Geology Reference
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dynamics of the various size fractions in detail
(Hairsine & Rose, 1992). However, compared with
the efforts that have historically been made to col-
lect data for model testing focusing on total soil
erosion, there are relatively few datasets available
that allow the testing of models describing size-
selectivity. One of the reasons is that traditional
field plots almost exclusively focused on areas
where net erosion occurs; while size-selectivity
may occur when sediment is detached, depositional
processes are potentially even more selective than
detachment processes (Beuselinck et al ., 1999).
Understanding deposition is also important because
deposition may lead to temporary storage and
remobilization of sediment (Krueger et al ., 2009).
As most field experiments on erosion were (and
probably still are) located on areas without net dep-
osition, the amount of data that is available to test
various approaches to selective deposition model-
ling is relatively limited, and only limited model
testing has been carried out (e.g. Beuselinck et al .,
1999). Sometimes the potential of a model to pre-
dict the movement of fine sediment and associated
soil constituents is evaluated using data collected
at the catchment outlet (Nasr et al ., 2007). It is evi-
dent that this approach holds potential, but that
the resulting model calibration may be very spe-
cific for the conditions tested due to the issues we
discussed earlier (see Chapter 13 as an example).
One of the major difficulties in deposition
modelling is the prediction of the actual sediment
size distribution of the transported sediment. It
has been known for a long time that eroded sedi-
ment is often transported in the form of (micro-)
aggregates, a fact that has profound implications.
The fact that colluvial soils often have a grain-size
composition similar to the eroding soils from
which they were derived can only be explained
by the fact that most of the colluvium was indeed
deposited in the form of (micro-)aggregates
(Beuselinck et al ., 2000). Understanding the dynam-
ics of aggregation during an erosion event is key to
understanding size selectivity and enrichment.
Recently developed measurement techniques offer
some perspective here: it is now for instance pos-
sible to measure the size distribution of transported
sediment on the fly using laser diffractometry
(Williams et al ., 2007). An additional complication
is that the (micro-)topography of depositional sur-
faces and therefore the runoff hydraulics may
change very rapidly once deposition occurs. Often,
one can observe depositional surfaces being re-
incised by consecutive events or even in the last
phase of the event causing the deposition in the
first place; the mobilization of previously depo-
sited material is possible when the sediment load
of the runoff water reaching the depositional zone
is well below sediment-transporting capacity
(Plate 3). Models accounting for these interactions
between sediment deposition, runoff hydraulics
and sediment load are at present non-existent.
At this point one might wonder why we should
develop our models further in order to include
these complicated processes. It is indeed unlikely
that further refinement of the description of dep-
ositional processes in process-based, dynamic
models will greatly improve our predictive capa-
bilities with respect to erosion rates and/or gross
sediment yield. Yet, the objective of models is not
only to predict the impact of future events or to
simulate past data correctly; models are also
scientific tools to enhance our understanding of
how landscapes function. Evaluating models for
sediment deposition and particle size-selectivity
using appropriate field and laboratory data will
allow us to judge to what extent we really under-
stand the processes we are interested in, which is
a valid scientific objective on its own. Also, bet-
ter understanding does not necessarily have to
lead to more complex process models, which are
often more difficult to calibrate and/or validate.
In some cases better understanding may also help
to decide where and how a model can be simpli-
fied without losing predictive power, thereby
allowing for more efficient model calibration.
Van Oost et al . (2004) were able to show that sedi-
ment re-entrainment was not important to
describe sediment deposition on, and export from,
eroding fields in Central Belgium: hence, a sim-
pler model structure not including re-entrainment
could also be used successfully. However, the
effect of such simplifications may strongly depend
on local conditions and they therefore need to be
applied with care. A better understanding of the
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