Geology Reference
In-Depth Information
The mechanism for this improvement seemed to
be that localized deposition occurred behind the
contour banks as a result of slight concentration of
flow down the slope, so that some points gener-
ated localized deposition when the flow reached
the contour bank. These areas of localized deposi-
tion behind the banks then created pools in which
flow down the grade along the contour bank could
then deposit sediment. Thus as the landform
evolved, it naturally created sediment-capture
regions along the grade of the contour bank that
did not exist in the original landform, where water
flowed freely down the drain behind the contour
bank. Clearly, a model that does not allow the
landform to evolve in response to erosion and dep-
osition cannot model this sediment capture mech-
anism; only an LEM can capture this behaviour.
The only assumption that was made in the
modelling was that the contour banks were not
overtopped at any stage during the simulation. To
verify this assumption, post-processing of the
simulation data indicated that deposition behind
the banks did not fill up the storage capacity
behind the contour bank. However, the simula-
tions did not examine the effects of extreme
events. Thus the simulations did not capture large
events where the conveyance capacity of the
channel behind the contour bank would be
exceeded. This is not a fundamental limitation of
LEMs, but reflected the limitations of the SIBERIA
LEM used for the study, which could not model
event-scale hydraulics. The authors are unaware
of any LEM that can realistically model event-
scale dynamics while also modelling landform
evolution over many years.
slope. This principle has been long understood, at
least qualitatively (e.g. Toy & Hadley, 1987). LEMs
allow us to quantify this concavity in terms of the
hydrology and erosion processes on the slope.
LEMs can then be used to assess the long-term
fate of those slopes relative to other design options
(Loch & Willgoose, 2000a,b; Vasey
et al
., 2000) so
that an objective judgement can be made of the
costs and benefits of the various alternatives.
One important point to note here is that in
landform design the absolute erosion rate is not
always important, because we are comparing the
relative efficacy of design alternatives. Generally,
a good landform design alternative is good no mat-
ter what the erodibility and absolute erosion rate
of the materials used to construct the slope. Of
course, if the objective is simply to meet a tonnes
per hectare threshold then the erodibility will be
important. More critical is the area-slope relation-
ship for the materials (Willgoose
et al
., 1991b;
Willgoose, 1994).
As noted above, a common regulatory require-
ment of an above-ground containment structure
is that it covers as little land area as possible to
reduce the amount of impacted area to a mini-
mum. The maximum volume of stored waste
per unit area covered is provided by a structure
that is convex. Yet natural landscapes are only
convex near the hilltops and concave down-
stream. As we have noted, this convex-concave
hillslope profile (commonly referred to as a cat-
ena profile) results from the balance between
soil creep and fluvial erosion processes. A struc-
ture that is convex everywhere is therefore not a
good match with natural landscapes, and is not a
good approximation of an equilibrium hillslope
profile. Figure 18.6 shows the longitudinal pro-
file of three hillslopes covering the range of con-
cavities typically observed in the field. The
convex slope has 25% more volume stored in it
compared with the planar slope, while the con-
cave has 30% less material stored in it than the
planar. The three slopes each have the same
average slope, so if the profile is averaged as, for
example, in USLE, then they would deliver the
same sediment load. However, at the base of the
slope the convex profile will yield the highest
18.3.3
Example 3: Geomorphic design
In applications where a landform is to be designed,
the erosion assessment task is not only to mini-
mize erosion (or alternatively make it the same as
occurs naturally) by the use of surface treatments
(e.g. vegetation or rock cover), but also to design a
landform that minimizes erosion. For instance,
we know that natural catchment long profiles are
concave, so a concave slope is naturally more geo-
morphologically stable than a planar or convex
