Environmental Engineering Reference
In-Depth Information
such as trees and shrubs on the floodplain are useful to increase the resistance and control the flow
velocity. With the roughness elements the overbank flood flow has higher stage but lower velocity, thus,
the channel and floodplain may remain stable. One consequence of the high roughness is high flood stages,
which may be solved by raising the grand levees. In general, high velocity rather than high stage poses a
threat to levees.
Fig. 11/19
Reclamation of floodplain with grand levees and narrowed flood channels with roughness elements
11.2
Sediment and River Morphology Management
A river system is a web of channels, of which all parts are mutually inter-affected. When considering the
human role in relation to changing river channels, at least five challenges persist. First, because
prediction of the nature and amount of likely change at a particular location is not certain, and because
the contrasting responses of humid and arid systems needs to be considered, modeling is required to
reduce uncertainty (Burkham, 1981). Second, feedback effects incorporated within the relation between
changes at channel, reach, and network scales have considerable implications, especially because
changes may have occurred, or have been initiated, under different environmental conditions. Third,
consideration of global climate change is imperative when considering channel sensitivity and responses
to threshold conditions. Fourth, channel design involving geomorphology should be an integral part of
restoration procedures. Fifth, better understanding of the ways in which the perception of the human role
in changing river channels varies with culture as well as varying over time should improve application of
design for river channel landscapes.
11.2.1 River Networks
Horton's law (Horton, 1945) is regarded as the central principle of stream network research. Numerous
investigations have shown that the linear rule of Horton's law is approximately valid in many natural
drainage networks (Ciccacci et al., 1992; Kinner and Moody, 2005). Further, some artificial stream
networks based on the random walk model also nearly obey Hortion's law in a similar manner (Shreve,
1966). Thus, many scholars believe that Horton's law is a universal description of stream networks.
However, arguments about the Horton's law still persist.
Horton's ratios are defined by Eqs. (1.1)-(1.4) as follows:
N
R
Z
e
B
(11.3)
B
N
Z
1
L
Z
1
D
R
e
(11.4)
L
L
Z
A
H
R
Z
1
e
(11.5)
A
A
Z
where
R
B
is the bifurcation ratio;
R
L
is the length ratio; and
R
A
is the area ratio. If the ratios are constant,
Horton's law may rewritten as:
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