ANUGA – Free & Open Source 2-D Hydrodynamic Model, with
Hydrologic Capability
R. VanDrie1 , and P. Milevski2
1
VANDRIE Hydraulics Laboratory
Burrapine, NSW 2447
AUSTRALIA
2
Wollongong City Council
Wollongong East, NSW 2520
AUSTRALIA
E-mail: rudy@balancernd.com.au
In the latter part of 2006, the Australian National University and Geoscience Australia released their
2D hydrodynamic model ANUGA to the free and open source software world. While initially developed
to simulate coastal inundation by a tsunami or storm surge, ANUGA is a fully generalised
hydrodynamic model with potential as a tool for simulation of any flow that is predominantly 2D in
nature.
In 2008 a paper was delivered at the “Water Down Under” conference detailing trials for both Urban
and Rural catchments. This paper was primarily aimed at identifying the hydraulic capabilities of the
model in a flooding context showing an example of hydrograph applied to a rural catchment and a
urban dam break scenario.
ANUGA is mostly coded in Python, a very powerful but easy to learn (free and open source) object
oriented language. As such, extensions or amendments to the model are very easy to develop and
implement. The authors have recently added the ability to model distributed rainfall, infiltration and
drainage structures such as bridges and culverts. In addition currently there is work being planned to
accommodate underground piped networks.
As such it is considered worthy to bring this to the attention of others that may be interested in using a
free highly capable 2D hydraulic model that is truly capable of hydrologic simulation. The paper will
highlight the use of ANUGA as a combined hydrologic/ 2D hydraulic model, with extensive examples
of rural, urban and mixed catchments. This is now a proven hydrology model simulating rainfall directly
on the 3D terrain model. The ANUGA Model is to be show cased by “The New Inventors” on the ABC
on the 10th of June 2009.
1. INTRODUCTION
In December 2006 The Australian National University and Geoscience Australia released to the public
a Free 2D Unstructured Grid, Finite Volume, Hydrodynamic Model. The model was a resultant of a
Mandate put to GA by the Australian Federal Government to build capacity to identify and manage
Hazard and Risk. This was interpreted and actioned by providing a software tool to aid in assessing
the impact of tsunami. This being the case the model is therefore well adapted to providing a robust
modelling solution to all forms general fluid flow based on the Shallow Water Wave Equation.
Therefore it is capable of modeling, not only Ocean Inundation but also Riverine Flooding and the
combination of both these.
The model was released as Free & Open Source Software (FOSS) meaning that every user has
access to the computational code. This allows every user with the capability and will, to add or
improve the content of the original code. The original code is then updated on the source forge web
site making the enhancements available to every one.
A compact yet full description of ANUGA has been covered by others. (Rigby et al, 2008) (Nielsen et
al). For more details, please refer to the following link. http://sourceforge.net/projects/anuga/
Traditionally catchment analysis and stream analysis has been reliant on a two-step process.
-
Undertaking Hydrologic Analysis of the catchment
Then using the flow estimates from the hydrology to undertake hydraulic analysis
The hydraulics was commonly derived using 1Dimensional flow models, however relatively recently
there has been a strong move to 2-Dimensional flow models.
The question has been raised “can 2D models be used to model hydrologic response?”.
Recent research papers (Clark et al, 2008) concluded that models such as TUFLOW and SOBEK
were un able to replicate the hydrologic response when compared to well accepted hydrologic models.
However both these models are fixed grid 2D models.
By contrast, this paper will show that ANUGA which is an unstructured grid model does not appear to
share that fate. In fact many catchments have now been modeled for hydrologic response with
ANUGA with impressive outcomes.
2. TRADITIONAL HYDROLOGY & HYDRAULICS
The most common approach to modeling catchment behaviour or even the flow characteristics of a
segment of a stream, is to initially estimate the flow using a hydrologic model. Once the flow estimate
is determined it is then required to undertake a second task that of determine the hydraulic response.
The once industry standard was to use HEC-2 and then HEC-RAS. These are 1-Dimensional flow
models built around resolving the Energy Equation. More recently there has been a shift to using 2Dimensional flow models, such as FLO-2D, HYDRO-AS2D, SOBEK, TUFLOW and the like. A relative
new model is a model developed by Geoscience Australia in conjunction with the Australian National
University. The model is known as ANUGA.
2.1.
Lumped Hydrologic Models
Step one of the traditional approach to hydrologic modeling is to use a lumped model such as WBNM
or RAFTS or RORB. With this methodology the entire catchment is broken into smaller sub catchment
as determined by characteristics such as land use or roughness. In this way the response of each sub
catchment can be controlled to best reflect reality. The hydrologic model does not know the detail of
the terrain as it is simply not required for that approach.
The analysis produces hydrographs for each sub catchment. In fact is usual for the models to produce
several hydrographs at each sub area. For instance WBNM produces the following hydrographs:
-
A. LOCAL PERVIOUS AREA Hydrograph
B. LOCAL IMPERVIOUS AREA Hydrograph
C. UPSTREAM Hydrograph entering the sub area from any up stream areas
D. Hydrograph leaving the area routed through a stream segment
E. Hydrograph that may be altered due to a lumped storage at the outlet of the sub area
As will be discussed these multiple hydrographs are useful in how they allow flows to be included into
the hydraulic models.
Lumped hydrologic models are widely used and well validated, and as such present a robust
comparative tool. The model to be used is WBNM2007. (Boyd et al, 2008)
Figure 1 Typical sub division of a catchment using a lumped hydrologic model
2.2.
Linking Hydrology to Hydraulic Models
It is usually the case that hydrologic models are used to produce output in the form of flow estimates
which are in turn used as input in hydraulic models. In older models such as HEC-RAS the simplest
approach is to enter the peak flow estimated from the hydrologic model into the segment of stream
where the hydraulic behaviour is under scrutiny. It is also possible to pass the entire hydrograph.
Similarly in 2-Dimensional flow models it is possible to enter both the peak flow in a stream segment or
a hydrograph. It is also possible to pass the hydrographs A,B and C above into the model. This is
usually done by combining hydrographs A and B and applying them either distributed over a sub area,
or input at a sub area centroid. Hydrograph C is applied at the up stream reach of the sub area stream
segment. In this way the hydraulics of the stream segment is able to reflect the diffuse local sub area
flow entering the stream.
2.2.1. Is there an alternative approach available?
Ideally one would think it should be possible to input rainfall directly over a terrain model to derive the
hydrologic response, which by default is also the hydraulic response.
3. ALTERNATE APPROACH TO HYDROLOGY & HYDRAULICS
As mentioned if the 2-Dimensional model was extended to encapsulate the entire catchment, then
surely applying rainfall over that catchment should represent the ultimate way to produce the
hydrologic response from the catchment. The flow response is matched exactly to the terrain. As long
as the user has a method to apply the appropriate roughness to the terrain the hydrologic response
may in fact be better than that produced by traditional lumped hydrologic models. As Clark et al has
shown that using a fixed grid model produces unpredictable result. They conclude that:“The 2D hydraulic models were sensitive to adjustments to a
number of model parameters. While it was expected that the
models would be sensitive to adjustments to parameters such as
the Manning’s ‘n’ roughness coefficient value, the degree of
sensitivity to changes to grid cell size and timestep parameters
was not anticipated.”
Further:
“Given the dependence of model estimates on adjustments to
parameters and other inputs, based on this investigation, these
models are too variable for their generic application and it is not
possible to give guidance on their general use.”
However the ANUGA model is not a FIXED GRID model it is an UNSTRUCTURED GRID model that
allows refinement of the grid around the watercourse for instance (or in fact around buildings).
Figure 2 Comparison of Fixed Grid and Unstructured Grid Approaches (Note flexibility of
refinement)
As such potentially ANUGA may not be plagued with the same problems as those identified by using
FIXED grid models.
3.1.
Hydrologic Modeling with 2D Hydrodynamic Models
ANUGA was not originally developed with the capability of modeling rainfall, nor was the intent of the
authors to model rainfall or catchment hydrology. However, the development language adopted to
code the model is PYTHON. Python is a very flexible pure object oriented language. As a result the
structure of the ANUGA code is such that it is relatively easy to define a new type of forcing function.
As mentioned in the introduction the developers of ANUGA have made the source code available and
thereby allow the developer community for this product to grow.
This is exactly what has occurred to provide the model with the capability of modeling the effect of
rainfall directly on the 3D terrain model. In 2007 Van Drie et al added a rainfall module.
Again due to the pure object nature of the code once a new forcing function is defined it will inherit all
of the functionality of the general forcing function. Therefore the rainfall module includes a host of
methods to apply rainfall including:
-
Rainfall applied universally over the computational domain
Rainfall applied via a polygon
Rainfall applied via multiple polygons (Spatially varying)
Rainfall read from Files
Rainfall developed from user defined functions.
The flexibility is almost endless.
Similarly the application of the roughness parameter follows exactly the same ideology.
Roughness can be applied by all of the methods described above. This allows a depth varying
roughness function to be created.
This applies to every aspect of ANUGA.
A series of general function allows the functions to be used across the full range of needs. For
example altering the terrain to include buildings is again relatively simple. Once the polygon of a
building is defined it can be used to directly alter the terrain in the model (within ANUGA). This
removes the need to alter the base terrain which is usually based on Airborne Laser Survey (ALS)
data.
So therefore ANUGA has the ability to:
-
Create a detailed 3D terrain model of the catchment
read Rainfall time varying patterns from multiple Files
Based on Polygons define where varies rainfall is to be applied
Based on yet other polygons define where surface roughness varies. Note that this may be a
function based on flow depth that allows roughness to vary with depth.
4. CASE STUDY OF 2-DIMENSIONAL SWW (HYDROLOGIC) APPROACH
Using the methods briefly described above a small urban catchment of approximately 12.5ha in area
with an average slope of approximately 13% was set up in ANUGA.
4.1.
Setting up the Model
In order to setup the model the terrain data is required. This was based on ALS data. The extent of the
catchment can be seen in the following figure.
Figure 3 Overview of terrain covering within which the test catchment is located
As described roughness polygons can be (and were) defined to allow input of various roughness
values for :
- Roads
- Residential Land
- Park Land
- Etc.
In addition if required specific adjustment of the terrain can be undertaken by the ANUGA model itself.
Figure 4 Roughness polygons to vary Manning’s n and buildings to be extruded
By defining the polygons of the outline of buildings the terrain can be adjusted to create 3-Dimensional
buildings. This is quite different to the current popular approach of using the polygons to increase the
roughness locally or increasing the roughness regionally in urban or residential areas. The benefits of
this approach has previously been shown to be significant in properly addressing the flow velocity and
hence the hazard presented by the flow (VanDrie Et al, 2007).
Figure 5 Terrain adjusted by ANUGA to include several buildings in the flow path
The benefit of utilizing ANUGA to provide the hydrologic response is that the 2-Dimensional
application of the Shallow Water Wave Equation is robust enough to properly account for the flow
velocity over all of the terrain as the model is resolving the flow over the entire model accounting for
the rainfall and correctly accumulating the flow in every small swale and depression. Also of course,
the model will account properly for all storages within the catchment. Therefore unlike the lumped
hydrologic approach no storage needs to be specifically defined at roadway culverts where both small
and significant storages can occur.
The shallow water wave equation correctly resolves the flow velocity over the continuously varying
slope down the catchment. This also therefore correctly accounts for all hydrograph timing over the
entire model. No specific lag parameters are required for stream segments!
Figure 6 Overview of the main stream slope down the catchment
4.2.
Running ANUGA
Once all the input data is prepared, an ANUGA script is written in PYTHON. Essentially the script
assembles all of the data, applies the parameters, including the rainfall and then “EVOLVES” the
solution. ANUGA basically takes care of the rest. The time step is adjusted as necessary to ensure the
model is never unstable regardless of flow complexity. This means that the model will easily deal with
hydraulic jumps, waves and even reflective waves (should they occur). Although this may not be
necessary for a typical hydrologic analysis, it may be necessary for say a DAM BREAK scenario. Such
a scenario was shown recently (10-06-2009) on the ABC’s “The New Inventers” program, showcasing
the ANUGA model in the context of a Natural Disaster Management tool.
Figure 7 Overview of the completed model as seen by ANUGA
ANUGA is remarkably robust. The authors are not aware of any model ever going unstable or
producing results that were questionable. It is often the case in other models that a parameter may
start to go unstable resulting in widely varying velocities or depths in areas of the model. These are
usually controlled in those models by forced fudging factors that smooth out the instability. This is
often why those models cannot resolve hydraulic jumps properly.
However in order to ensure this level of stability, the ANUGA runs can take some time to run. Run
times are generally controlled by the number of triangles and the size of those triangles. It should be
noted that the user has considerable control over these aspects. Therefore fast models can be built if
one is patient enough to refine the model. This is one area of ongoing development. The vision is to
create an AUTO-ADAPTIVE-MESH SCHEME that will automate mesh refinement at each time step.
4.3.
ANUGA RESULTS
ANUGA is provided with a remarkable viewing capability. The model results can be reviewed whilst
“flying over” the catchment. The entire storm can be run forward, backward, stepped forward and
backward and paused. This allows a unique insight into the flow behavior. Hydraulic jumps if they
occur become visible as shocks and standing waves.
In addition there is a host of ways in which the results can be viewed or ported to any GIS application.
There are also methods that can export time series at any location or time series over a cross section.
In this manner time history of velocity can be plotted or a hydrograph at any location.
Figure 8 Overview of the RESULTS in the ANUGA Viewer (Note buildings)
The conserved quantities in ANUGA are :
- Ground Elevation
- Depth
- Momentum X
- Momentum Y
From these any other pertinent parameter or quantity can be derived and mapped. For example it is
possible to extract, Velocity, Froude Number, or HAZARD directly. The following example shows the
flood level (stage) of the above flood simulation.
Figure 9 Alternate view via GIS of colour coded Flood Level
5. COMPARING ANUGA TO WBNM
As mentioned the aim of this paper is to highlight that the 2-Dimensional UNSTRUCTURED GRID
model ANUGA, can genuinely reflect the correct hydrologic response of a catchment. This will be
shown by comparative analysis to a well validated and accepted lumped hydrologic model
WBNM2007.
Both models (ANUGA & WBNM2007) have been run for a series of storms ranging from a 5 year ARI
to PMF event. The output of the models has been compared by comparing the hydrograph at a known
location. The results are regarded as being very good as shown below.
Figure 10 showing 5yr and 10 yr ARI hydrographs compared
The shape of the hydrograph, the timing of the peak and overall storm volume are all comparable. In
fact it may be the case that ANUGA produces a more realistic outcome as shown by the initial delay in
flow at the start of the event. Worthy of a mention is the fact that WBNM2007 reproduces almost
identical hydrographs with (1) a single lumped sub area or with (2) a refined 31 sub catchment model.
Figure 11 showing 20yr and 50 yr ARI hydrographs compared
The consistent reproduction of the WBNM2007 results for the full range of storm events by ANUGA is
in stark contrast to the findings of Clark et al using 2 fixed grid models.
Figure 12 showing 100yr and PMF event hydrographs compared
Clark found that “when TUFLOW was run with a 5 metre grid, the model became unstable and
crashed. Although the model ran stably when the timestep interval was reduced from 1 to 0.5
second…”
Further Clark states that, “Both 2D models are sensitive to changes to the DEM computational grid cell
size. There is a trend in the SOBEK results for the peak flow rate to decrease as grid cell size
increases. The trend for TUFLOW is the opposite, with the exception of the 100 metre grid cell size
results, the peak flow rate tends to increase as grid cell size increases. Despite the differences in
trends, estimates from both models appear to converge to the WBNM estimates as grid cell size
decreases. The volume of rainfall excess discharged from the catchment is also highly affected by
adjustments to this parameter.”
It appears that ANUGA does not have these tendencies with model grid size down to 0.1m2 running
stably. The case study model has triangles down to 0.1m2 and a total of 60,000 triangles.
A very concerning result reported by Clark is concerning the TUFLOW results, it was found that:
“results indicate that TUFLOW estimates are highly sensitive to adjustments to the timestep
parameter. When run with a 2 second timestep, 34% more rainfall drains from the catchment than was
actually applied as rainfall excess, this is indicative of mass errors occurring. When run with a 0.5
second timestep, the recession limb of the hydrograph stops attenuating after approximately 4 hours
to become a constant flow rate of 55m3/s, longer run times at this timestep could potentially lead to
the generation of mass”. Again ANUGA is not impacted by any issue related to mass balance or time
steps. The time step is computed internally and the FINITE VOLUME methodology ensures mass
balance is maintained correctly.
Yet another concern raised by the Clark paper is the trend due to the impact of varying the roughness.
SOBEK reportedly varied in discharge by 23% and TUFLOW by 32% by doubling roughness from
0.04 to 0.08. This is certainly not the finding using ANUGA.
Figure 13 Impact of adjusting Manning’s n to Hydrograph Shape in ANUGA
This will be the subject of a further research paper in the near future.
6. OTHER APPLICATIONS UNDERTAKEN
To date the two authors of this paper between them have applied ANUGA to around 100 catchments
as a combined hydrologic / hydraulic model, with areas ranging up to around 15,000 ha. In all cases
ANUGA ran stably and produced sensible results when compared to WBNM2007.
Figure 14 Large catchment with steep escarpment, showing variable size grid applied
These large catchments with highly refined meshes produce very large output files (> 1Gb), however
the viewer is not impacted with regard to the speed and smoothness of the visualization.
Figure 15 Largish 110km2 Catchment model (hydrologic/hydraulic)with ANUGA
7. CONCLUSIONS
It is concluded that as a 2-Dimensional UNSTRUCTURED GRID model ANUGA is capable of
reproducing the correct hydrologic response for entire catchments when compared to an industry
accepted lumped hydrologic model. This appears in stark contrast to several well known FIXED GRID
models.
In fact the comparison of hydrographs shows a remarkable fit over a range of events from 5 year ARI
to the PMF. The effect of adjusting Manning’s n appears much more realistic when compared to the
results of adjusted roughness reported by Clark.
ANUGA does not display any of the instability problems reported by Clark when using small grid cells.
It is therefore concluded that anyone interested in attempting to merge hydrology and detailed
hydraulics would be well served in down loading ANUGA and reviewing the relative ease with which it
can be applied. For details refer to the following link. http://sourceforge.net/projects/anuga/
8. ACKNOWLEDGMENTS
This work was conducted in collaboration with the Design Section of Wollongong City Council, and
with the input and advice from Ole Nielsen of Geoscience Australia.
9. REFERENCES
1. Boyd M. J., Rigby E.H. and Van Drie R. (2007) WBNM User Manual,
www.uow.edu.au/eng/cme/research/wbnm.html
2. Clark, K. Ball, J. Babister, M. (2008), Can Fixed Grid 2D Hydraulic Models be Used as Hydrologic
Models?. Water Down Under 2008, 31st Hydrology and Water Resources Symposium and the 4th
International Conference on Water Resources and Environment Research Conference, Adelaide, April
4-17, 2008.
3. Nielsen 0, Roberts S, Gray D, McPherson A and Hitchman A (2005) Hydrodynamic modelling of
coastal inundation, MODSIM 2005 International Congress on Modelling and Simulation, Modelling and
Simulation Society of Australia & New Zealand, 518-523,
www.mssanz.org.au/modsim05/papers/nielsen.pdf
4. Rigby, E. VanDrie, R. (2008), ANUGA – A New Free & Open Source Hydrodynamic Model.
Water Down Under 2008, 31st Hydrology and Water Resources Symposium and the 4th International
Conference on Water Resources and Environment Research Conference, Adelaide, April 14-17, 2008.
5. VanDrie, R. Simon, M. Schymitzek, I. (2008), HAZARD:- Is there a better definition? & Impact of Not
accounting for buildings! Preprints of the 48th NSW Flood Plain Management Authorities Conference,
Wollongong, February 26 - 29, 2008.