Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 277

energy transmission, w t , is positive in the main flow region and negative in the bed region.
Bakhmeteff and Allan (1946) indicated that an inflection point M on the curve of the energy
distribution divides the flow into two regions. The region above point M is called the main flow region;
that below it is called the near-bed flow region. For natural streams, the near-bed flow region amounts to
about 1/10 of the total water depth. Of the energy provided by the flow, 92% comes from the main flow
region, and 90% of it is transmitted to the near-bed flow region and lost to resistance there. The local
energy loss in the main flow region accounts for only 8% of the total.
The near-bed flow region is not only the region where most of the loss of flow energy is concentrated,
but it is also the place where the exchanges between the bed material and bed load, and between the bed
load and suspended load, take place. If the vertical distribution of sediment concentration has a gradient,
this gradient normally reaches a maximum in this region. The existence of a gradient for the sediment
concentration affects the local flow characteristics. Thus, from the viewpoint of sediment movement, the
near-bed flow region is the most important region, unfortunately, however, it is also the most difficult
region in which to take measurements.

Fig. 5.18 Vertical distributions of the energy provided by flow, w t , local energy loss, w s, and energy transmitted to
the boundary w b , in a unit time step ( w 0 = J sU, where U is the mean velocity over the vertical) (after Qian and Wan, 1983)

In summary the complete process of energy transformation is simply described in Fig. 5.19.

Fig. 5.19 Energy transformation in open channel flow (after Qian and Wan, 1983)

At any solid boundary, the sediment particles on an alluvial stream bed cause surface friction that is
called the grain friction. Nikuradse glued uniform sediment particles to the inner surface of a pipe for his
classical study of the resistance of flow through pipes. The particle diameter was taken as the size of the
roughness in the pipe. For a natural stream with a nonuniform bed material, a representative particle size