Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 146

value of velocity and the crosses are the minimum and maximum values of velocity. The results support
the conclusions of Wohl and Thompson (2000) that the bed-generated turbulence predominates at step
lips and upstream from steps and in runs. The pool section is dominated by wake-generated turbulence.
The shearing occurs primarily at mid-profile and across the channel rather than throughout the profile,
even at a distance of 1.5 m downstream from the step.
Although steps composed of boulders are the most common type, they can also form in bedrock
(Hayward, 1980; Wohl and Grodek, 1994). In this case the steps consist of rocks and there are gravel and
fine material in the pools. Figures 3.26(a) and 3.26(b) show bedrock step pool systems in steep mountain
streams. The step pool systems in bed rock channels function similarly in energy consumption and
resistance maximization as those in gravel bed channels. The flow velocity in the channel with step-pools
is effectively reduced and the impact and risk of flash flood are mitigated. Step-pool systems have been
reported from a wide range of humid and arid environments (Chin, 2002), and analogous forms have
even been observed in supraglacial streams (Knighton, 1984). They, thus, appear to be a fundamental
element of steep fluvial systems.

(a) (b)
Fig. 3.26 (a) A step pool on a bedrock channel in Hong Kang; (b) A bed rock step-pool system in a steep mountain
stream in southwestern China

Step-pool systems can also form through accumulation of large woody debris in heavily forested
catchments (Keller and Swanson, 1979). In this case the steps consist of large woody debris and there are
fine materials in the pools. Figure 3.27 shows a step-pool system in the Jiuzhai Creek in northern
Sichuan. The step-pool system is developed on a landslide dam on which good vegetation has already
developed. Large woody debris form the steps and created high resistance, which protects the landslide
dam from erosion.

3.3.2 Development of a Step-Pool System

3.3.2.1 Development Degree of a Step-Pool System
The most important hydraulic feature of step-pool systems is the extremely high bed roughness, which
maximizes the resistance and reduces the flow velocity. To represent the bed roughness of a step-pool
system a parameter, S P , is introduced, which may be used to describe the development degree of a
step-pool system: