4.3 GENERAL CONSIDERATIONS
The amount of material transported or deposited in the stream under a given set of conditions
is the result of the interaction of two groups of variables. In the first group are those variables
that influence the quantity and quality of the sediment brought down to that section of the
stream. In the second group are variables that influence the capacity of the stream to transport
that sediment. A list of these variables is given as follows.
Group 1 - Sediment brought down to the stream depends on the geology and topography of
watershed; magnitude, intensity, duration, distribution, and season of rainfall; soil moisture
conditions; vegetal cover; cultivation and grazing; surface erosion and bank cutting.
Group 2 The capacity of a stream to transport sediment depends on hydraulic properties of
the stream channel. These are fluid properties, slope, roughness, hydraulic radius,
discharge, velocity, velocity distribution, turbulence, tractive force, viscosity and density of
the fluid sediment mixture, and size and gradation of the sediment.
These variables are not all independent and, in some cases, their effect is not definitely
known. The variables which control the amount of sediment brought down to the stream are
subject to so much variation, not only between streams but at a given point of a single
stream, that the quantitative analysis of any particular case is extremely difficult. It is
practicable, however, to measure the sediment discharge over a long period of time and
record the results, and from these records to determine a soil loss from the area.
The variables that deal with the capacity of the stream to transport solids are subject to
mathematical analysis. These variables are closely related to the hydraulic variables
controlling the capacity of the stream to carry water.
4.3.1 Source of Sediment Transport
Einstein (1964) stated that:
Every sediment particle which passes a particular cross-section of the stream
must satisfy the following two conditions: (1) it must have been eroded
somewhere in the watershed above the cross-section; (2) it must be
transported by the flow from the place of erosion to the cross-section.
Each of these two conditions may limit the sediment rate at the cross-section,
depending on the relative magnitude of two controls: the availability of the
material in the watershed and the transporting ability of the stream. In most
streams, the finer part of the load, i.e., the part which the flow can easily carry
in large quantities, is limited by its availability in the watershed. This part of
the load is designated as washload. The coarser part of the load, i.e., the part
that is more difficult to move by flowing water, is limited in its rate by the
transporting ability of the flow between the source and the section. This part
of the load is designated as bed sediment load.
Thus, for engineering purposes, the two sources of sediment transported by a stream are: (1)
bed material that makes up the stream bed; and (2) fine material that comes from the banks
and the watershed (washload). Geologically both materials come from the watershed. But for
4.3
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