Quantcast Causes of System Instability

 
  
 
Fundamentals of Fluvial Geomorphology and Channel Processes
(meander wavelength, sinuosity, etc.). Depending upon the magnitude of the change and the
basin characteristics (bed and bank materials, hydrology, geologic or man-made controls,
sediments sources, etc.) these adjustments can propagate throughout the entire watershed and
even into neighboring systems. For this reason, the disruption of the equilibrium condition will
be referred to as system instability.
As defined above system instability is a broad term describing the dis-equilibrium
condition in a watershed. System instability may be evidenced by channel aggradation,
degradation, or plan form changes. This manual does not attempt to provide a complete
discussion of all aspects of channel response, but rather, focuses primarily on the
degradational and plan form processes because these have the most significant impact on bank
stability. For a more complete discussion of channel processes, the reader is referred to
Simons and Sentrk (1992), Schumm (1972), Richards (1982), Knighton (1984), and Thorne
et al. (1997).
Before the specific causes are addressed a brief discussion of the consequences of
system instability is necessary. The consequences of system instability can generally be
discussed in terms of two components: (1) hydraulic consequences, and (2) geotechnical
consequences. The consequences of system instability are illustrated in Figure 2.15. The
hydraulic consequences of system instability are usually reflected in increased energy
(discharge and slope) which result in excessive scour and erosion of the bed and banks. This
erosion endangers bridges, buildings, roads, and other infrastructure, undermines pipeline and
utility crossings, results in the loss of lands 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.
2.2.2.1 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
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