Environmental Engineering Reference
In-Depth Information
Fig. 1.10 Size distribution curves of sediments: The abscissa is the cumulative percentage of particles finer than the
corresponding diameter on the ordinate ( D -coordinates) (after Wang et al., 2001)
Size distribution curves can reflect distinguishing features of sediment samples, but such curves may
not be convenient to use in the quantitative descriptions and comparisons of one example with another.
Therefore scientists have proposed the use of a variety of characteristic parameters to describe features of
the sediment size distribution and to carry out statistical analyses based on those parameters. The most useful
parameter is the median diameter D 50 which is the value on the ordinate of the point on the accumulative
curve corresponding to the 50% on the abscissa. The sorting coefficient defined by ( D 84 / D 16 ) 0.5 is also
often used to represent the uniformity of the sediment mixture.
1.1.4 Sediment Loads and Bed Forms
Sediment loads are the sediments carried by the flow or the sediments in motion. Sediment loads can be
classified according to their patterns of motion as contact load (rolling or sliding), saltation load, suspended
load, and laminated load. In these, contact load, saltation load, and laminated load all belong to the category
of bed load.
1.1.4.1 Bed Load
Figure 1.11 shows various patterns of motion of sediment load and their relation with the bed. The
particles on the bed are subjected to drag force by the flow and slide or roll forward making contact with
the bed frequently. These particles are called contact load. If the flow velocity is high, the force acting
on the bottom surface of the particle is enlarged because the particle has moved a small distance away
from the bed, and the bottom surface area on which the static pressure acts increases. Therefore, the lift
force becomes much greater. In other words, the particle experiences an abrupt increase in the lift force at
the instant of rolling away from the bed. Consequently, it may jump into the flow from the bed. As the
particle rises to some height, the effect of the increasing horizontal velocity component of the particle is
greater than the effect of the flow velocity, and the relative movement between the particle and the flow
begins to reduce. In general, as the particle reaches its highest point, its velocity is close to the local flow
velocity. From this point, the particle descends. Particles moving in such a way produce what is called
saltation load. Contact load and saltation load are bed load (Chien et al., 1998).
If the saltation height of a particle is large, the particle gains much momentum from the flow, and it
may rebound after it falls down and strikes the bed. In some cases the particle does not only rebound, but
it may also induce other bed particles that it hits to jump into flow. The height of particle saltation is
proportional to the density difference between the particles and the fluid. The natural sand particles usually
have the same density (2.65 g/cm 3 ). Since the density of water is more than 800 times larger than that of
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