Geology Reference
In-Depth Information
on the hillslope by reducing hillslope lengths and
(2) capturing sediment behind the bank. Both of
these processes can be satisfactorily modelled
with traditional models. However, as sediment
accumulates behind the contour bank the hydrau-
lic conveyance of the channel along the contour
bank is reduced. Eventually, if the channel is not
periodically maintained, water in a storm will
overflow the contour bank as a result of channel
blockage. At this stage the contour bank will be
eroded and the flow behind the contour bank
concentrated into a gully that develops below the
bank. Modelling post-failure contour banks is
well suited to LEM application. However, to
date, post-failure analyses have not been com-
mon because they require being able to simulate
the hydraulics of the flow behind the contour
bank within a rainfall event. To date, within-
event hydraulics has been seen as being too com-
putationally intensive in the context of the
multi-year erosion simulations carried out with
a LEM.
Geomorphologically stable landforms have
long been an objective of landform rehabilitation
design. These landforms would degrade less
rapidly (and presumably re-establish a natural eco-
system more quickly), have less off-site impact,
and look more natural. LEMs allow us to model
the evolution of these landforms, and quantita-
tively assess their stability and how far they are
from equilibrium. For instance, we know that flu-
vial erosion leads to concave hillslopes, and the
link between the erosion process and its natural
concavity is known. Providing quantitative pre-
dictions of the stability of a concave landform is
important because waste disposal structures are
normally designed with a convex (not concave)
profile, because this profile minimizes the land-
form footprint for a given volume of waste. This
reduced footprint is by default one of the main
design criteria for waste disposal rehabilitation,
and LEMs have assisted in making the case that
the increased footprint of a structure with con-
cave-up components is a compromise worth mak-
ing. We will talk about some of the real world
challenges of building a geomorphic design in the
examples below.
Finally, by their nature LEMS calculate the
sediment transport balance on a DEM (either
gridded elevations or a triangulated mesh), simu-
lating erosion and deposition at a point from the
sediment transport balance at that point. Thus
they are inherently spatially distributed. Many
modern traditional models are also spatially dis-
tributed, even if their landform does not evolve.
Either way, because they are spatially distributed
they are excellent candidates for inclusion as com-
ponents of GIS or CAD design tools. Most GIS,
however, are not good at handling an evolving
landform (more typically GIS are used to display
some evolving property on a fixed landform), so
LEM capabilities are not fully exploited by GIS
interfaces. Accordingly most have been integrated
into scientific visualization tools (where an evolv-
ing landform is not a constraint) rather than GIS,
or stand-alone custom interfaces. Unfortunately
this has compromised their ability to integrate
into existing land management tools, which are
mostly GIS-based, although there are some initia-
tives on the horizon that might (at least partially)
address this problem (e.g. CSDMS: Syvitski
et al
.,
2004; TelluSim: Willgoose, 2009).
This section has summarized some of the sim-
ilarities and differences between LEMs and tradi-
tional erosion models, and highlighted how those
differences influence the scope of applications for
LEMs. To make these general statements more
concrete, we now look at some examples of appli-
cations of LEMs over the last decade. These
examples will highlight how LEM capability has
elucidated aspects of the erosion assessment that
are either difficult or impossible to do using tradi-
tional approaches.
18.3
Application Case Studies
18.3.1
Example 1: Encapsulation structures
In many industries there is a need to build earth-
or rock-covered structures to encapsulate waste
that may otherwise be a potential danger to the
environment or life. These wastes include min-
ing (tailings, below-grade ore, and waste rock),
chemical, nuclear, and household solid waste.
