Environmental Engineering Reference
In-Depth Information
Preproject
Postproject
FIGURE 8.30 Preproject and postproject conditions for the Cedar River Watershed aquatic restoration
LWD project plan: Rock Creek above the 40 road. (From Bohle, T., CRW Aquatic Restoration LWD Project
Plan : Rock Creek above the 40 Road , Seattle Public Utilities, Cedar River Watershed, 2005.)
some cases where jams of LWD create a blockage to ish passage, some removal may be necessary.
This should be done with caution and often only selective removal, or removal of small amounts of
the LWD, is necessary to remove the blockage.
8.4.18
M eander c reatIon
8.4.18.1 Causes and Considerations
A major cause of altered sinuosity is, of course, channel “realignment,” which is the intentional
straightening of rivers and streams for navigation or other purposes. However, any project changes
that tend to alter the channel width, mainly increased channel-forming discharges, tend also to alter
the meander dimensions in the course of time (USACE 1994).
Natural rivers are so rarely straight that straight natural rivers are essentially nonexistent and
straight reaches of rivers rarely exceed 10 times the channel width (Leopold and Wolman 1957).
Even in a straight stretch of river channel, the river thalweg (main channel) is seldom straight, hav-
ing more commonly a sinuous path between areas of aggradation and degradation. Therefore, river
restoration commonly involves putting the “curves” back into a river, such as for those previously
“realigned.” As discussed previously, one common measure of the “curvature” is the channel sinu-
osity, deined by the channel length divided by the valley length. Leopold et al. (1964) designated
single channels with sinuosities greater than 1.50 as meandering where an absolutely straight chan-
nel would have a sinuosity of 1.0, so that restoration involves increasing sinuosity and meanders.
Another consideration for restoration design is that natural rivers are often in a “dynamic equilib-
rium.” Channels commonly change due to, for example, time-varying responses to lows and sedi-
ment loads. So, while conditions may change (be dynamic), the river may still be geomorphically
stable where its properties (depth, slope, etc.) do not change on average over long periods (such as
10 years or more; Shields et al. 2003b). Such dynamic changes may result in, or in some cases be
required for, a healthy ecosystem. However, while restoring that dynamic nature may be desirable
from an ecological perspective, it may not be a socially acceptable outcome, particularly in urban
environments or where threats are posed to riparian resources or infrastructure (Shields et al. 2003b)
due to erosion and looding. Therefore, stability is often a key component in restoration design.
Putting curves back into rivers is complicated by the fact that the curves, or meanders, not only
result from the low but also the sediment transport characteristics. For example, Ackers (1982)
demonstrated that meandering was more common in channels with high sediment concentrations
than in those with low concentrations, implying that the restoration of meandering channels must
Search WWH ::




Custom Search