Environmental Engineering Reference
In-Depth Information
Fig. 3.59
Channel degradation and riparian forest disappearance along streams from semi-desert areas of the
western United States. Weirs are used to raise the water table and rehabilitate a riparian buffer strip (after DeBano and
Schmidt, 1989)
3.4.2.4
Artificial Step-pool System
In most cases channel incision cannot be controlled. And in many cases, the control structures induce
some negative effects. Therefore, remedial strategies are more practical and environmentally sound. One
strategy is to create a step-pool system or an armored bed that will resist entrainment, and, thus, prevent
incision. Figure 3.60(a) shows the artificial step-pool system on an incised river in Taiwan, China applied
to protect a bridge (50 m from the front step). The piers of the bridge are endangered by incision of the
channel. Huge stones of diameter up to 2 m were placed on the river bed to form steps. The flow velocity
is reduced and the bridge scour is stopped. In Germany many rivers are incised by sediment-starved
flows. The government spent 40 million Euros to construct the artificial step-pools on the Mangfall River
(Fig. 3.60(b)), which have effectively controlled incision of the channel, although the step-pools have
been partly broken by a flood. The riprap steps look natural and form no barrier to fish migration.
When incision results from channel confinement and increased unit stream power, the channel may be
modified to reduce unit stream power, basically by reducing gradient or increasing width. The incision
control structures discussed above, of course, can reduce gradient by concentrating the river's fall in
discrete, controlled steps. The channel gradient can also be reduced by artificially increasing the
channel's sinuosity. Width can be increased by mechanically enlarging the channel, or by permitting the
river to erode its banks, thereby contributing to the bed load sediment supply as well.
Another strategy to reduce bed shear stress from channel confinement in incised channels is to
excavate a new, lower floodplain adjacent to the low-flow channel at a level that will frequently flood.
Such “compound” channels have other benefits, because riparian vegetation can be established on the
floodplain, increasing habitat value, and the low-flow channel can be permitted to erode its banks and
migrate within the compound channel (Haltiner et al., 1996). This approach had been applied to the
Meuse River in the Netherlands (Klassan et al., 1998)
3.4.2.5
Bed Load Supply
Artificial step-pools can be used on small and high slope streams, but would probably not be applicable
for large rivers with high unit stream power where even coarse particles are likely to be mobilized (Kern,
1997). In this case bed load supply is effective to control incision. As shown in Fig. 3.61, on the Rhine
River below the Barrage Iffezheim, an annual average of 170,000 t of sand and gravel are dumped on the
river bed from barges to compensate for trapping of bed load by upstream dams (Kuhl, 1992). Key factors
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