IMPACT ON EROSION UPSTREAM AND DOWNSTREAM

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The final step in evaluating the effectiveness of alternative methods of bank

stabilization is to consider potential impacts on channel erosion upstream and downstream of

the project.

For comprehensive bank stabilization projects, assessing the long-term impacts of the

project on the channel system is a difficult and imprecise task. Projects which also involve

bed stabilization, basin management, or regulation of reservoir releases add further

complexity, because they are indeed intended to have significant impacts on channel behavior

upstream and/or downstream, as discussed in Chapter 4. Defining these impacts is a

necessary part of project justification.

Here our focus is much narrower, addressing the topic only as it applies to the

potential impacts of alternative site-specific bank protection methods on erosion in adjacent

reaches. Fortunately, predicting these impacts is usually less harrowing than predicting the

impact of a stabilization project on channel capacity, not because the impacts on erosion in

adjacent reaches can be predicted precisely, but because the range of possible responses is

more limited, and the impacts are more likely to be local. Therefore, the sensitivity of the

prediction to erroneous assumptions is less critical.

A cautious statement can be made that stabilizing a riverbank is not likely to have

significant detrimental impacts upstream. It is more likely that the stabilization project itself

will be threatened by future channel migration upstream of the project.

The possibility that preventing erosion in one reach will affect erosion downstream

is of more concern. Although total erosion downstream is not likely to increase, that may not

comfort a landowner who sees a stabilized channel upstream perpetuating the impingement

of erosive flows on his or her property, whereas before stabilization, the bar upstream may

have been migrating downstream, holding the promise not only of cessation of erosion on the

landowners property, but perhaps even having the potential for eventually creating additional

useable property by deposition of sediments. That landowner's concern will be even more

acute if he or she has no interest in events even farther downstream, where the long-term

potential for erosion may be reduced by the project, through a reduction in the rate of

meander development.

This dilemma cannot be resolved through the selection of a particular stabilization

method, since the very act of arresting channel migration changes future events to some

extent, regardless of the method chosen.

In summary:

Potential problems due to project-induced changes should be acknowledged,

and geomorphic concepts should be used to predict the impacts of bank

stabilization on channel erosion upstream and downstream. The less a