Fundamentals of Fluvial Geomorphology and Channel Processes
generally it is most significant during the first few years following closure of the dam. In
some situations, a channel may shift from a degradational to an aggradational phase in
response to slope flattening due to degradation, increased sediment inputs from tributaries and
bed and bank erosion, and reduction in the dominant discharge.
System instability can also be introduced by the diversion of water into or out of the
stream. Channel diversion structures are designed to divert a portion of the water and/or
sediment from a stream and deliver it to another location. Diversions are often needed for
water supply, irrigation, hydropower, flood control, or environmental reasons. The system
effects and complexities are similar to those downstream of major dams. According to Lane's
balance the sediment load in the receiving stream will be increased due to extra, transport
capacity of the increased discharge. In time, the erosion of bed sediments decreases as the
slope is reduced through bed degradation.
An increase in discharge due to a flow diversion can have a significant impact on the
channel plan form as well as the vertical stability. Schumm (1977) proposed a qualitative
relation similar to Lane's that included meander wavelength. His relation states that:
bdL
Q'
S
where Q is the discharge, b is the width, d is the depth, S is the slope, and L is the meander
wavelength. The above relation indicates that an increase in discharge may result in an
increase in the meander wavelength which would be accomplished through accelerated
erosion of the streambanks. Therefore, whenever diversions such as this are proposed the
potential for increased meander activity must be considered. If a stream is in the process of
increasing meander wavelength, then stabilization of the bends along the existing alignment
is likely to be unsuccessful and is not recommended.
Basin Wide Factors. Sometimes the changes in the controlling variables can not be
attributed to a specific upstream or downstream factor, but rather are occurring on a
basin-wide basis. This often results from a major land use change or urbanization. These
changes can significantly modify the incoming discharge and sediment loads to a channel
system. For example, urbanization can increase peak flows and reduce sediment delivery, both
of which would tend to cause channel degradation in the channel system. A land use change
from forest to row crop on the other hand might cause a significant increase in the sediment
loading resulting in aggradation of the channel system. Unfortunately, it is difficult, if not
impossible, to predict when basin wide changes such as these will occur. Therefore, the best
the designer can do in most cases is to simply try to design the bank protection measures to
accommodate the most likely future changes in the watershed. For instance, if there is a
possibility of future urbanization in the upper watershed, then additional launching stone may
be needed to protect the bank from the destabilizing impact of any future bed lowering.
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