becomes narrower, sediment transport
becomes more efficient, when narrowing of a channel occurs because of
deposition on the bank, erosion of bed is likely to result. Again, when the
deposition of the channel bed occurs, the erosive forces on the bank
increase resulting widening of the stream.
d. Channel Shape:
The ratio of width to depth is used to describe the channel shape. The
interaction of discharge, amount of sediment, slope and local factors such as
bank erodability and channel alignment determine channel shape.
Longitudinal Profile of a Stream:
The longitudinal profile of a stream is a graphic outline of the stream’s gradient
along its course. It provides information of the elevation of the streambed at any
location, the elevation being measured with respect to some fixed datum (e.g.
Mean Sea Level).
Figure: Schematic diagram of longitudinal profile of a stream
The longitudinal profile is a function of the following variables:
Discharge Flow resistance
Load delivered to the channel Depth of flow
Size of debris, Width of the channel
Velocity of flow Slope of the channel
Note: The base level of a river or stream is the lowest point to which it can flow
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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Equation of Longitudinal Profile:
Assumption:
The tendency of a stream to erode at any particular point along its profile is directly
proportional to the height of the stream above the base level.
dH
dX
Here, is slope. At upstream, elevation is higher and hence slope is greater, so
erodability is high. Here flash flood occurs as the water goes down very quickly. On
the other hand, at downstream, slope is less, erodability is less but long time
stagnation of water takes place while flood.
Readjustment of the Stream Grade:
A graded stream delicately adjusted to its environment of supply of water and rock
waste from the upstream sources is highly sensitive to changes in those controls.
Changes in climate and in land surface of the watershed bring changes in
discharge and load at downstream points and these changes in turn require
channel readjustment.
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
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1. Effect of Increase in bed load:
Increase of bed load beyond stream capacity
Accumulation of coarse sediment on the stream bed
o Elevation of stream bed
Aggradation
As consequence of aggradation:
a. In the upstream direction:
Reduction of the channel slope in the upstream direction
Reduction of the stream capacity in the reach
o Accumulation of bed materials in the upstream direction
b. In the downstream direction:
Increase in the channel slope
Increase in velocity
o Bed material is dragged downstream
Aggradation changes the channel cross
section from a narrow and deep form to a
wide and shallow one. Continuous formation
of bars divides the flow into multiple
directions which gives rise to ‘braided channel’.
2. Effect of Decrease in bed load:
Due to the decrease of the bed load, a channel precisely graded is no longer in
equilibrium. As a result:
Scouring of stream
Trenching into the alluvium and lowering of stream profile
o Degradation (channel trenching)
As a result of this, the channel takes a narrower cross section and develops steep,
wall-like banks of alluvium. After this channel trenching, the former floodplain is free
from annual flooding and becomes a ‘stream terrace’.
After degradation has taken place, the stream will normally attain a new and lower
profile of equilibrium. When the new equilibrium is established the stream will usually
develop meanders and will shift laterally to form a new floodplain. In many cases
alternate cycles of aggradation and degradation produce a very complex series of
terraces. These reflect the changing response of the stream to climatic changes.
Md. Shafiul Alam (MSA)
Lecturer, Dept. of Civil Engineering
