Engineering Geology

Chapter 3
In the early stages of river development, erosion tends to be greatest in the lower part of the
drainage basin. However, as the basin develops, the zone of maximum erosion moves
upstream, and it is concentrated along the divides in the later stages. The amount of erosion
accomplished by a river in a given time depends on its volume and velocity of flow; the character
and size of its load; the rock type and geological structure over which it flows; the infiltration
capacity of the area it drains; the vegetation (which affects soil stability); and the
permeability of the soil. The volume and velocity of a river influence the quantity of energy it
possesses. When flooding occurs, the volume of a river is increased significantly, which leads
to an increase in its velocity and competence. However, much energy is spent in overcoming
the friction between the river and its channel so that energy losses increase with any
increase in channel roughness. Obstructions, changing forms on a river bed such as sandbars
and vegetation, offer added resistance to flow. Bends in a river also increase friction.
Each of these factors causes deflection of the flow that dissipates energy by creating eddies,
secondary circulation and increased shear rate.
The load that a river carries is transported in four different ways. Firstly, there is traction, that is,
rolling of the coarsest fragments along the river bed. Secondly, smaller material, when caught
in turbulent upward-moving eddies, proceeds downstream in a jumping motion referred to as
saltation. Thirdly, fine sand, silt and mud may be transported in suspension. Fourthly, soluble
material is carried in solution.
Sediment yield may be determined by sampling both the suspended load and the bedload.
It can also be derived from the amount of deposition that takes place when a river enters
a relatively still body of water such as a lake or a reservoir.
The competence of a river to transport its load is demonstrated by the largest boulder it is
capable of moving; it varies according to the velocity of a river and its volume, being at a maximum
during flood. It has been calculated that the competence of a river varies as the sixth
power of its velocity. The capacity of a river refers to the total amount of sediment that it carries.
It varies according to the size of the soil and rock material that form the load, and the
velocity of the river. When the load consists of fine particles, the capacity is greater than when
it is comprised of coarse material. Usually, the capacity of a river varies as the third power of
its velocity.
Both the competence and capacity of a river are influenced by changes in the weather, and
the lithology and structure of the rocks over which it flows, as well as by vegetative cover
and land use. Because the discharge of a river varies, all sediments are not transported
continuously, for instance, boulders may be moved only a few metres during a single flood.
Sediments that are deposited over a flood plain may be regarded as being stored there
temporarily.
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