Fundamentals of Fluvial Geomorphology and Channel Processes
the cutoffs 52% of the channels showed no change, 32% showed improvement, and 16% exhibited channel
degradation. This study indicates that predicting the channel response to cutoffs is not nearly as simple as
might be inferred from Lane's balance. Therefore, the designer should always be aware of the considerable
uncertainties that exist when attempting to predict, even in qualitative terms, the behavior of river systems.
Previous discussions have focused primarily on the initial response of a channel to various alterations
in the watershed. However, it must be remembered that the entire watershed is connected and that changes
in one location can, and often do, affect the channel stability at other locations, which in turn provides a
feedback mechanism whereby the original channel response may be altered. For example, the initial
response to a base level lowering due to channelization may be channel degradation. However, as this
degradation migrates upstream the sediment supply to the downstream reach may be significantly increased
due to the upstream bed and bank erosion. This increased sediment load coupled with the slope flattening
due to the past degradation may convert the channel from a degradational to an aggradational phase.
Multiple response to a single alteration has been referred to as complex response by Schumm (1977).
Another complicating factor in assessing the cause and effect of system instability is that very rarely
is the instability a result of a single factor. In a watershed where numerous alterations (dams, levees,
channelization, land use changes, etc.) have occurred, the channel morphology will reflect the integration
of all these factors. Unfortunately, it is extremely difficult and often impossible to sort out the precise
contributions of each of these components to the system instability. The interaction of these individual
factors coupled with the potential for complex response makes assessing the channel stability and
recommending channel improvement features, such as bank protection, extremely difficult. There are
numerous qualitative and quantitative procedures that are available. Regardless of the procedure used, the
designer should always recognize the limitations of the procedure, and the inherent uncertainties with
respect to predicting the behavior of complex river systems.
3.4.3 LOCAL INSTABILITY
For this discussion local instability refers to bank erosion that is not symptomatic of a
dis-equilibrium condition in the watershed (i.e., system instability) but results from site-specific factors and
processes. Perhaps the most common form of local instability is bank erosion along the concave bank in
a meander bend which is occurring as part of the natural meander process. Local instability does not imply
that bank erosion in a channel system is occurring at only one location or that the consequences of this
erosion are minimal. As discussed earlier, erosion can occur along the banks of a river in dynamic
equilibrium. In these instances the local erosion problems are amenable to local protection works such as
bank stabilization measures. However, local instability can also exist in channels where severe system
instability exists. In these situations the local erosion problems will probably be accelerated due to the
system instability, and a more comprehensive treatment plan will be necessary.