Quantcast Questions to be Developed and Answered (Cont.)

 
  
 
Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
Geomorphic characteristics such as bank geometry play a major part in the employment
of bioengineering. Banks that have been eroded and undercut to a very steep, unplantable
slope require grading prior to planting (Edminster et al. 1949 and Edminster 1949). The
angle required varies with the soil, equipment used, and several other factors. Sand, for
instance, has an angle of repose of about 30 degrees whereas clay can stand on a much
steeper angle (Gray 1977). Most slopes that accommodate revegetation are less than 1-1.2
V:1 H. On steep banks where undercutting may be a problem, the toe of the bank may need
protecting with riprap or other hard, structural treatments. Special structural treatments other
than vegetation and drainage structures may be necessary where geomorphic features
contribute to internal erosion of the bank, called piping or sapping. This is where water can
seep into the bank from higher elevations through porous strata and cause bank failure when
the erodible strata are gone. Sometimes, bioengineering with appropriate geotextile filters
can treat piping problems, but not always.
The edaphic component includes all the soil parameters: texture, structure, fertility,
erodability, chemistry, etc. Soil texture, structure, and depth all affect the water holding
capacity of a soil and need to be considered when determining water retention requirements
or supplemental irrigation requirements during dry periods of the year. In addition to
ensuring proper bank slopes and bank toe protection, attention should be given to the edaphic
component that may in turn require some site preparation activities. It is desirable to have
slopes covered with at least a 10-cm layer of topsoil high in organic matter; this can be
stockpiled prior to any grading. Movement of soil, however, is expensive and must be
considered in light of the economic practicality. In lieu of moving rich topsoil, the existing
substrate may be amended with fertilizer and mulch to help produce a better soil. In any case,
plants need a growing medium that supports the plant and facilitates nutrient and water
uptake. The site may require other soil amendments such as lime, gypsum, or other special
nutrients depending upon the soil's pH and fertility. Soil tests should be conducted prior to
revegetation to determine any amendments needed.
The biological component is one of the most important components and is interdependent
with the other components. It includes habitat requirements of animal and plant species and
the plan can be modified to some extent to meet these requirements if the life requisites of
these species are known. This component also includes the availability of suitable plant
species that, in part, make up the habitat for various riparian animals. Choices must be made
between native and introduced species, plants obtained from commercial nurseries, or from
the wild. This component also includes the propagation and cultural practice for the plants,
planting, and aftercare. It includes plant diseases, insects, predators, and the presence or
absence of grazing animals. An example of spider mite damage is presented in the case study
of Court Creek, Illinois, Volume II, where willow had to be sprayed with an insecticide to
control damage. If spraying had not occurred, streambank protection with living willow
would not have been achieved. Protective screen sleeves or deer and grazing animal
exclosures must be provided if these risks are present. The potential for damage from insect,
rodent, deer, and other predation must be considered and protection provided to planted
wetland vegetation.
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