Based on the classification presented in Figures 5.11, 5.12, and 5.13, the first stream (a) is a
straight channel with alternate bars and sinuous thalweg. This stream has a relatively low
slope and low width-depth ratio. Figure 5.13 also indicates that it has an intermediate sediment
size, sediment load and moderate ratio of bedload to total load (between 3 - 10 percent). This
stream can be classified as relatively stable.
The second stream (b) is a sinuous point bar stream which is somewhat wider at bends. The
meander bends are expected to shift gradually with possible neck cutoff. The stream is then
relatively stable.
The third stream (c) is a braided sand-bed stream with multiple bars and channels. Bars are
likely to be comprised of coarse sand and the large flow velocities and stream power will
generate large sediment load with a large proportion transported as contact load. The overall
stability of this braided channel is very low, channel shifting, and avulsions are certainly
common.
5.9.3 PROBLEM 3 Channel Response to Changes in Watershed Conditions
Determine the effect of watershed deforestation, bank erosion and headcutting on channel
stability and gradation changes in an alluvial stream.
Referring to Table 5.8, deforestation generally causes aggradation problems and therefore
channel instability. The reason for this is that deforestation increases runoff and peak runoff
discharge as well as sediment transport from upland areas.
Headcutting is a degradation process. Upstream migration of headcuts induces bank failure
and channel stability problems. Bank erosion has basically the same consequence on channel
stability as that of headcutting. The gradation changes, however, are more difficult to assess
because bank erosion changes the width-depth ratio.
5.9.4 PROBLEM 4 Channel Migration Rate
(a) Determine the bank erosion rate and the erosion index of the sinuous point bar stream
sketched in Figure 5.33.
The best method to estimate the rate of bank erosion is to compare two sets of aerial
photographs. However, a first assessment can be obtained from Figure 5.26. Entering the
figure with an average width around 150 m (492 ft), the median erosion rate should be around
2 meters (6.5 ft) per year. Wide bend streams have slightly larger erosion rates than given by
the dashed line. Entering Figure 5.28 with a sinuosity of 1.2, the erosion index might be as
large as 18 indicating channel instability. Erosion is to be expected on the concave side of the
bends. Other means for assessing lateral migration rates such as study of past aerial
photographs, scroll formation (Figure 5.12(b)) and field studies should be undertaken if any
bridge crossing was to be built across this river.
(b) Sketch the likely future changes in the meandering river shown in the sketch below (Figure
5.34).
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