River Ecology and Restoration - River Dynamics and Integrated River Management - page 697

Engineered Log Jams —Woody debris within a stream can often influence the instream channel
structure by increasing the occurrence of pools and riffles. As a result, streams with woody debris typically
have less erosion, slower routing of organic detritus (the main food source for aquatic invertebrates), and
greater habitat diversity than straight, even-gradient streams with no debris. Woody debris also provides
habitat cover for aquatic species and characteristics ideally suited for fish spawning.
Reintroduction of woody debris, or log-jams, has been extensive, but limited understanding of woody
debris stability has hampered many of these efforts. Engineered log jams can restore riverine habitat and
in some situations can provide effective bank protection. Even in large alluvial channels that migrate at
rates of 10 m/yr, jams can persist for centuries, creating a mosaic of stable sites that in turn host the large
trees necessary to initiate stable jams (Abbe et al., 1997). Engineered log jams are designed to emulate
natural jams and can meet management or restoration objectives such as habitat restoration and bank
protection.
After learning about the uncertainty and potential risks of creating man-made log jams, landowners
near Packwood, Washington, U.S. decided the potential environmental, economic, and aesthetic benefits
outweighed the risks. An experimental project consisting of three engineered log jams was implemented
to control severe erosion along 420 m of the upper Cowlitz River. Five weeks after constructing the log
jams, the project experienced a 20-year return period flood (850 m 3 /s). The engineered log jam remained
intact and met design objectives by transforming an eroding shoreline into a local depositional environment
(accreting shoreline). Approximately 93 tons of woody debris that was in transport during the flood was
trapped by the jams, alleviating downstream hazards and enhancing structure stability (Abbe et al., 1997).
Landowners have been delighted by the experiment.
10.4.3 streamRestoration
In some cases, it might be desirable to divert a straightened stream into a meandering alignment for
restoration purposes. For cases where the designed channel will carry only a small amount of bed
material load, bed slope and channel dimensions may be selected to carry the design discharge at a
velocity that will be great enough to prevent suspended sediment deposition and small enough to prevent
erosion of the bed. Increasing the sinuosity of a stream may create better habitats for faunal communities.
Meanders can then be created with length ranges from 4 to 9 times the channel widths. Meanders should
not be uniform. For instance, the incised, straightened channel of the River Backwater in the U.K. was
restored to a meandering form by excavating a new low-level floodplain about 50 to 65 feet wide
containing a sinuous channel about 16 feet wide and 3 feet deep (Hey, 1995). Preliminary calculations
indicated that the bed of the channel was only slightly mobile at bank-full discharge, and sediment loads
were low.
For small rivers restoration practices also involve change of the channel dimensions, in which the
average values for width and depth of the channel were determined in the design. These determinations
are based on the imposed water and sediment discharge, bed sediment size, bank vegetation, resistance,
and average bed slope. However, both width and depth may be constrained by site factors, which the
designer must consider once stability criteria are met. Perhaps the simplest approach to selecting channel
width and depth is to use dimensions from stable reaches elsewhere in the watershed or from similar
reaches in the region. A reference reach is a reach with a desired biological condition, which will be used
as a target to strive for when comparing various restoration options. A reference reach used for stable
channel design should be evaluated to make sure that it is stable and has a desirable ecological condition.
In addition, it must be similar to the desired project reach in hydrology, sediment load, and bed and bank
material. Often a stable reach upstream of or downstream from the reach to be restored is selected as the
reference reach.