Causes of System Instability - 7

7_StreambankManual
Page Navigation
29

(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