Geology Reference
In-Depth Information
practitioners to dominate for the first few metres
of flow, which is normally below the resolution
of most LEMs. Most LEMs discretize the catch-
ment elevation onto a grid or triangulation with a
resolution of tens of metres, so that rainsplash is
only active at a sub-grid resolution. Applications
with metre-scale resolution are rare because of
computer limitations.
In addition to fluvial erosion, most LEMs
include a variety of other processes including soil
creep, landsliding and debris flows. These proc-
esses are not normally included in traditional
models (but see Chapter 14), and represent one of
the most significant process differences between
LEMs and traditional models. One problem in
discussing LEMs in general is that not all LEMs
include all of these non-fluvial processes. Which
of these processes are modelled is typically a
reflection of the research heritage of the model.
Furthermore, which of these processes is impor-
tant for any particular application is also some-
times difficult to judge, even for researchers.
However, we do know that to generate naturally
occurring rolling soil-mantled hillslopes with
convex hilltops, it is necessary to include at least
soil creep and rainsplash (called 'diffusive' proc-
esses because they tend to smooth the landform)
in addition to fluvial erosion (which generates the
concave-up parts of the landscape). To state it
more generally, we know that to generate natural
landscapes with regions of convex and concave
hillslopes we need at least two processes: fluvial
and some other 'diffusive' process. Which diffu-
sive process applies is site-specific, but for soil-
mantled landscapes some form of creep or soil
mass movement process normally needs to be
modelled.
All LEM fluvial erosion models have a sedi-
ment transport process that increases in flux/unit
width with increasing water discharge/unit
width. This means that if flow converges as it
moves downstream as, for instance, the result of
the development of a rill, then the erosion within
the rill will increase relative to the adjacent uni-
form sheetflow. Outside of the rill, erosion will
decrease relative to uniform sheetflow because
discharge/unit width is decreasing. This means
that, in the absence of other processes (e.g. rains-
plash), rills and gullies develop as result of the
positive reinforcement of the physics of fluvial
erosion. This is the competition between rill and
inter-rill erosion in traditional models. In princi-
ple, LEMs can directly model this process where
the processes interact to change an initially
smooth landform surface into a rilled surface,
while traditional models, where the landform
does not change in response to the erosion, can-
not. Moreover, if the dependence of sediment
transport is superlinear with discharge/unit width
(i.e. transport
discharge m where m >1), then the
sediment transport from a given width of rilled/
gullied catchment (i.e. one where there are series
of rills separated by inter-rills along the contour)
will be higher than for the same width of uniform
depth sheetflow, so that the erosion rate is higher.
Many traditional models use empirical rilling fac-
tors that allow for this increase in erosion as a
result of rilling which, in principle, are unneces-
sary for LEMs because the rilling process can be
modelled directly.
Gully erosion has a further important conse-
quence. Many man-made containment structures
for potentially hazardous and nuclear waste cover
the waste material with a layer of benign low
erodibility material - the capping layer. Since
failure of the structure will occur if any of the
waste is released, the capping layer must resist
erosion. As we will show in the examples below,
when overland flow convergence can occur as a
result of landform evolution, the landscape
becomes covered in a network of high erosion
regions (the gullies) separated by the low erosion
regions (the ridges). Failure occurs when the gul-
lies penetrate through the capping. It is of little
consequence to know that the average erosion
across the whole structure is less than the thick-
ness of the capping if the point of deepest erosion
determines failure. The deepest erosion is much
higher than the average because the average
includes all the intervening ridges where erosion
is very low.
In farmland and rehabilitated mine sites, con-
tour banks are a major form of erosion control.
They work by (1) reducing the sediment discharge
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