Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
Create Benches
The creation of slope breaks can
reduce slope length. These breaks
include benches, steps, or trenches
cut into the slope. The reduced
gradient at these breaks slows
the velocity of overland flow and
collects sediments (Figure 5-56).
If either practice is applied in a long
continuous line, they must be constructed
on contour to the slope. If
they are not level, water can collect
and move along the structure, much like a channel, and eventually spill onto the slope below,
creating rills and gullies. (The exception to this is the construction of live pole drains to redirect
water off-site for slope stability [see Section 10.3.3.4, Live Fascines]). When placed properly
along the contour, slope distances are shortened and the structures collect sediments and
create areas for plant growth. Plants that are grown above barriers or near the bottom edge
of benches can take advantage of water and sediments that collect during rainstorms. Native
vegetation can be incorporated into many of these designs to take advantage of increased
soil moisture and sediment accumulation. The distance that these structures are placed apart
from each other should be less than the critical distance where rills are expected.
5.7 SLOPE STABILITY
This discussion is directed to non-engineers to simplify and make accessible basic slope stability
concepts that must be understood in developing revegetation strategies. It is by no means
a substitute for professional engineering expertise. Technical references for slope stability
are many (including Carson and Kirby 1972; Spangler and Handy 1973; Brunsden and Prior
1984; Denning and others 1994), to which the reader is referred for a comprehensive review
of this subject. For a detailed evaluation of the role of vegetation in slope stability, the reader
is referred to Gray and Leiser (1982).
Creating stable slopes is essential for establishing healthy plant communities, but the reverse
is equally true – establishing native vegetation is critical for stabilizing slopes. The following
discussion takes the latter perspective of how to create the most favorable environment for
establishing vegetation on potentially unstable slopes and how this approach can be integrated
into the overall strategy of slope stabilization. A vegetated slope adds stability to slopes by
holding the soil together through a network of root systems and by removing water from the
soil, which is the primary driving force behind most landslides.
Figure 5-56 | Structures that
shorten slope length
Structures that shorten the slope length
can slow surface runoff, collect sediments
and increase soil moisture. Typical
treatments include: a) placement of fiber
rolls, logs, straw waddles, and compost
berms; b) benches, steps, and trenches;
c) willow waddles; and d) willow brush
layers. Strategic placement of plants
can take advantage of increased soil
moisture by planting where roots can
access the additional moisture. Most
species do not respond well to being
buried by sediment and should be
planted above or below depositional
areas (A and B). However, some species,
such as willow, root where the stems are
buried, and these species can be planted
where sediments are expected to be
deposited (C and D).
Slumps
• moderate slope gradients
• circular failure movement
• cohesive soils
• deep soils
• slow moving
Debris Slides
• steep slope gradients
• planar failure movement
• non-cohesive soils
• shallow
• fast moving
Figure 5-57 | Slumps and slides
Common landslides typically associated
with road construction. Modified after
Varnes (1978) and Bedrossian (1983).
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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