Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 13

Fig. 1.11 Various sediment loads in stream flows
air at room temperature, the height of saltation in wind is about 800 times larger than that in water flow if
the particles jump from the bed at same initial velocity. The difference is of profound significance. For
movement of wind-blown sand, particles jump quite high and gain much more energy from the wind. And
when they fall on the bed they splash more particles into the flow. The chain reaction results in a sharp
increase in the rate of sediment transportation soon after sand motion is initiated in desert. Although the
saltation load in flowing water is more important than the contact load, the height of saltation is usually
only a few times the particle diameter in water flow, and the kinetic energy it possesses when falling back
down onto the bed is not enough to induce such chain reactions.
If the drag force of the flow is extremely high a laminated load motion may occur. As the surface of
the riverbed is neither compact nor impermeable, and is composed of granular materials, the shearing
force of the flow may transmit into the bed. If the resistance of the surface layer of the bed due both to
the submerged weight of the particles resting on the bed surface and to the extra pressure exerted by the
saltation load and contact load, is not sufficient to overcome the shearing stress acting on that layer, the
layer next to the surface is bound to move. The motion may penetrate into the bed gradually, following a
progressive increase in stream power. Since the grains are closely packed in the bed, they can move only
in layers, and in the process of movement, the moving bed is dilated to a certain extent so as to attain
more freedom of mobility. There is no turbulence within the layers because the shear between layers checks
the development of turbulent eddies (Wang and Qian, 1987). Laminated load motion was first observed
in flume experiments and debris flows (Wang and Qian, 1985). Recently, the author observed laminated
load motion in a mountain stream on the Yunnan-Guizhou Plateau. The bed slope of the stream is around
0.09. In flood season numerous layers of bed sediment were initiated and moved in layers as laminated
load motion. The laminated load particles are relatively uniform with diameter about 1-5 mm. The
average velocity is only about 0.5 m/s but the sediment transporting capacity is extremely high. Laminated
load motion is a special form of bed load motion with an extremely high intensity.
1.1.4.2 Suspended Load
Flows at high velocity are turbulent and have eddies of various sizes. If a particle jumping from the bed
enters such an eddy, it may be carried far away from the bed. In order to carry a particle, the size of the
eddy must be much larger than the particle and its upward velocity component must be higher than the
fall velocity of the particle. If an eddy is of about the same size as a particle, the particle is liable to fall
out of the eddy; hence, the eddy would no longer affect the movement of the particle. On the contrary, if
an eddy is much greater than a particle, the eddy may carry the particle for a long time. And by the time
the particle falls out of the eddy, it may already have been carried into the region of the main flow.
Obviously, the transport of suspended particles is mainly the effect of large-scale eddies. These particles,
carried by eddies and moving downstream at the same velocity with the flow, are called the suspended
load, as shown in Fig. 1.11. Suspension of particles takes a certain amount of energy from the turbulent