Unstable Banks With Slow To Moderate Erosion Rate. If a bank is partly graded (smooth
slope) the degree of instability is difficult to assess and reliance is placed mainly on vegetation.
The grading of a bank typically begins with the accumulation of slumped material at the base
such that a slope is formed, and progresses by smoothing of the slope and the establishment
Stable Banks With Very Slow Erosion Rate. Stable banks tend to be graded to a smooth slope
and the slope angle is usually less than about 30 percent. In most regions of the United
States, the upper parts of stable banks are vegetated, but the lower part may be bare at
normal stage, depending on bank height and flow regime of the stream. Where banks are low,
dense vegetation may extend to the water's edge at normal stage. Mature trees on a graded
bank slope are particularly convincing evidence for bank stability. Where banks are high,
occasional slumps may occur on even the most stable graded banks. Shallow mountain
streams that transport coarse bed sediment tend to have stable banks.
Field information on lateral migration rates for channels of different sizes has been compiled by
Brice (1982). Bank erosion rates tend to increase with increasing stream size. In Figure 5.26
channel width is taken as a measure of stream size. The dashed line is drawn arbitrarily to
have a slope of 1 and a position (intercept) to separate most equiwidth streams from most
wide-bend and braided point-bar streams. For a given channel width, equiwidth streams tend
to have the lowest erosion rates, and braided point-bar streams the highest. Braided streams
without point bars (diamond symbol, Figure 5.26) plot well below the arbitrary curve because
their channels are very wide relative to their discharges. Channel width is an imperfect
measure of stream size, as are drainage area and discharge, particularly for the comparison of
streams in arid and semiarid regions with streams in humid regions. If braided streams and
braided point-bar streams (which are uncommon in most parts of the United States) are
excluded, the dashed curve in Figure 5.26 provides a preliminary estimate of erosion rates that
may be encountered at a particular site. An example on the use of these results is presented
in Section 5.9, Problem 4.
5.8.2 Stability Problems Associated With Channel Relocation
For some highway encroachments, a change in the river channel alignment is advantageous.
When a river crossing site is so constrained by non-hydraulic factors that consideration of
alternative sites is not possible, the engineer must attempt to improve the local situation to
meet specific needs. Also, the engineer may be forced to make channel improvements in
order to maintain and protect existing highway structures in or adjacent to the river.
Suppose a meandering river is to be crossed with a highway, as shown in Figure 5.27a.
Assume that the alignment is fixed by constraints in the acquisition of the right-of-way. To
create better flow alignment with the bridge, consideration is given to channel improvement as
shown in Figure 5.27b. Similarly, consideration for improvement to the channel would also be
advisable for a hypothetical lateral encroachment of a highway as depicted in Figure 5.27c. In
either case, the designer's questions are how to realign the channel, and what criteria to use to
establish the cross-sectional dimensions.