Integrated River Management - River Dynamics and Integrated River Management - page 728

construction of new channels. Other channelization procedures classified as river channel maintenance
include dredging, cutting, or the removal of obstructions.
The application of conventional channelization methods can adversely affect the morphological and
biological characteristics of river channels. In many rivers, the river banks are hardened and smoothened
with concrete and stones, to enhance the flow velocity for quickly draining flood water. This practice is
against Principle I and does not really provide better flood safety. The smooth banks have much smaller
roughness than the natural vegetated banks, so high velocity current may directly assault the banks and
break the levees. Figure 11.11(a) shows the concrete bank of the Lijiang River at Guilin, China. The
hardened banks are too smooth so that the high flow velocity during flooding directly assaults the banks
and has caused bank erosion and has broken the concrete bank. Figure 11.11(b) shows the hardened bank
of the Blue Nile River in Sudan. The bank is roughened by sticking stones on the surface, thus the bank
roughness is increased, which creates high resistance to the flow and moves high velocity current far
from the banks. Figure 11.11(c) shows that channelization of the Zheduo River at Kangding resulted in
high flow velocity and elimination of fish and aquatic plants. Only a few species of invertebrates live in
the bed consisting of cobles and boulders.
Floodplain vegetation is a type of resistance against flood flow. Such vegetation behaves as an obvious
obstacle to the flood propagation. A number of studies on flood flow and sediment transport in rivers
with riparian vegetation have been conducted to understand the mechanical effects of riparian vegetation
on river hydraulics and geomorphology (Ikeda and Izumi, 1990; Thorne, 1990; Ikeda et al., 1991;
Thornton et al., 2000; Carollo et al., 2002). Riparian vegetation promotes geomorphic stability via
increased flow resistance and reduced near-bank velocity (Andrews, 1984; Hey and Thorne, 1986).
Riparian vegetation also increases the strength of bank and flood plain materials via buttressing, arching,
and root reinforcement (Gray and Leiser, 1982). Communities of riparian vegetation can promote
entrapment and deposition of fine sediment around the river bank and on flood plains (Abt et al., 1994;
Lee et al., 1999; Elliott, 2000). In some places the floodplain vegetation is cleared for increasing flood
velocity and reducing flood stage. This practice is against Principle I and should be abandoned.
Principle ē —Controlling Erosion and Reducing Sediment Transportation
Sediment is the core for the fluvial processes. Sediment movement starts from soil erosion. Therefore,
erosion control is essential for stabilization of the river network. The sediment load of rivers comes from
mountainous areas due to slope erosion, rill erosion, gully erosions and channel bed and bank erosions.
Erosion is also the essential cause of the geological disasters of landslides and debris flows (Chapter 4).
After the sediment is eroded from the upstream watershed and transported by the river flow, it will also
be deposited somewhere in the river basin or at the river mouth. Erosion changes the upstream landscape
and impairs or even destroys the vegetation, and sediment deposition changes the river morphology and
buries the substrate of the aquatic habitat.
Construction of sediment trapping dams on the Loess Plateau of China is in accordance with Principle
II. More than ten thousand sediment trapping dams have been constructed, which remarkably reduce the
sediment load into the Yellow River. Most of the Yllow River's sediment load comes from the Loess
Plateau. The reduction in sediment load also is mainly attributed to the reduction in sediment supply
from the Loess Plateau. Figure 11.12 shows the variation of annual runoff and annual sediment load
measured at the Tangnaihai station in the upper reaches on the Qinghai-Tibet Plateau, the Tongguan
station in the middle reaches on the Loess Plateau, and the Lijin station on the Yellow River delta (Liu et
al., 2008). No dams and very few human structures are in the watershed upstream of the Tangnaihai
station. The runoff and sediment load at the Tangnaihai station have no reduction trend except for
fluctuations due to variation in precipitation. The annual sediment load fluctuates with the variation of