5.8.3 Assessment of Stability for Relocated Streams
Brice (1980) reported case histories for channel stability of relocated streams in different
regions of the United States. Based on his study, the recommendations and conclusions
presented here apply to specific aspects of the planning and construction of channel relocation.
They are intended for assessment of the risk of instability and for reduction of the degree of
instability connected with relocation. Serious instability resulting from relocation can be
observed either when the prior natural channel is unstable or when floods of high recurrence
interval occur during or soon after construction. Although there is an element of uncertainty in
channel stability, the experience represented by Brice's study sites provides useful guidelines
for improvement in the performance of channels relocated by highway agencies.
Consideration of the following aspects of the channel relocation is recommended.
Channel Stability Prior To Relocation. Assessment of the stability of a channel prior to
relocation is needed to assess erosion-control measures and risk of instability. An unstable
channel is likely to respond unfavorably to relocation. Bank stability is assessed by field study
and the stereoscopic examination of aerial photographs. The most useful indicators of bank
instability are cut or slumped banks, fallen trees along the bankline, and wide, unvegetated,
exposed point bars. Bank recession rates are measured by comparison of time-sequential
aerial photographs. Vertical instability is equally important but more difficult to determine. It is
indicated by changes in channel elevation at bridges and gaging stations. Serious degradation
is usually accompanied by generally cut or slumped banks along a channel.
Erosion Resistance Of Channel Boundary Materials. The stability of a channel, whether
natural or relocated, is partly determined by the erosion resistance of materials that form the
wetted perimeter of the channel. Resistant bedrock outcrops, which extend out into the
channel bottom, or that lie at shallow depths, will provide protection against degradation. Not
all bedrock is resistant. Erosion of shale, or of other sedimentary rock types interbedded with
shale, has been observed. Degradation was slight or undetected at most sites where bed
sediment was of cobble and boulder size. However, serious degradation may result from
relocation. Degradation may result from the relocation of any alluvial channel, whatever the
size of bed material, but the incidence of serious degradation of channels relocated by highway
agencies is slight.
The cohesion and erosion resistance of banks tend to increase with clay content. Banks of
weakly coherent sand or silt are clearly subject to rapid erosion, unless protected with
vegetation. No consistent relation was found between channel stability and the cohesion of
bank materials, probably because of the effects of vegetation.
Length Of Relocation. The length of relocation contributes significantly to channel instability at
sites where its value exceeded 250 channel widths. When the value is below 100 channel
widths, the effects of length of relocation are dominated by other factors. The probability of
local bank erosion at some point along a channel increases with the length of the channel. The
importance of vegetation, both in appearance and in erosion control, would seem to justify a
serious and possibly sustained effort to establish it as soon as possible on the graded banks.
Bank Revetment. Revetment makes a critical contribution to stability at many sites where it is
placed at bends and along roadway embankments. Rock riprap is by far the most commonly
used and effective revetment. Concrete slope paving is prone to failure. Articulated concrete
block can be effective especially when vegetation can establish in the interstices between
blocks. Bank revetment is discussed in detail in the next chapter.