Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 145

between two steps or two pools, L , is inversely proportional to the average slope of the stream, S :
1.19

0.31 L (3.2)

where L is in meters. The decrease in step length is rapid as the slope increases up to about 0.15. The
influence of bed slope on the adjustment of step-pool morphology is further illustrated by a relation
between the average step steepness (H s /L) and slope obtained by Abrahams et al. (1995) from field and
laboratory data;

s HL s (3.3)
This relation indicates that the average elevation loss due to steps is about 1.5 times the average elevation
loss along a reach, which implies that about one third of the step height is the result of pool scour
(Knighton, 1984).
Wohl and Thompson (2000) measured the vertical velocity profiles of flow in a small mountain
channel with step-pool bed forms. The results suggest that locations downstream from bed-steps are
dominated by wake turbulence from mid-profile shear layers. Locations upstream from steps, at steps,
and in runs are dominated by bed-generated turbulence. Adverse pressure gradients above and below
steps may enhance turbulence generation, whereas favorable pressure gradients at steps suppress
turbulence. The wake-generated turbulence leads to higher energy dissipation in step-pool reaches
relative to more uniform-gradient reaches. The bed-generated turbulence that predominates at step lips
and upstream from steps, and in runs, is analogous to the turbulence that dominates riffles and runs in
pool-riffle channels. The wake-generated turbulence in step-pools is also analogous to the shear
associated with lateral eddies in larger pools, except that in step-pools the shearing occurs primarily at
mid-profile and across the channel rather than throughout the profile and along the channel margins.
Wang et al. (2009) measured the velocity distributions in the pool section and step section of Shengou
Creek with a current meter. Shengou Creek is located in the upper reaches of the Yangtze River basin in
southwestern China, and there is a fully developed step-pool system on the creek. Measurements were
done at the step-lips, in the pools, and in reaches of normal run. Roller eddies occur immediately
downstream of the step making it difficult to measure the velocity profile. The velocity profiles in the
pools were measured at a distance of 1.5 m from the steps. Figure 3.25 shows the velocity profiles
measured at a step-lip, 1.5 m downstream of the step in the pool section, and 2 m above the step where
the flow is almost in a normal run. The step is about 0.7 m high. The points in the figure are the mean

/~1 5

Fig. 3.25 Velocity profiles measured in Shengou Creek: (a) profile at a step-lip, where the water depth is 0.32 m;
(b) profile 1.5 m downstream of the step in a pool where the water depth is 0.61 m; (c) profile 2 m above the step,
where the water depth is 0.42 m. The range of “+ ü +” shows the variation range of the mean velocity at the
measurement point, which indicates the intensity of fluctuation of velocity