Integrated River Management - River Dynamics and Integrated River Management - page 735

shapes may accommodate the normal discharge variation and their channel chapes remain unchanged.
During floods the discharge may be ten times higher than the average discharge. The complex channel
shapes provide enough water conveyance capacity and keep the velocity below a critical value to prevent
the banks from being eroded.

Average velocity of the Yangtze River from Yibin to Yichang at a discharge of 30,000 m 3 /s Fig. 11.17

Figure 11.18 shows the typical complex channel shapes of channels and floodplains. Engineers
consider the floodplain to be any part of the valley floor subject to occasional floods that threaten life and
property. As flood discharge exceeds the bankfull discharge the wet area of the cross sections sharply
increases with stage, thus, the flow velocity remains equal to or lower than the limit velocity. However,
in many cases human structures constrain the channel within a narrow levee-defined valley and reclaim
the floodplain for agriculture and residential development. Various channel improvements or impoundments
are used to restrict the natural process of overbank flow. The limit velocity law is broken and the velocity
during high floods is much higher than 2.5 m/s. In this case many organisms are stressed and the ecosystem
is impaired. Moreover, high flow velocity scours the bed and banks, thus, the risk of bank failure is high.
The limit velocity law should be taken into consideration when engineers design levees.

Fig. 11.18 Typical complex channel shapes of alluvial rivers

Figure 11.19 shows floodplain reclamation. Grand levees are constructed to constrain the flood water
within a narrower valley. In order to control the flow velocity below the limit velocity, roughness elements,