Quantcast Complexities and Multiple Factors

 
  
 
Fundamentals of Fluvial Geomorphology and Channel Processes
2.2.2.2 Complexities and Multiple Factors
Lane's balance and other geomorphic analyses of initial morphological response to
system disturbance provide a simple qualitative method for predicting the channel response
to an altered condition. However, it does not take into account the magnitude of the change
and the existing morphologic condition of the stream. For instance according to Lane's
balance a channel cutoff should induce degradation. While this is often the case, there are
many examples where there may be no observable change in the channel morphology
following the construction of cutoffs. Brice (1981) documented the stability of streams at 103
sites in different regions of the United States where channels had been relocated. He found
that following 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 water-
shed 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.
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