Environmental Engineering Reference
In-Depth Information
calibration and verification data are available, and
that an experienced modeller undertakes the
work, available 1-D sediment models like iSIS
Sediment remain good predictive sediment tools
for application to flood risk management.
4 AfixedManning's n is used and this is applied to
both channel resistance and bed shear stress used
in sediment transport calculations.
5 The effect of bedforms (dunes, pebble clusters)
on resistance and sediment mixing is not included
explicitly.
6 Simulation of gravel traps that are small relative
to cross-section spacing can be difficult.
7 Steep rivers with high Froude numbers may
affect model stability and breach the applicability
of the available sediment transport equations.
8 Accurate simulation of armouring effects is
dependent on the criteria selected for deciding on
active-layer thickness, which are poorly defined.
9 Some of the more widely preferred sediment
transport formulations for gravel-bed rivers,
which have been incorporated into the research
version of iSIS, are unavailable in the standard
program.
10 Bank erosion/instability, its sediment yield
and its morphological impacts are not well repre-
sented in the standard model.
11 Selection of the method for updating cross-
section geometry is subjective, but can strongly
influence the modelling results.
12 Reliable interpretation of results depends to a
degree on user experience.
The conclusion that must be drawn is that
uncertainties associated with 1-D sediment
modelling are high and that this uncertainty is
derived from lack of knowledge concerning sedi-
ment transport mechanics and the sparse avail-
ability of field measurements of bed sediment size
distributions and sediment loads in UK rivers, as
well as limitations in the performance of 1-D
models themselves. As uncertainties in multi-
dimensional models are also conditioned by lim-
ited knowledge of processes and data with which
to characterize sediment properties and dynamics,
they are likely to be as great as or greater than those
in 1-D modelling. This suggests that the use of
simple, fast-running 1-D sediment models within
stochastic or probabilistic frameworks may at
present be the best way to handle uncertainty
when predicting future sediment dynamics.
The fact remains though that, as Bradley
et al. (1998) conclude, provided that reasonable
Cellular Automaton Evolutionary Slope and
River Model (CAESAR)
Background and basis
The Cellular Automaton Evolutionary Slope and
River Model (CAESAR) is a two-dimensional flow
and sediment transport model that can simulate
morphological changes at the catchment or reach
scales, on a flood by flood basis, over periods up to
several thousands of years. To date, CAESAR has
been applied to over 20 different catchments and
reaches, at spatial scales ranging from that of a
500m reach to that of a 500 km
2
catchment, and
over timescales ranging from that of an individual
flood to 10,000 years.
CAESAR is a cellularmodel thatmay be classed
as a 'reduced complexity' model. It fits in the gap
between multi-dimensional hydrodynamic and
Computational Fluid Dynamics (CFD) models
that can readily be applied to sediment-related
analyses of small reaches over short timescales,
and coarse-resolution landscape evolutionmodels
that can simulate changes in regional landforms
over thousands of years. Cellular landscape mod-
els work by representing the terrain using a grid of
cells, within which landscape development is de-
termined by fluxes of water and sediment between
the cells that are simulated using rules based on
simplifications of the governing physics (Nicho-
las 2005). In fluvial geomorphology, cellular mod-
els use simplified or 'relaxed' versions of the
complex flow equations used inCFDmodels. This
allows a substantial increase in speed of operation,
which, in turn, enables them to be applied to
extensive reaches and large catchments over long
timescales.
CAESAR was inspired by a model of river
braiding reported by Murray and Paola (1994).
Based on some of Murray and Paola's approaches,
