Integrated River Management - River Dynamics and Integrated River Management - page 769

steps, downstream of steps in the pools, and in the backwater zones, enabling the monitoring of siltation
or erosion in the test reach. The cross sections and longitudinal profile were surveyed by an electronic
theodolite. Figure 11.54 shows the monitoring reach on which 12 artificial steps are constructed.

Fig. 11.54 Sketch of experimental reach and location of measurement cross sections

The long-term average rate of bed incision was estimated at 5 cm/yr before the experiment. A bridge
was constructed 40 years ago and the bottom sill of the bridge became by 2.2 m over the river bed in 2006.
Incision and scouring have been especially pronounced in some sections, not only inducing bank failure,
landslides, and debris flow, but also causing damage to local infrastructure and aquatic ecosystems. After
installation of the artificial step-pool system the streambed incision was effectively controlled (Fig. 11.55).
At the initial stage the stream bed rose about 20 cm. In general, the most intense aggradation occurred
upstream of steps, where the depth of siltation was 20 to 50 cm. This was mainly affected by the elevation
difference between the stream bed and the crest of the step lip. Subsequently, the streambed gradually
became stable. As a comparison, the reach downstream of the test reach continued to incise (Fig. 11.55(b)
to (e)).
Figure 11.56(a) shows the bed elevation variation of a typical crosssection in the experimental reach
before and after the installation of the artificial step-pool system. The crosssection was incised for about
0.7 m from June 2003 to June 2006, and threatened the road bed of the newly-built Kunming-Dongchuan
highway on the right channel bank. After the artificial step-pools were installed, incision was stopped and
a part of the channel bed silted up. Subsequently, the streambed gradually became stable. As a
comparison the bed elevation of a cross-section located downstream of the test reach continued to incise
as shown in Fig. 11.56(b).
The resistance caused by the step-pool system composes the main flow resistance. Manning's roughness
coefficient, n , increases with the development degree of step-pool systems, S R P (Wang et al., 2009). In this
experiment, S R P R increased from 0.13 for the natural channel to 0.21 after construction of the artificial
steps. The increasing S R P R represents high dissipation of flow energy, thereby reducing bed-load transport,
and protecting the bed from erosion. Figure 11.57 shows the velocity profiles upstream of an artificial
step, on the step lip, and in a pool downstream of the step. Strong turbulence occurred in the pool, which
acted as an energy dissipation pool. The flow energy was dissipated, and, thus, the flow could not scour