The Clarence River has flooded areas around Grafton NSW four times since 2009, significantly affecting rural properties.Photo: NSW State Emergency Service, Clarence Nambucca Region
This article originally appeared in the Autumn 2016 edition of Fire Australia magazine. By Freya Jones.
Predicting water depth and its velocity is key for timely and accurate flood forecasts, and Bushfire and Natural Hazards CRC research along the Clarence River in northern New South Wales has recently undertaken a hydrological survey to improve flood forecasting in the area.
Using a HydroSurveyor, including an echo sounder, Doppler velocity profiler and GPS antenna, the research team has built a three-dimensional map of the river bed which can be maintained as conditions change.
This is a significant benefit for the Clarence Valley, and will be used to calculate the capacity of the river to deal with incoming flows, says Associate Professor Valentijn Pauwels, who is leading the CRC project, Improving flood forecast skill using remote sensing data.
“With this information we can predict water depth and velocity at any point in the river valley,” says Associate Professor Pauwels.
“The availability of timely and accurate flood forecasts will allow for time-effective warnings, the implementation of evacuation plans and the set-up of safe recovery and storage areas,” he added.
This forecasting relies on the data to predict the arrival time, water depth and speed of a flood using two main models, hydrologic and hydraulic. Associate Professor Pauwels says the models each predict different aspects of a flood that are then applied together.
“The hydrologic model determines the flow of water that is entering a river network using rainfall and catchment conditions, while the hydraulic model predicts how that water will travel downstream along the river system,” he says.
Although these models have come a long way in terms or capabilities, they do not yet provide all the answers.
“It is challenging to provide accurate flood warnings because of errors or uncertainties in the model structure and the model parameters,” says Associate Professor Pauwels.
This is where remotely sensed data may be applied to improve the accuracy of predictions.
The research is looking at how remotely sensed data can be assimilated operationally within existing models to improve the accuracy of flood forecasting. Remote sensing involves using satellite technology to capture information about a particular area from far afield. This means that for regions which are dangerous or inaccessible at ground level, aerial data can be collected to fill in the gaps and aid predictions.
Behind flood forecasting is a complex science which is constantly adapting to new technologies. The current models rely on rainfall stations to measure the amount of rainfall on particular catchments. The hydrologic model then calculates how much of that rainfall will be absorbed by the soil depending on current soil moisture levels. The Bureau of Meteorology’s Manager of Policy and Strategy Unit, Soori Sooriyakumaran, is one of the project’s lead end-users and explains that there are particular limitations with the existing methods.
“There are parts of Australia where our rainfall station coverage is quite sparse due to the area being large and remote,” he said. “Remotely sensed rainfall data will help understand the rainfall variability across such areas.”
“Remotely sensed data also has its own problems, but even with those it can add value to the input that goes into the modelling,” Mr Sooriyakumaran added.
Remotely sensed soil moisture products have a great potential for calibrating and updating hydrologic models - remote observations of flood extent and water levels can be used to correct and constrain in real time the prediction of the flooded area and depth generated by the hydraulic model.
A challenge to the project lies in combining the satellite data with data collected on-the-ground in a way that will minimise errors.
“The spatial and temporal resolutions with which the on-ground and remotely sensed data are observed are different so there are some challenges in bringing them together,” Mr Sooriyakumaran said.
To overcome this and to minimise overall errors when combining the data sets, error characteristics of the data are analysed, with the desired goal being more precise and robust outcome for flood forecasting and flood warnings.
“What we are trying to do with this research project is to come up with the best combination of satellite and ground data so that we can have as accurate as possible input and constraint into the modelling”.
The application of remotely sensed data can be compared to a missing piece of the puzzle for flood forecasting, joining together information from different sources to form the bigger picture. Mr Sooriyakumaran says this technology will only improve over time.
“Satellite remote sensing is an expanding new field, and we are going to have better and better data coming through in the future with higher resolution and higher frequency. This is one of the technologies that is going to keep improving flood forecasting into the future.”
This research will be beneficial to the emergency management by supplying them with a more comprehensive depiction of conditions.
“[The satellite data] could give us better situational awareness by showing areas under inundation, explained Mr Sooriyakumaran.
“To forecast future water levels we need good information on what is happening on the ground now, which we can present to the emergency services so they can plan their emergency response better.”
Back on the Clarence River, Clarence Valley Council Local Emergency Management Officer, Kieran McAndrew, says the river is the heart of the council area, and has been affected significantly by flooding in recent years - 2009, 2011, 2012 and 2013 all saw serious floods.
"It is the largest of all NSW coastal rivers in terms of catchment area and river discharge, which means flooding is part of life for the community of around 50,000 people," he says.
The Monash University-based research team, comprising of Associate Professor Pauwels, Professor Jeffrey Walker, Dr Stefania Grimaldi, Dr Yuan Li and Ashley Wright, believes the research they conduct will have positive impacts on emergency warnings for floods and contribute to lessening the damage and potential costs to communities.
“It is estimated that in Australia floods cost an average $377 million per year,” highlighted Associate Professor Pauwels.
“An improved flood forecasting system will enhance the emergency management capability, thus reducing the flood-related financial costs and community distress.”
Mr McAndrew says the council wanted to support the research in any way they could.
"The Clarence Valley community relies on warnings to prepare for imminent flooding, so there is a real benefit to be gained from the research.”
The next steps for the project will be to bring all the existing data together with the new satellite information and put it into operational use.
“Up until now we have focused on collecting the data, from here we will start improving the models,” says Associate Professor Pauwels.
This process will be made easier with the Bureau of Meteorology’s recently upgraded platform for modelling, Hydrological Forecasting System or HyFS, which uses a framework that allows users to easily plug in new models says Mr Sooriyakumaran.
However, Mr Sooriyakumaran believes that the benefits of this research will not be immediate.
“We are not expecting dramatic changes to happen overnight, they will take time,” he says.
“But as technology advances and the research is applied into an operational setting, the true impact will be realised”
“The upgrade of our flood forecasting modelling system means we are able to bring research learnings into operations much more readily, a capability that we did not have before.”