Each of these protection systems has its own unique mode of failure and threshold capability
for erosion resistance. When using these systems, careful attention should be placed on the
design of termination details at the crest, sides, and toe of the embankment. Powledge, et
al. (1989a, 1989b) note that many of these systems were originally designed for uses other
than the protection of embankments during overtopping flow, and have been adapted to this
application as a result of a recognized need. For application of the Factor of Safety method
(Section 6.5) to the design of riprap for the complex geometries often associated with
overtopping flows, reference to Julien (1995) is suggested.
6.9 ENVIRONMENTAL CONSIDERATIONS
Streambank protection projects should be planned, designed and constructed with
consideration being given to environmental quality factors and project objectives.
Environmental quality should address preservation or restoration of environmental resources
within the project boundaries and avoidance of adverse impacts associated with the project.
Some environmental factors are project specific and are necessarily defined during the
planning phase while others are mandated by existing regulations. Streambank protection
projects should strive to preserve or restore existing environmental quality to the extent
possible.
6.9.1 Environmental Impacts
Impacts of streambank protection projects are dependent on project location and regional
characteristics. For example, in arid regions of the western United States, forested habitat may
be restricted to riparian areas and be directly and extensively impacted by project construction;
whereas in the southeastern United States, forested habitat may be more extensive, but
changes in stream hydraulics caused by the project can result in adverse impacts throughout
the floodplain ecosystem. While general categories of impacts may be stated, site-specific and
regional considerations and individual project features are critical in determining the magnitude
and type of environmental impact. In some cases streambank protection is performed in
conjunction with other projects having different purposes, and it is difficult to isolate impacts
due to streambank protection alone. Categories of environmental impacts associated with
streambank protection projects include aesthetic, physical, water quality, and biological.
Asthetic impacts most often occur because the natural appearance of the project area is
disturbed or changed and replaced by an artificial appearance due to structures or channel
alignment. The physical impacts of streambank protection can affect channel morphology,
sediment-carrying capacity of the stream, and stream hydraulics. These physical effects tend
to manifest themselves as changes in landscape diversity and associated aquatic habitat
diversity or quality; for example, loss of side channels or shallow areas or replacement of
natural bank with revetment. Losses or changes in habitat will affect wildlife and aquatic life
either by a reduction or change in community structure; however, changes in habitat
composition for a specific project can be either detrimental or beneficial depending on
circumstances.
Water quality impacts from changes in turbidity together with alteration of riparian habitat (e.g.,
shading) affect stream temperature and photosynthetic activities that in turn may affect algae
or aquatic plant populations, dissolved oxygen, and other water quality parameters.
Temporary changes in water quality may occur as a result of construction activities.
6.53
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