Case study

The Challenge

The Wivenhoe Alliance was formed to reduce the flood risk to the communities downstream of the Wivenhoe Dam. The key objectives of the Alliance were to:

• Allow Wivenhoe Dam to safely pass the latest estimate of the Probable Maximum Flood (PMF);
• Preserve the flood mitigation benefits of Wivenhoe and Somerset Dam for more frequent flood events;
• Ensure that outflows are less than inflows for all flood events;
• Limit the frequency of operation of the auxiliary spillway to reduce downstream damage; and
• Minimise the cost of the upgrade.

Approach and Key Activities

Design discharges for the Brisbane River were estimated for rare and extreme flood events using various hydrological models, including URBS. The predicted inflows to Wivenhoe and Somerset dams together with the outflows from the Bremer River and Lockyer Creek were simulated through the WIVOPS program, which uses the complex gate operating procedures to predict the dam outflows.

Hydraulic (MIKE11) modelling of the Brisbane River was undertaken to estimate design flood levels and flood extents along the lower Brisbane River downstream of the Wivenhoe Dam. These models were used to assess the impact on design flows and peak flood levels of various auxiliary spillway options. They were then used to optimise the design and to assess the impact of the preferred option.

The impacts on design flood discharges of both spatial and temporal variations in rainfall together with dam starting water levels were evaluated. The gate operational procedures of the Wivenhoe Dam were modified to include the auxiliary spillway.

Outcomes

The use of calibrated and defensible hydrological and hydraulic models to optimise the spillway design leading to a significant reduction in flood risk to the communities below the Wivenhoe Dam.