General Principles of Erosion Protection
6.1.1.1 Prediction of Channel Migration
The key to success in choosing the upstream and downstream limits of the work lies
in the prediction of channel migration. The basic parameters of channel migration, or
meandering, are shown in Figure 6.1. Note that both sinuous and straight streams exhibit
characteristic patterns and spacings of bars, pools, and crossings. The one particular
characteristic of these patterns which is an invaluable aid in a correct determination of the
siting of stabilization work is that the movement of bars, pools, and crossings has components
both perpendicular to the axis of the meander belt and downvalley. As a rule, the greatest
movement is usually downvalley.
While this is a sound general rule, in nature the variability of the bed and bank
materials usually distorts the actual pattern from the ideal pattern of movement to some
degree, as shown on Figure 6.2. Therefore, it is important to obtain some verification of
recent migration trends for each specific location.
There are four potential sources of data which can be used in this verification. Listed
in approximate descending order of reliability, they are:
(a) Historical geomorphology based on documentary information on channel
evolution from hydrographic surveys, topographic maps, and/or scaled aerial
photographs. With the position of the stream channel documented at two or more
points in time, the length of bank which has been subject to erosion can be
identified.
(b) Interpretation of existing planform ("process geomorphology"). If data are
available only for the present point in time, the principles of downvalley migration
and increase in bend amplitude, together with experience derived from similar
situations on other streams, can be used to predict likely locations for continued
erosion if the bank is not stabilized.
(c) Historical narrative accounts of channel shifting based on interviews with local
residents, landowners, and interested individuals. While these observers may not
be scientists, and may not be completely unbiased in their opinions, they can
provide useful information on historical erosion and channel changes.
(d) Numerical or physical morphological modeling. Numerical modeling of
meandering is a developing science that shows promise, but unfortunately, reliable
prediction of future migration requires that the model be verified using past
migration trends as documented by one or more of the first three sources of
information listed above. Therefore, to some extent, the necessary information
must already be available before numerical modeling can be undertaken. The
same is true of physical modeling, with the additional disadvantages of requiring
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