REAL-TIME FLOOD RISK MANAGEMENT – PUTTING CERTAINTY INTO
THE UNCERTAIN
Cameron Druery,
Associate,
Patterson Britton & Partners
David McConnell,
Principal,
Patterson Britton & Partners
Lucas Adair,
Supervising Hydraulic Engineer,
Gold Coast City Council
Abstract
By their very nature, floods are unpredictable. Flood risks can be quantified and assessed utilising
hydraulic modelling results from a range of design flood events. However, actual flood events very rarely
ever match these design floods. The challenge, therefore, is to understand the likely behaviour of a real
flood, utilising the understanding of design floods.
Logan City Council and Gold Coast City Council have used GIS technology to develop a real-time flood
forecasting system for the Logan and Albert River system. The system utilises a library of design flood
events (from the 1 in 2 year ARI event to the PMF event) to predict a forecasted peak flood surface from a
number of predictions at various gauges in the catchments.
Predicted peak flood levels at the various gauges are entered into the system. A predicted flood surface is
then interpolated using the library of design flood events, and mapped to the DEM “on the fly” (ie in real
time) to provide realistic flood extents and depths. The surface is warped to account for varying degrees of
flooding in tributaries. The surface is presented against various GIS datasets such as aerial photography
and cadastre to provide a highly visual, and interrogatable, view of what the predicted flood may look like.
The predicted surface is then integrated with a number of GIS datasets to provide additional flood
management information such as:
 Closure of evacuation routes
 Inoperation of critical infrastructure (such as hospitals, emergency headquarters, power
substations etc)
 Individual property flooding
 Predicted flood damages
 Reporting of flood affected properties
Real-Time Flood Risk Management- Putting Certainty into The Uncertain
Druery
1. INTRODUCTION
By their very nature, floods are unpredictable. Flood risks can be quantified and assessed utilising
hydraulic modelling results from a range of design flood events. However, actual flood events very rarely
ever match these design floods. The challenge, therefore, is to garner an appreciation of the likely
behaviour of a real flood, utilising our understanding of design floods.
Gold Coast City Council (GCCC) and Logan City Council (LCC) commissioned Patterson Britton &
Partners (PBP) to develop a real-time, GIS based flood forecasting system for the Logan and Albert River
system. The system was to utilise design flood hydraulic modelling results to provide an indication of the
likely flood surface for a series of peak flood level predictions at gauges throughout the catchment. It was
important that the system be easy to use and highly intuitive as one of its intended uses is under the high
pressure situation of an actual flood event.
The Logan and Albert Rivers catchment comprises approximately 3,850 km 2 of South East Queensland.
The study area for this project covers an area 32km x 22km, extending from Logan City in the upstream
reaches to the outlet of the Logan River at the Northern End of the Gold Coast, as shown in
Figure 1.
Approximately 20,000 properties are subject to flooding by the Logan and Albert Rivers. Of those, close to
16,000 are flooded above floor level.
Logan City
Beenleigh
Figure 1 – Logan and Albert Rivers, Flood Affected Areas.
In Queensland, the responsibility for managing flood emergencies rests with the Local Government
Authorities and the Queensland State Government Counter Disaster Organisation. Each Local Council
maintains its own local division of the State Emergency Service (SES). Each Council operates a Disaster
Coordination Centre which facilitates the implementation of disaster management plans and coordination of
local emergency services.
This project was carried out using funding secured by LCC and GCCC under the Federal Natural Disaster
Mitigation Plan.
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2. EXISITING INFORMATION
2.1. Flood Model
In 1997, a one dimensional (1D) MIKE11 model of the Logan and Albert Rivers catchment was developed
as part of the Logan and Albert Rivers Floodplain Modelling Study. Model results were available for the 1 in
2, 5, 10, 20, 50, 100, 200, 500 year ARI and PMF design flood events.
In order to utilise the 1D model results within a GIS environment, a two dimensional (2D) framework was
developed for the 1D model results. This involved locating the model cross sections in real world
coordinates, followed by triangulation between adjacent cross sections. The triangulation facilitated the
creation of a continuous, pseudo 2D flood surface where linear interpolation between upstream and
downstream cross sections could be used to determine flood parameters at any location in the catchment,
rather than just at cross section locations. The creation of the pseudo 2D surface was carried out using
PBP’s waterRIDE™ suite of products. Once created, the pseudo 2D surface provided access to hydraulic
parameters such as water level, depth, velocity, velocity times depth, hazard etc, at any location in the
floodplain. Figure 2 shows an indicative triangulation of a 1D MIKE11 results set into a 2D flood surface.
Figure 2 Triangulation between adjacent cross sections in a MIKE11 model
to provide a continuous, pseudo 2D flood surface.
The 2D flood surface developed using the aforementioned procedure uses the MIKE11 cross sections to
define terrain. To facilitate a better indication of flood extents, depth and hazards, the flood surface was
then mapped over a finer scale Digital Elevation Model (DEM) derived from Councils’ Aerial Laser
Scanning (ALS) dataset.
2.2 Aerial Laser Scanning
Prior to this project, LCC and GCCC had commissioned extensive Aerial Laser Scanning (ALS) of the
combined LGA’s, providing spot heights and high quality orthorectified aerial imagery. For such a large
floodplain, the ALS data contained a very large number of points and it was necessary to use thinning
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algorithms to reduce this to a more manageable number without sacrificing any of the important
topographic features from a local flooding view point.
2.3 Property and Cadastral Database
LCC and GCCC carried out an extensive “drive by” survey of the approximately 20,000 flood affected
properties in the Logan and Albert River catchments. Supplementing the GIS property layer containing
standard property information such as address, owner, zoning etc, the “drive by” survey yielded the
following additional information:
 Floor level
 Ground level
 Habitable floor level height above ground
 Structure type
 Construction material
 Property photo
2.4 Critical Infrastructure and Evacuation Routes
As part of this project, LCC and GCCC developed a GIS based list of critical infrastructure and evacuation
routes. The critical infrastructure list included facilities such as power sub stations, telephone exchanges,
Police and emergency services headquarters, Council chambers, hospitals, nursing homes and schools.
For each critical infrastructure item, a critical level was determined as being the level above which, if
inundated, the facility would be considered inoperable.
The low points on primary evacuation routes were identified and surveyed. This data was compiled to
provide information on likely evacuation route closure.
2.5 Bureau of Meteorology
The Bureau of Meteorology (BoM) is the sole authority for flood forecasting and warning in Australia. In the
Logan and Albert River catchments, BoM operates a number of rainfall and river height observation
systems, including: volunteer rain and river height observers (where volunteers phone in observed levels
during the early stages of an event), as well as automated telemetry stations operated in conjunction with
the Queensland Department of Natural Resources and Mines.
In the Logan and Albert River catchments, BoM (from its Flood Warning Centre in Brisbane) issues
predicted flood levels at 5 gauges, namely: Slacks Creek, Waterford, Eagleby, Wolfdene and Riedel Rd.
BoM will issue a flood warning if the predicted flood levels are greater than 6.0m AHD at the Waterford
gauge, in the upstream reaches of the Logan River. Figure 3 shows the location of the 5 gauges.
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Figure 3 – Location of gauges at which predicted flood levels are issued.
3 FLOOD SURFACE PREDICTION - THEORY
3.1 General
Known water surface profiles can be used to interpolate an entire flood surface from a series of gauge
levels (either predicted or recorded). Generally, known water surface profiles are derived from hydraulic
modelling of a range design flood events. This provides a ‘library’ of flood behaviour for different magnitude
events from which to derive a representation of a predicted flood surface.
Water surface profiles along a river usually have a gradient. It is therefore important that any recorded flood
levels at one location use this water surface gradient to determine levels at other locations, rather than
simply applying the predicted gauge level as a flat water surface. It is also important to consider the fact
that the water surface slope is likely to vary between design flood events of different magnitude.
Figure 4, overleaf, illustrates how a predicted flood level at a single gauge on a river can be used to
interpolate a predicted flood surface for the river. The predicted level at the gauge is used to find the two
design water surfaces that the level is between (ie one above and one below) from the library of design
flood events (ie the MIKE11 modelling results). The ratio between the predicted level and the upper and
lower surfaces is then used to interpolate predicted flood levels at every node (or cell) in the water surface
network.
The water surface network is then ‘mapped’, on the fly, to the finer scale Digital Elevation Model to provide
a better representation of the predicted flood extent and depths. This provides the user with access to the
predicted water level and depth surface, for the extent of flooding.
Using a library of design surfaces with the predicted flood levels allows the water surface slope to be
incorporated into the predicted flood surface. It also allows the impact of a variation in water surface slope
between different design flood events to be included. For example, a PMF event in a region may have a flat
water surface in a particular location due to backwater effects, but the 1 in 10 year ARI event may have a
steep water surface profile. Such an effect is illustrated in Figure 5.
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Figure 4
Create library
of design
flood events
Integrate with
GIS layers
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Gauge Location
PMF
Interpolated
1 in 100
1 in 50
1 in 20
1 in 10
Figure 5 – Preservation of Water Surface Slope in Predicted Flood Surface.
3.2 Multiple Gauges
As the spatial distribution of rainfall during a storm event is not necessarily uniform across a river
system’s catchment, it is likely that different parts of the system would be subjected to differing amounts
of rainfall, and consequently, different degrees of flooding. For example, one part of catchment may be
subject to a 1 in 46 year ARI flood event whilst another is subject to a 1 in 87 year ARI flood event.
This variance adds a layer of complexity to the predicted surface interpolation routine in that the surface
must be warped to account for the different flood magnitudes in various parts of the catchment. To
facilitate the warping, the ability to enter predicted levels at a number of gauges throughout the
catchment needed to be incorporated into the system.
The influence of a predicted flood level at each gauge was accounted for by developing a zone of
influence for each gauge in the catchment. These zones reflect the region where water levels are
determined solely from the prediction at the gauge within the zone.
Where predicted water levels are considered to be influenced by a combination of gauges, buffer zones
between the zones of influence are created and allow a predicted water surface to be calculated in these
regions that incorporates an influence from each gauge. The relative influence from each gauge is
determined using an average function, weighted to inverse distance from adjacent zones of influence
within the buffer zone.
Figure 2 shows the zones of influence determined for the 5 gauges (Eagleby, Riedel Road, Slacks Creek,
Waterford and Wolfdene). Figure 2 also shows the two buffer zones used to transition between the zones
of influence.
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Buffer Zones
Figure 6 – Zones Of Influence.
4 FLOOD SURFACE PREDICTION – PRACTICAL APPLICATION
During a flood event, BoM issues predicted flood levels at each of the five gauges. These values are then
entered into the system to derive an indicative predicted flood surface. Once created, the surface can be
interrogated and integrated with a variety of GIS and non-spatial datasets to provide flood emergency
managers with timely access to a host of valuable information.
LCC and GCCC desired to use the system both during a flood emergency (ie in an operation context)
and also as a flood emergency planning and management tool. In this manner, the tool provides a
common and familiar interface for flood emergency managers.
4.2 Predicted Flood Surface Interrogation
As the predicted flood surface is mapped, in real time, to the finer scale ALS DEM, the predicted flood
extent, flood levels and depths can be readily seen and interrogated at any location. The system provides
standard GIS tools facilitating zooming, panning and spot polling which allow the flood emergency
manager to drill down and examine flood predicted behaviour in specific areas of interest. Figure 7 and
Figure 8 show the interpolated flood extent and flood depths, for a series of predictions at the gauges,
respectively.
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Figure 7 – Predicted Flood Surface – Flood Extent.
Figure 8 – Predicted Flood Surface – Flood Depth.
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4.3 Property Inundation
A requirement of the system was to provide LCC and GCCC with an indication of the degree of over floor
flooding for all flood affected properties to assist in evacuation planning. To facilitate this, when a forecast
surface is determined, the system automatically reads the predicted flood surface at each property. This
is then combined with the floor level information contained in the property information GIS layer to
calculate a depth of over floor flooding. Figure 9 shows a thematic map of the depth of over floor flooding
for a selected group of properties.
Figure 9 – Depth of Over Floor Flooding.
The system provides the ability to extract additional information on flood affected properties from the
underlying GIS table, including a linked photo of the property. Such information may be invaluable in
assisting emergency management personnel in the identification of properties to evacuate.
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4.4 Critical Infrastructure
In a similar process to the property inundation, a depth above (or freeboard to) a critical level for various
infrastructure items can be determined by integrating the critical infrastructure GIS layer with the
predicted flood surface. Such an integration provides flood emergency managers with an indication of
any infrastructure inoperability issues that may affect the safe and efficient management of a flood
emergency (such as loss of power or telecommunications). Figure 10 shows the status of a number of
items of infrastructure for a flood event.
Figure 10 – Status of Critical Infrastructure (green = operational, red = inoperable).
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4.5 Evacuation Routes
The system integrates the predicted flood surface with the evacuation route GIS layer (containing the
levels of low points on evacuation routes) to determine the status of an evacuation route. When the depth
is generally greater than 0.3m, the evacuation route could be considered “cut” to normal vehicular traffic
such as cars. Such information is clearly crucial for flood emergency managers to plan evacuation of
residents likely to be affected by the oncoming flood. Figure 11 shows the status of a number evacuation
routes for a flood event.
Figure 11 – Status of Evacuation Routes.
4.6 Reporting
To facilitate a complete and structured evacuation process, the system can prepare hard copy reports for
the predicted flood surface. Such reports may include information such as:
 A list of all properties likely to be inundated above floor level, ordered by street address, including
contact details for evacuation
 A list of all aged care facilities likely to require evacuation
 A list of likely flood affected properties requiring a formal flood warning communication
Hard copy reports provide a clear means of executing a flood emergency management plan. Such
reports could be equally prepared during the flood event, or during flood emergency management
planning processes.
4.7 Flood Damages
The depth above floor automatically calculated by the system was readily extended to the automatic
calculation of flood damages for a given predicted flood. LCC and GCCC provided flood damage curves
(depth of above floor flooding versus dollar damages, for a variety of structure types) which the system
uses to automatically carry out flood damages calculations. The damages are written to a field in the
property information GIS layer, providing access to thematic maps of damage and calculations such as
total damage for a flood event.
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Such information may be beneficial shortly after an event to provide an indication of the cost of the flood
event to the community and could be used to accelerate disaster support funding applications.
4.8 Community Education
LCC and GCCC intend to use the system as a tool to promote flood awareness in the community. The
simplicity and highly visual nature of the system facilitates clear understanding of flooding issues
amongst the community. It provides the community with an understanding of flooding in a practical sense,
linking the forecasts issued by BoM to a flood extent. When a predicted flood level of Xm at the Waterford
gauge is issued, the community will readily be able to determine what that means to them.
Increasing the flood awareness of the community has immeasurable benefits on a number of fronts: from
flood planning issues (such as development constraints) through to preparedness for evacuation during
flood events.
Although not adopted by LCC and GCCC, the system has the ability to “personalise” flooding at individual
properties. As photographs were taken of each property, along with floor levels, the photographs could be
‘scaled’. The predicted water surface can be superimposed on the photograph to provide a realistic
indication of what the predicted level means at an individual property.
Figure 12 – Predicted flood surface at a property – Minor Flooding
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Figure 13 – Predicted flood surface at a property – Major Flooding
5 FURTHER POSSIBILITIES
The focus of this project was to provide an indication of the likely peak flood surface for a predicted flood
event. As the waterRIDE™ system has the unique ability to analyse time varying hydraulic model results,
the time element could augment the predictive capabilities of the system. Using the full time-varying
results set for each design flood event in the library of design floods, time based interrogation can be
carried out, providing invaluable additional information such as:
 Time available for evacuation (including property inundation and evacuation route closure)
 Phasing of evacuation by regions or suburbs
 Rates of rise of floodwaters
 Duration of inundation – affecting flood damages and immediate disruption to the community
6 CONCLUDING REMARKS
The system developed for LCC and GCCC using waterRIDE™ provides a highly visual and intuitive tool
to assist flood emergency managers in both planning for a flood emergency, and during the high stress
environment of an actual flood.
The system overcomes problems associated with the fact that actual floods very rarely ever behave like
the design flood events. It draws from the rigorous effort and knowledge applied to design flood modelling
and takes a practical interpretation of the predicted flood event. It provides the missing link between a
flood prediction from BoM and what is means “on the ground”, greatly improving the understanding of
flood forecasts.
Generally, the information required to implement such as system is readily available for Councils and
flood emergency management authorities. That is:
 Flood modelling results
 Digital Elevation Models
 Property information/cadastre
 Aerial Photography
 Gauge locations
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7 ACKNOWLEDGEMENTS
The permission of Logan City Council and Gold Coast City Council to use its flood and GIS data as well
as examples from its flood forecast mapping interface are gratefully acknowledged.
8 REFERENCES
Flood Warning System for the Logan and Albert Rivers, Australian Bureau of Meteorology, 2005.
waterRIDE™ FLOOD Manager – Users Guide, Patterson Britton & Partners, 2007.
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