Tweed Valley Flood Study FAQs

A flood study is a comprehensive technical investigation of flood behaviour within a catchment, in this case, the Tweed Valley. It uses sophisticated computer models to simulate flooding, defining the nature of flood hazards across the floodplain and providing information on the extent, level and velocity of floodwaters and distribution of flood flows.

The flood study will produce maps that define flooding characteristics across the floodplain. These are used for community flood awareness, flood risk planning, development controls and emergency management.

The current Tweed Valley Flood Study, completed in 2009, is due to be updated. Since then, the Tweed Valley has experienced 2 record floods (2017 and 2022) which have provided many lessons learned and rich data to use for flood studies.

In addition, the Australian Rainfall and Runoff (the guidebook for all things hydrology and surface water hydraulics in Australia) underwent an extensive update in 2019. This new version revolutionises the practice of hydrology in Australia and this contemporary best practice has been fully incorporated into this flood study.

While this flood study does incorporate the 2022 flood, it is not a reaction to that flood event. The project to update and expand the study started in 2020. It also incorporates the 2017, 1989 and 2021 floods.

Flood insurance terms, conditions and premiums are set by the insurance industry. Insurers may base property assessments on Council’s flood information however, Council has no influence over flood insurance products or premiums.

See Insurance Council of Australia and Understanding insurance for more information.

The study follows the framework set by the NSW Flood Risk Management Manual. The Tweed Floodplain Management Advisory Committee has provided oversight of the project, while staff from Council, NSW Department of Planning and Environment and SES have provided direction on technical matters.

WMAwater, a recognised expert in flood and water management, was engaged by Council to help complete the study. They specialise in hydrological and hydraulic studies of waterways and floodplains and the development of long term strategies and designs for management of flood and water resource risks.

No. After the 2022 floods, the National Emergency Management Agency announced the Northern Rivers Resilience Initiative, which included funding for the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to collect suitable data and construct a whole-of-catchment model for the Richmond River and test different flood mitigation scenarios.

The CSIRO flood study component of this program is limited to the Richmond River only and does not include the Tweed River.

Council has formally requested that the Tweed River be included in the CSIRO’s project. However, CSIRO has advised that they do not have the resources to complete a study of the Tweed River at this time.

Under the NSW Flood Risk Management Manual process, a flood study only quantifies flood risk – it does not evaluate mitigation measures.

Evaluating risk management measures (such as dams, levees, bypass floodways, evacuation routes, house raising) is conducted as part of a Floodplain Risk Management Study which is the next step in the NSW Flood Risk Management Process.

For now, the Tweed Valley Floodplain Risk Management Study and Plan 2014 remains Council’s adopted strategy for addressing flood risk. However, this will be updated in due course.

AEP is the probability of an event being equalled or exceeded within a year. Therefore a 1% AEP event has a 1% chance of being equalled or exceeded in any year.

While the flood study computer models are calibrated to real flood events like 2022, the study outputs are ‘design’ events. These are theoretical events created from detailed hydrology procedures that best represent the scale of a certain probability flood at a specific location.

Real world flood events are highly variable (e.g. location of rainfall, timing of rainfall, catchment saturation, ocean/tide levels). Design flood events consider all of these variables and reduce flood study outputs to be consistent across the catchment.

In some areas, detailed analysis shows that the 2022 flood event exceeded a 1% AEP magnitude when compared to this flood study.

It remains open to Council and the community to select appropriate revised designated flood levels and flood planning levels. This will be a separate body of work following the completion of this flood study.

Not necessarily. While Designated Flood Levels are typically based on 1% AEP levels from flood studies, it’s open to Council and communities to adopt alternative levels. Upon completion of this study, Council will undertake an additional process to define appropriate designated flood levels and flood planning levels for the Tweed Valley.

The PMF is the worst case scenario for flooding and is used for emergency response planning and specialised land use planning such as the siting of critical infrastructure (e.g. hospitals). Most other planning decisions are based on the 100 year Average Recurrence Interval (ARI) flood and an allowance for uncertainty and the potential effects of climate change.

The PMF is a theoretical flood resulting from the largest rainfall that could possibly occur within the catchment and therefore is extremely rare and unlikely. It is also used to determine which areas are potentially flood prone (in the floodplain) and which areas are flood free (only land outside the PMF extent is truly ‘flood free’).

In mid-2021 an initial community survey seeking residents’ flood experiences was conducted. The information we received was used within the study to inform known flooding issues and assist in the calibration of the flood model.

The project has now reached the draft report stage and has been endorsed by the Tweed Floodplain Management Advisory Committee and elected Councillors to proceed to public exhibition.

The draft report is now being shared with the Tweed Community seeking feedback.

Upon completion of the public exhibition process, the project team will review and consider all feedback received. Where appropriate, the study will be amended to address any valid issues identified.

The study will then be updated and finalised. The final version of the study report will be presented to the Floodplain Management Advisory Committee and elected Council for adoption.