|
|
||
 Selection of Site-specific Stabilization Techniques
aquatic habitat. If obtaining and placing suitable fill material is a problem,
stone fill can be used in the same way, although the cost may be prohibitive
if a large quantity of stone is required to obtain geotechnical stability.
Channel relocation often creates valuable wetland and aquatic habitat in the
form of the abandoned channel. However, subsequent deposition usually
degrades the aquatic portion of that habitat significantly, even if determined
efforts are made to artificially preserve it. The rate and amount of
degradation depends upon many factors, as discussed by Gagliano (1984) and
Shields and Abt (1989). The relocated channel will be poor habitat initially
unless features are deliberately incorporated into the work, and its
construction may destroy valuable terrestrial habitat.
Selective clearing and snagging is sometimes used to achieve limited
improvement in hydraulic conveyance. In some cases, this concept can be
applied to bank stabilization as well, with cleared vegetation being used as
armor or indirect protection for the streambank, reinforced by living
vegetation, with a limited amount of earthwork as required. This approach
can be effective, but its application is limited by site conditions and by
available resources, since it is labor-intensive and may require conscientious
maintenance. Also, it is difficult to write a performance-type specification for
the work.
Selection considerations relating specifically to aquatic habitat are:
Protection methods which provide zones of slow currents are desirable. The
habits of the endemic species and the hydraulics of the stream will determine
how critical this is, and the season of the year when it is most critical. For
example:
Farabee (1986) reports limited sampling on the Upper Mississippi
River that found much greater numbers of fish on a revetment of
large, loosely placed stones than on a revetment of smaller, tightly
packed stones. Large stones were defined as having an average
diameter of 2 feet or more, and small stones as having an average
diameter of 1 to 2 feet.
Smooth armor materials and stone armor of small stones may create
near-bank velocities higher than on a natural bank, which may
adversely affect the upstream movement of salmon fry.
Structures which create a wider, shallower cross-section in bends, such as
bendway weirs and Iowa vanes, might improve aquatic habitat by increasing
the diversity of depth and current velocity available in the bends. The
structures would provide cover and a more diverse substrate.
104
1 4
|
||
|
||
