Fundamentals of Fluvial Geomorphology and Channel Processes
dynamic behavior of rivers. Oftentimes the engineer or scientist draws the erroneous conclusion that a
dis-equilibrium condition exist because natural cutoffs are occurring. However, this type of dynamic
behavior is quite common in rivers that are in a state of dynamic equilibrium. In this situation, as natural
cutoffs occur, the river may be obtaining additional length elsewhere through meandering, with the net result
being that the overall reach length, and therefore slope, remains unchanged.
In summary, a stable river, from a geomorphic perspective, is one that has adjusted its width, depth,
and slope such that there is no significant aggradation or degradation of the stream bed or significant
planform changes (meandering to braided, etc.) within the engineering time frame (generally less than about
50 years). By this definition, a stable river is not in a static condition, but rather is in a state of dynamic
equilibrium where it is free to adjust laterally through bank erosion and bar building. This geomorphic
definition of stability (dynamic equilibrium) is developed here to establish a reference point for the
discussion of system and local instability in the following sections.
3.4.2
SYSTEM INSTABILITY
The equilibrium of a river system can be disrupted by various factors. Once this occurs the channel
will attempt to re-gain equilibrium by making adjustments in the dependent variables. These adjustments
are generally reflected in channel aggradation (increasing bed elevation), degradation (decreasing bed
elevation), or changes in planform characteristics (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 planform 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 planform 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 3.20. 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
52
Previous Page
