the engineer, the distinction is important because the bed material is transported at the
capacity of the stream and is functionally related to measurable hydraulic variables. The
washload is not transported at the capacity of the stream. Instead, the washload depends on
availability and is not functionally related to measurable hydraulic variables.
No sharp demarcation exists between washload discharge and bed material discharge. As a
rule of thumb, many engineers assume that the bed material discharge is composed of sizes
(Ds) equal to or greater than 0.062 mm which is also the division point between sand and silt.
The sediment discharge consisting of grain sizes smaller than 0.062 mm is considered as
washload. A more reasonable criterion is to choose a sediment size finer than the smallest 10
percent of the bed material as the dividing size between washload and bed sediment load. It is
important to note that in a fast flowing mountain stream with a bed of cobbles the washload
may consist of coarse sand sizes. For these conditions, the transport of sand sizes is supply
limited. In contrast, if the bed of a channel is silt, the rate of bed load transport of the silt sizes
is less a question of supply than of capacity. The sediment transport capacity of silts has been
addressed in some transport equations. This invalidates the criterion based on Ds < 0.062 mm
for coarse bed sediment streams, however, the criterion based on D10 might still be applicable.
4.3.2 Mode of Sediment Transport
Sediment particles are transported by rolling or sliding on the bed (bed load or contact load) or
by suspension by the turbulence of the stream. Even as there is no sharp demarcation
between bed material discharge and washload there is no sharp line between contact load and
suspended sediment load. A particle may move part of the time in contact with the bed and at
other times be suspended by the flow. The distinction is important because the two modes of
transport follow different laws. The equations for estimating the total bed material discharge of
a stream are based on these laws.
A further subdivision of mode of transport of sediment, including a pictorial representation of
measured load and unmeasured load follows.
When a river reaches equilibrium, its transport capacities for water and sediment are in
balance with the rates supplied. In fact, most rivers are subject to some kind of control or
disturbance, natural or human-induced, that give rise to non-equilibrium conditions.
Total sediment load can be expressed by three equations as illustrated in Figure 4.1 (Julien
1995):
1. By type of movement
L T = Lb + L s
(4.1a)
2. By method of measurement
LT = Lm + Lu
(4.1b)
3. By source of sediment
L T = L w + L bm
(4.1c)
4.4
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