Geology Reference
In-Depth Information
Generalized rules for the design of these strips
will no doubt emerge as we develop more experi-
ence in field applications.
At the managerial and decision-maker level,
the main limitation to LEM application is that
LEMs are still an unfamiliar tool. The LEM user
may sometimes address this through education of
the manager. In other cases this unfamiliarity can
be important for highly political applications.
LEMs have not yet been proven in court and this
can be important for projects that will need to be
legally defended. In these latter applications the
LEMs unquestionably provide insight that cannot
be obtained by any other means. The problem is
that if millions of dollars are to be spent and justi-
fied using LEM simulations, some assurances are
needed that the model's predictions (sometimes
hundreds and thousands of years into the future)
are, if not correct, at least not potentially mislead-
ing. An important related issue is the need to
identify indicators based on short-term perform-
ance (e.g. 10-20 years of monitoring) that can be
used by regulators, as part of rehabilitation sign-
off, to provide confidence in long-term (e.g. 100-
1000 years) LEM predictions. Progress has been
slow in this area, but will be driven by practical
applications. These practical applications will
elucidate the types of validation required. These
validation test cases are qualitatively different
from the type of validation needed for traditional
erosion models. For instance, we believe more
work needs to be done to understand the area-
slope dependence of erosion models, particularly
with respect to case studies where concavity plays
a large role in the performance of the structure.
We will now concentrate on the science issues.
The main issue with LEMs is that many hillslope
properties that can be directly measured for an
existing hillslope actually evolve in concert with
an evolving hillslope. This complicates the mod-
elling of an evolving hillslope because this indi-
cates that we need submodels for the evolution of
these hillslope properties, and these submodels
must respond dynamically to the evolution of the
hillslope.
The best example of this need for submodels is
the evolution of the grading of soil on the surface
(a)
Maximum depth
of erosion
(b)
Reduced
maximum depth
of erosion
Armour
strip
Fig. 18.8 Using an armour strip to reduce gully and/or
rill erosion in the upstream part of a catchment or
hillslope. The heavy line is the original hillslope,
while the lighter line is the erosion after some time,
showing how the armour strip reduces the maximum
depth of erosion: (a) hillslope without the armour strip
protection; (b) hillslope with armour strip protection.
One emerging application of LEMs is in the
design of erosion protection of landforms using
rock armours. It can be very expensive to cover
an entire landform with a rock armour layer to
protect it from erosion. Yet in arid zones where it
can be difficult to establish vegetation cover, this
may be the only protection measure possible. It is
possible to protect an area from erosion incision
by creating a low erosion zone downstream of a
sensitive area that effectively stops the upward
propagation of gullies/rills, and constrains the
depth of gullies/rills upstream of the low erosion
zone (Fig. 18.8). A key feature is that the evolu-
tion of the landform is constrained by the low
erosion of the armour strip. This armour strip
pins the change in elevation at the mid-slope, and
thus constrains the change in elevation upslope
and downslope of the armour strip. LEM simula-
tions confirm this behaviour. Notably, LEM sim-
ulations show that both protected and unprotected
slopes initially have the same erosion rate, and it
is only after some time that erosion is reduced on
the hillslope with protection because the erosion
protection relies on the evolution (or lack of it) on
the armour strip relative to the rest of the slope.
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