planning or maintenance of a highway crossing during the service life of the structure. The
kinds of changes considered here are: (1) lateral bank erosion; (2) degradation or aggradation
of the streambed that continues progressively over a period of years; and (3) natural short-term
fluctuations of streambed elevation that are usually associated with the passage of a flood
(scour and fill). Stability is inferred mainly from the nature of point bars, the presence or
absence of cut banks, and the variability of stream width (see also HEC-20, Lagasse et al.
2001).
5.8.1 Bank Stability
On a laterally unstable channel, or at actively migrating bends on an otherwise stable channel,
the point bars are usually wide and unvegetated and the bank opposite to a point bar is cut and
often scalloped by erosion. The crescentic scars of slumping may be visible from place to
place along the bankline. The presence of a cut bank opposite of a point bar is evidence of
instability, even if the point bar is vegetated. Sand or gravel on the bar appears as a light tone
on airphotos. The unvegetated condition of the point bar is attributed to a rate of growth that is
too rapid for vegetation to become established. However, the establishment of vegetation on a
point bar is dependent on other factors besides rate of growth, such as climate and the timing
of floods. If the width of an unvegetated point bar is considered as part of the channel width,
the channel tends to be wider at bends. Streams whose width at bends is about twice or more
the width at straight reaches are called wide-bend streams.
Oxbow lakes are formed by the cutoff of meander loops, which occurs either by gradual
closure of the neck (neck cutoffs) or by a chute that cuts across the neck (chute cutoffs). Neck
cutoffs are associated with relatively stable channels, and chute cutoffs with relatively unstable
channels. Recently formed oxbow lakes along a channel are evidence of recent lateral
migration. A recently formed lake is usually immediately adjacent to the channel and it
transmits flow at high river stages. Commonly, a new meander loop soon forms at the point of
cutoff and grows in the same direction as the previous meander. Cutoffs tend to induce rapid
bank erosion at adjacent meander loops. The presence of abundant oxbow lakes on a
floodplain does not necessarily indicate a rapid channel migration rate, because an oxbow lake
may persist for hundreds of years.
Along an unstable channel, bank erosion tends to be localized at bends, and straight reaches
tend to be relatively stable. However, meandering of the thalweg in a straight reach is likely to
be a precursor of instability. Bars that occur alternately from one side to the other of a straight
reach are somewhat analogous to point bars and are indicative of a meandering thalweg.
The following paragraphs summarize the characteristics of unstable and stable banks. For a
more detailed discussion of bank stability and the mechanics of bank failure see HEC-20
(Lagasse et al. 2001).
Unstable Banks With Moderate To High Erosion Rate. The slope angle of unstable banks
usually exceeds 30 percent, and a cover of woody vegetation is rarely present. At a bend, the
point bar opposite of an unstable cut bank is likely to be bare at normal stage, but it may be
covered with annual vegetation and low woody vegetation, especially willows. Where very rapid
erosion is occurring, the bankline may have irregular indentations. Fissures, which represent
the boundaries of actual or potential slump blocks along the bankline indicate the potential for
very rapid bank erosion.
5.53
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