Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
require less effort to establish vegetation than those along intermittent flowing streams in dry
climates. In desert climates, where fewer plants in the inventory can be chosen than in humid
climates, learning these plants' life requisites is essential for successful planting. The
probability for bioengineering project failure is higher with fewer species planted and where
growth stresses are greater.
The physical component includes physical parameters of a project: site stability such as
subsidence or accretion; aspect (direction slope faces), which in turn influences environmental
factors such as temperature (south and southwest facing sites are hotter and
evapotranspiration is higher than on other directions); hydrodynamic aspects such as water
flooding relationship to bank height; fluvial geomorphology, such as historical stream
meander, pattern, cross-sectional and longitudinal profiles, and energy sources such as wave
and current action; and geomorphic features, such as landforms and terrain influences, e.g.,
impacts of off-site water sources.
From the above list of physical parameters, hydrologic and geomorphic factors are
particularly important. For purposes of determining where to use vegetation on the bank and
the kinds of vegetation to use and when to plant, one needs a knowledge of the stream's
hydrographic and fluvial geomorphic characteristics. If stream guage data are not available,
one will have to rely on high water marks, the knowledge of persons living in the areas, and
any other data derived from local vegetation and soils that indicate flood periodicity. Table
1 gives an example of hydrologic characteristics of the upper Missouri River. It shows the
frequency of various flows and their duration with 25,000 cfs being the normal flow from late
spring through fall. A 40,000 cfs flow with a duration of 6 months can be expected to occur
once every 10 years. Figure 2 subsequently shows the approximate water level corresponding
to various river flows using the level of 25,000 cfs as the reference. At a flow of 40,000 cfs,
the river level will be approximately 3 feet above the reference level. From other data, we
also know that flows exceed normal usually in June or July; therefore, planting should occur
in early spring or fall.
This data also gives information that leads to forming vegetation planting zones. We
know that for this example, a daily high flow of 35,000 cfs translates to a zone 2-ft high on
the bank that could occur once for 60 days every two years. This means that this zone will
have to be vegetated with extremely flood-tolerant vegetation, e.g., emergent aquatic species,
willow (Salix spp.), and is equivalent to a "splash zone" that will be discussed later.