Tweed Valley flood study update and expansion

We want to extend our sincere thanks to the community for participating in the Tweed Valley Flood Study Update and Expansion community consultations. Your valuable feedback has helped shape the final report, which Council has now adopted. This collaborative effort allows us to understand flood risks better and improve planning for the future.

The updated flood study incorporates the latest best practices in hydrology and reflects the lessons learned from recent major flood events, including those in 2017 and 2022. The comprehensive flood mapping now covers the entire Tweed Valley, providing critical information for flood risk management, planning, and emergency response.

Previous community consultation

We saught feedback on the draft flood study to ensure it aligns with the community's understanding of flooding in the valley.

The draft flood study covers the entire Tweed Valley and is on public exhibition.

To help the community understand the draft report, we provided the following before the community information sessions:

Community information sessions

Drop-in, face-to-face sessions were held in the afternoon, from 4 pm to 6 pm, in the following areas:

Those who couldn't make it in person had the option to join the online session held at 5 pm on Thursday 7 March 2024.

Locality specific areas

We prepared information specific to some key locations to give the community a better understanding of flooding in the following key areas:

What is the Tweed Valley Flood Study update and expansion?

Tweed Shire Council, with support from the NSW Department of Planning and Environment (DPE), has engaged flood specialists WMAwater to update and expand the Tweed Valley Flood Study. The previous flood study was completed in 2009 and only covered the area from Murwillumbah to Tweed Heads.

A flood study is a comprehensive technical investigation of flood behaviour within a catchment, in this case, the Tweed Valley. It defines the nature of flood hazards across the floodplain by providing information on the extent, level and velocity of floodwaters and the distribution of flood flows.

The flood study uses sophisticated computer models to simulate flooding across the Tweed Valley. These models are calibrated to actual recorded floods (2017, 2022, 2021, 1989) to ensure they accurately represent real-world flooding.

The study simulates a broad range of 'design' flood events from very minor/frequent floods up to extreme/very rare floods and future climate change scenarios. The study outputs include extensive mapping for each of these events such as peak level, depth, velocity, depth x velocity, hazard and hydraulic categories. These maps define flooding characteristics across the floodplain and are used for flood risk planning, development controls and emergency management.

This flood study does not consider measures to alleviate or mitigate flood risk (e.g. levees, bypass floodways, evacuation route upgrades, house raising). Under the NSW Flood Risk Management Manual model, a flood study simply quantifies flooding. It is a Floodplain Risk Management Study that evaluates options to mitigate flood risk. This is a subsequent step in the NSW process. For now, the Tweed Valley Floodplain Risk Management Study and Plan 2014 remains Council's adopted strategy for the management of flood risk in the valley. However, updates to that study and plan are intended to follow from the completion of this flood study.

This project was funded under the NSW and Commonwealth Government's Natural Disaster Resilience Program.

Why do we need a new flood study?

Due to software and computing technology limitations at the time, the 2009 flood study produced mapping for the lower valley only – the area from Murwillumbah to Tweed Heads. This study will produce mapping for the entire Tweed Valley.

The Tweed has experienced the two biggest floods on record (2017 and 2022) since the 2009 flood study. Lessons learned from these events have increased our understanding of flood behaviour in the valley. Additionally, a wealth of data has been collected from these major floods, helping us to calibrate our computer flood models.

Australian Rainfall and Runoff, the guidebook for all things hydrology and surface water hydraulics in Australia, has undergone an extensive update since the 2009 flood study. The 2019 version of the guidebook revolutionises the practice of hydrology in Australia and this contemporary best practice has been fully incorporated into this flood study.

What has changed since the 2009 Study?

A flood study is a highly complex process. The new study follows different industry guidance (Australian Rainfall and Runoff 2019) to the 2009 study and also enjoys the benefit of lessons learned in the 2017 and 2022 major floods. Therefore, the results of this flood study do not match those of the 2009 study precisely.

The report contains an entire chapter on "what has changed" comparing the draft results to those of 2009 study.