Environmental Engineering Reference
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(a)
(b)
Fig. 3.22 (a) Step-pool system in a small ravine in the upper Yangtze River basin (b) Step-pool system on the
Xiaojinchuan River in Sichuan, China
channel (Abrahams et al., 1995). Pool-pool or step crest-step crest spacing has an average of 2 4 stream
widths in two Oregon streams and is also rather variable because of an uneven distribution of bedrock
outcrops and boulder deposits along the channels (Grant et al., 1990). Step structure appears to be better
defined and more regular on steeper slopes (Judd and Peterson, 1969; Whittaker, 1987; Chin, 2002;
Grant et al., 1990; Wohl and Grodek, 1994), which implies that, if step height is controlled by the largest
particles an increase in the bed slope must be accommodated by more closely spaced steps.
The flow over the boulders forming each step is supercritical (Froude number, Fr > 1) and changes to
sub-critical ( F r < 1) in the pool, causing a considerable amount of energy dissipation through turbulent
mixing (Hayward, 1980; Whittaker and Jaeggi Martin, 1982). In addition, energy is expended as a result
of the form drag exerted by the large particles that make up the steps. Thus, step-pool systems have an
important resistance role, which is of particular significance in mountain streams where alternative forms
of energy dissipation, such as lateral adjustment, are inhibited by the narrowness of the valleys, and
where the large amounts of potential energy generated by the steep slopes could otherwise lead to
extreme erosion.
A step-pool system is the best ecologically sound riverbed pattern in mountain streams. Large fishes
may swim up through the steps and small fishes may swim up against low velocity in the gaps between
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