Copyright 2011 Tom Coulthard, University of Hull, UK. T.Coulthard@hull.ac.uk
INTRODUCTION, HISTORY AND BACKGROUND OF CAESAR CAESAR is a two dimensional flow and sediment transport model. It can simulate
morphological changes in river catchments or reaches, on a flood by flood basis, over
periods up to several thousands of years. It is free do download and use under a GNU
licence. It was initially designed to simulate the geomorphic response of river catchments
to changes in climate and/or land cover. Its purpose was to address the debate as to
whether changes in climate or anthropogenic changes in land cover had led to changes in
UK river behaviour over the Holocene. In order to answer these research questions the
model had several requirements:
1. To simulate the relevant time scales (10's to 10 000 years - Holocene).
2. Cover suitable spatial scales, e.g. a long reach of river or a catchment
3. To incorporate the relevant processes and parameters that were operating over the
above scales (e.g. hydrologicial, fluvial and slope processes)
4. To simulate the whole catchment. There are many feedbacks and interactions between
processes operating within a river catchment, and it is important to try and include this
within the model (CAESAR now also models river reaches).
This remit was initially part of a PhD project, and since then the model has grown, both in
sophistication and application. CAESAR has proved to be very flexible, and to date has
been applied to over 20 different catchments and reaches on scales ranging from 500m to
500km2, and over time scales from individual floods to 10 000 years.
One of the main early findings from CAESAR was that the behaviour of river
catchments (in this example in the UK) was mainly controlled by shifts in climate, but
was modulated (both increased and decreased) by changes in land cover. Coulthard et
al., (2000) found that climate and land cover individually could lead to a c.150%
increase in sediment yield, but when combined this could rise to 1300%.
This is also shown to the left in a figure from Coulthard and Macklin (2001) where
simulations from the River Swale showed that as land cover decreases closer to
present day, the size of sediment discharge from similar size 'wetter' periods increases.
This work (Coulthard and Macklin, 2001) also allowed some validation of the model
results, by comparing the simulated sediment discharge - driven by independent
climate and land cover records - to frequency distributions of 14C dated flood
deposits in Northern England.
This work also showed that the sediment output from river catchments was far from
linear - something which would be a theme investigated by future work. Following on
from this work in the Swale, the CAESAR model was altered to create the 'TRACER'
model that allowed the tracking on heavy metal contaminated sediment down
through the river system - this work was published in Geology (see images below and
Coulthard and Macklin, 2003) and showed how contamination patterns were
geomorphically controlled, and developed into hot spots of high concentrations.
Though initial applications of CAESAR were modelling river catchments - it
became clear that the basic CAESAR model had the capability to simulate erosion
and deposition over river reaches. This led to the development of 'reach mode'
whereby water could be input at a point within the model (e.g. at the top of a
reach) and erosion and deposition on a more detailed DEM of a reach could be
simulated. In addition, catchments and reaches can be linked - so output from a
coarser scale river catchment model can be fed directly into another higher
resolution CAESAR model of a reach.
Technical developments in CAESAR have continued, with the recent addition of
lateral erosion - allowing the channel to meander. This is carried out using a novel
edge counting algorithm that counts the number of wet and dry cells next to a
river bank and then uses this to calculate whether it is on the inside or outside of a
Other developments from 2005 onwards included the addition of suspended
sediment, how shear stress is calculated as well as how slope processes are
determined. Recent work by Welsh et al., (in press) also relates the rate of slope
processes within CAESAR to the hydrological model - so during wetter periods
landslips and soil erosion increase.
Recent work (Van De Wiel and Coulthard 2010 -
see left) has used CAESAR to explore the
non-linear output of sediment from river
catchments. The output of sediment from a
river cathcment is hard to predict and seems to
contain a large amount of noise or scatter. Using
CAESAR in various studies we have found that
this non-lienarity may be caused by the
breaching of bed armour layers.
In most recent work, we suggest that sediment
output may be controlled by Self Organised
Criticality (SOC) which may mean that sediment
delivery is effectively unpredictable, and that
we may not be able to infer past environmental
records (e.g. climate or land cover) from
Case Studies and on-going research projects: CAESAR has been applied to over 100 different reaches and catchments by researchers accross the world. These range from UK rivers, to
those in Australia, New Zealand, Spain, Romania, France and Switzerland. It has been applied to whole catchments ranging from 1 to 1000
km2, and reaches up to 40km in length.
CAESAR is presently being used to simulate how changes in bed morphology will influence flood risk in Carlisle, as part of the EPSRC Flood
Risk Management Research Consortium (FRMRC II) program. It is also in use as part of a Swiss project SED-RIVER to investigate the role of
climate change on alpine catchment morphology and fish habitat. Furthermore, it is being used to establish rates of geomorphic change and
the influence of extreme rainfall on rehabilitation plans for the Ranger uranium mine, NT, Australia.
The most recent developments (Spring 2011) bring a 2d hydrodynamic flow model to CAESAR. This is based on the Lisflood-ACC code (Bates et
al., 2010) that adds an inertia term to the established lisflood flow model. This aids stability and greatly speeds up the code (especially on
smaller grid cells) which means for the first time, there is a flow model that operates fast enough to realistically be used in a landscape
This flow model was intially added to existing CAESAR erosion and depositon functions - but this afforded a chance to re-write 80% of the
original code (some that dated back to 1996) and tidy up many of the operations. CAESAR-lisflood has the same feel and look as CAESAR but
operates with a correct hydrodynamic non steady state flow. One further advantage of the lisflood-acc flow model is that it is readily run in
parallell. The old scanning CAESAR flow routing algorithm was sequential, preventing this. As a result, CAESAR now makes full use of
multi-core windows machines giving significant increases in operational speed. An early example of the flow model in operation can be seen in
the video clip below: