Environmental Engineering Reference
In-Depth Information
Simon and Hupp (1986). If a channel is disturbed by some drastic change such as forest clearing,
urbanization, dam construction, or channel dredging, then instability sets in with scouring of the
bed, and destructive bank erosion and channel widening occur due to the collapse of sections of the
bank. The banks will continue to cave into the stream, widening the channel, and the stream also
begins to aggrade, or ill in, with sediment from eroding channel sections upstream. In the inal
stage, aggradation continues to ill the channel, reequilibrium occurs, bank erosion ceases, and
riparian vegetation once again becomes established. See Chapter 3 for a more detailed discussion.
An important impact of destabilization is the consequent increase in destructive bank erosion, as
illustrated in studies of the Goodwin Creek Watershed, Mississippi, by the National Sedimentation
Laboratory (Simon 2009; Figure 5.14). For such a system, the contribution of sediments from bank
erosion can often far exceed the contribution of sediments from channel sources, as illustrated in
Table 5.1.
It is not only the magnitude of the sediments that is an issue, but also their size distribution.
Some of the commonly used sediment size categories are listed in Table 5.2. The adverse effect of
ine sediment deposits on benthic organisms is well documented in the literature. Suspended ine
sediments reduce light transmission, which can limit the growth of aquatic plants, while deposited
ine sediments can alter benthic habitats and entomb benthic organisms. In addition, contaminants
are most closely associated with cohesive sediments, such as silts and clays.
The size distribution of sediments also impacts sediment transport. For example, the settling
of particles in water is impacted by their size and density, so that in a high-energy stream chan-
nel, the bottom may be armored (coarser particles overlying iner) or consist primarily of coarse
particles. Finer particles are more typically found in areas of lower energy, such as the inside
of bends. As the characteristics of rivers change from their headwaters, changes also occur in
sediment grain size, with bed material grain size decreasing with increasing distance from the
headwaters (FigureĀ 5.15).
FIGURE 5.14
Bank erosion in the Goodwin Creek watershed. (From Simon, A., Adjustment processes in lu-
vial systems implications for streambank instability and control, National Sedimentation Laboratory, Oxford,
MS, from a workshop presented at the EWRI Water Resource Congress 2009, Kansas City, MO, 2009.)
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