Key Takeaways
- Wivenhoe Dam, south-east Queensland’s biggest reservoir, is nearly full, but its operators say water levels are exactly where they need to be.
- The dam provides drinking water and flood protection for much of the state’s south-east, and its secondary function is to mitigate flooding by controlling the flow of the Brisbane and Stanley Rivers.
- Wivenhoe Dam has a total capacity of 1.2 million megalitres, but it can hold far more than that during a flood event, thanks to an additional compartment located above the drinking water.
- The dam’s floodgates are used to control the flow of water during rain events, and the decision to open them is based on complex software, real-time data from hundreds of sensors, and weather forecasts from the Bureau of Meteorology.
Introduction to Wivenhoe Dam
Wivenhoe Dam, located north-west of Brisbane, is one of 26 dams operated by Seqwater and is by far the biggest of the lot. Completed in 1984, the dam was built with the primary purpose of providing drinking water to the millions of people who live in south-east Queensland. Its secondary function is to mitigate flooding by controlling the flow of both the Brisbane and Stanley Rivers, which pass through some of the heaviest rainfall areas in the country. Data on flooding in the city of Brisbane dating back to the 1890s shows that the dam has been very successful in this regard.
Understanding Wivenhoe Dam’s Capacity
When deemed ‘full’, Wivenhoe Dam can hold just under 1.2 million megalitres. However, to use official jargon, that means "operational supply level" and is a percentage of its total drinking water supply level, not its actual capacity. During a flood event, Wivenhoe can hold far more than 1.2 million megalitres, thanks to an additional compartment located above the drinking water. This additional compartment is double the size of the drinking water compartment, providing an additional 2 million megalitres of flood storage available.
Flood Operations and Management
The decision to open Wivenhoe Dam’s floodgates is based on complex software, real-time data from hundreds of sensors, and weather forecasts from the Bureau of Meteorology. Seqwater’s principal dam safety and operations manager, Kevin Bartlett, explains that the flood operations centre uses this data to monitor rainfall and river conditions and determine when to open the gates. The engineers rely on an array of hundreds of sensors dotted across the catchment, which provide real-time data on rainfall, run-off, flow rates, and dam levels. Forecasts from the Bureau of Meteorology are also used to model how a flood event will play out.
Lessons from the 2011 Floods
The 2011 floods led to accusations that the operators of Wivenhoe Dam had breached their duty by failing to follow the flood manual. In 2019, the Supreme Court of New South Wales found Seqwater and others had been negligent, but Seqwater later won an appeal against that decision. The floods prompted several reviews of the flood manual, and today’s engineers are devoted to its stipulations. The floods also prompted political calls to drain Wivenhoe to below full supply level ahead of forecast heavy rain, but the manual does not allow for that.
The Wivenhoe Myth
Operators say that even if Wivenhoe Dam is operated in full accordance with the manual, it will not be able to stop a major flood from affecting those downstream. This is because only the upper Brisbane and Stanley Rivers are controlled by the Wivenhoe and Somerset dams, while other waterways, including the volatile Lockyer Creek, Bremer River, and lower Brisbane River, are all unregulated. During a particularly severe event, dam operators lose some of the limited control they have, because Wivenhoe is designed to spill over once it reaches a certain level to prevent a catastrophic structural failure.
Staying Alert and Prepared
Floods can devastate large areas, and it’s essential to know the risk of flooding at your home. Seqwater notifies local governments and disaster management teams as early as possible regarding releases, but advanced notice is not always possible. The community needs to be prepared and react quickly to changing weather conditions. Dam releases regularly cause Brisbane River crossings to be impassable, and dangerously charge downstream waterways. Staying alert and prepared is crucial to minimizing the impact of floods.