Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 150

The channel bed developed into three kinds of bed form in different cases. In the first case, the flow
power was small but the clear water flow could remove the fine bed materials. Only coarse particles
remained in the top layer of the bed, and, thus, an armor layer had developed, which might resist the
shear stress of the flow. The roughness of the bed became larger and the velocity reduced. In the second
case, not many cobbles and large gravel were present in the bed material. The flow might remove the
coarsest particles as well, and the armor layer had been destroyed. The moving particles formed dunes
that generated form drag, which caused reduction in the flow velocity. In the third case, cobbles and large
gravel were present in the bed material, and the flow power was high enough to remove all individual
particles. However, the largest particles acted as keystones and prevented other large and small particles
from moving. A sequence of frameworks of tightly interlocking structures developed on the channel bed.
The interlocking structures exhibited an inherent stability, which could resist the assaults of the flow and
composed the steps. Behind the steps the bed was scoured into pools. The flow over the steps was
supercritical, and it changed to subcritical in the pools. Hydraulic jumps formed in the pools, which
functioned as energy dissipation pools and greatly reduced the flow velocity. Thus, the step-pool system
obviously increased the resistance and the flow depth, which was of particular significance for the
stability of the channel bed. The form drag and the hydraulic jumps consumed a large amount of flow
energy, which otherwise could lead to extreme erosion.
Figure 3.32 shows the development process of the step-pool system: (a) an armor layer formed in a
(a) (b)
(c) (d)
Fig. 3.32 Development process of a step-pool system: (a) armor layer; (b) step-pool; (c) dry bed-form after the
experiment; and (d) hydraulic jump at a step-pool section