Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
Water Input – How to Assess
See discussions under Section 5.2, Water Input, for how to assess water input.
Mitigating for High Water Input
Proper Surface Drainage
Increased water is the driving force behind slumps and debris slides. Designing proper water
drainage is probably the most important measure to implement for slope stability (Gedney
and Weber 1978). Where road water is inadvertently routed into potentially unstable areas,
there is a greater potential for slope failure (Fredricksen and Harr 1981). Road projects that
are designed to move storm water away from or out of unstable slopes as quickly as possible
through road and slope drainage structures increase slope stability. In some cases, installing
a curb at the top of the cut slope will effectively move water away from unstable slopes below.
Coordinate with engineers to insure that surface runoff, during construction of phased projects,
is addressed prior to completion of final stormwater system.
Species Selection
Select species that have adapted to wet soils. These include sedges (Carex spp.), rushes
(Juncus spp.), bulrushes (Scirpus spp.), willows (Salix spp.), cottonwoods (Populus spp.), and
cedars (Thuja spp. and Chamaecyparis spp.). Since each species has a unique way, or strategy,
of modifying the moisture regime of a site, planting a mixture of species is a way of ensuring
that all strategies are represented on the site. For instance, willows establish quickly and can
draw large quantities of moisture from the soil, but only when the willows have leaves. Cedars,
on the other hand, are slower to establish, but longer lived. They can withdraw moisture from
the soil during the winter, unlike deciduous species. When they are well established, they
intercept large amounts of water in the crowns, preventing precipitation from reaching the
soil. Trees have a great ability to significantly deplete moisture at considerable depths (Gray
and Leiser 1982). Wetland species, such as rushes and sedges, unlike many tree and shrub
species, grow well in saturated soils.
Live Pole Drains
The live pole drain (Polster 1997) is a biotechnical engineering technique where continuous
willow bundles (Figure 5-61) are placed across a slope, much like an open drain, to redirect water
to a more stable area, such as channels and draws. Where small slump failures have occurred
during or after construction, live pole drains can be installed to increase slope drainage, add
root strength, and remove soil moisture from the slide mass.
5.7.4 SLOPE LENGTH
Slope length is important for stability because the longer the slope, the more water concentrates
in the lower portion of the slope. Increased water increases pore water pressure and soil
weights, thereby decreasing the stability of the mid to lower sections of long slopes. This is one
reason why slumps are often observed in the mid to lower slope positions of longer fill slopes.
Slope Length – How to Assess
Slope length can be obtained from road plans or measured directly in the field using a tape.
Mitigation for Long Slopes
Live Pole Drains
Using live pole drains (discussed in the previous section), shortens the distance water is
transmitted through the hill slope by intercepting surface and subsurface water in ditches
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