Fundamentals of Fluvial Geomorphology and Channel Processes
adjacent to the stream, and generates a significant amount of sediment that is ultimately deposited
downstream in navigation and flood control channels. The geotechnical consequences of system instability
are a direct function of the hydraulic consequences of bed lowering. As degradation proceeds through a
system, the channel bank heights and angles are increased, which reduces the bank stability with respect
to mass failures under gravity. If degradation continues, eventually the banks become unstable and fail.
Bank failures may then no longer be localized in the bendways, but rather may also be occurring along both
banks in straight reaches on a system-wide basis. When this occurs, conventional bank stabilization
measures are generally not suitable, and a more comprehensive treatment plan involving grade control or
flow control dams, diversion structures, etc., is required.
3.4.2.1 Causes of System Instability
The stability of a channel system can be affected by a number of natural or man-induced factors.
Natural geologic processes obviously cause dramatic changes but these changes generally occur over
thousands or perhaps millions of years and, therefore, are not often a direct concern to the individual trying
to stabilize a streambank. However, channel systems are significantly impacted within the engineering time
span by the natural forces of earthquakes or volcanic eruptions. Although these phenomenon may have
catastrophic consequences and receive considerable media attention, the most commonly encountered
system instability problems can generally be attributed, at least in part, to man's activities.
Any time one or more of the controlling variables (runoff, sediment loads, sediment size, channel
slope, etc.) in a watershed are altered there is a potential for inducing system instability. The particular
system response will reflect the magnitude of change and the existing morphological sensitivity of the
system. Therefore, each system is unique and there is no standard response that applies to all situations.
With this in mind, it is not practical to attempt to discuss all the possible scenarios of channel response.
Rather, the aim of this discussion is to present some of the more common factors causing system instability,
and to illustrate how a particular channel response might be anticipated using the stability concepts
discussed earlier.
A list and brief discussion of some of the more common causes of system instability are presented
in the following sections. For this discussion the causes have been grouped into three categories: (1)
downstream factors, (2) upstream factors, and (3) basin-wide factors. Following this, a brief discussion is
presented concerning complex response and the complications involved when a system is subjected to
multiple factors.
Downstream Factors. The stability of a channel system can be significantly affected by a
downstream base level lowering. Base level refers to the downstream controlling water surface or bed
elevation for a stream. One of the most common causes of base level lowering is the implementation of
cutoffs or channelization as part of channel improvement projects (Figure 3.21). As indicated by Lane's
relation (Figure 3.19), the increased slope must be offset by one of the other variables. Consequently, there
is an imbalance between the sediment transport capacity and supply. If the discharge and bed material are
assumed to remain constant (which may not always be the case), then the channel must adjust to the
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