Rehabilitation of the Bulimba Creek Oxbow, and associated floodplain, as part of the Port of Brisbane Motorway project (PoBM) has proven that environmentalists and industry can work together to achieve an outcome that is beneficial to all. The rehabilitation strategy they came to support has significantly raised the benchmark for water quality treatment and environmental management on major road projects in Queensland. Water management strategies included the adoption of treatment devices not normally associated with motorway stormwater systems. These devices are now recycling motorway runoff, which is providing a valuable inflow, rejuvenating the existing wetland, and enhancing an ecosystem that supports native flora, fauna and migratory birds. In addition, sustainable water quality treatments and environmental management strategies were applied to the entire length of the project.

Background

Three years in the making, the first stage of the PoBM was opened in December 2002—well ahead of time and under budget, and with minimal disruption to the community and environment. The project was a massive undertaking that resulted in 5 km (3 miles) of motorway, 12 major new bridges and Queensland’s first tri-level interchange.

The design and construction was delivered by the PoBM Alliance, which was made up of Queensland Motorways Limited, Leighton Contractors, PB, and Coffey Geosciences. PB led the design team responsible for many of the breakthrough outcomes that allowed the Alliance to achieve its goals, and was responsible for environment, community consultation, permits and approvals and technical site supervision. We recognised and embraced the fact that sustainable development was vital to the achievement of excellent engineering outcomes.

Community Concerns about the Bulimba Creek Oxbow

The Bulimba Creek Oxbow is bound on three sides by Bulimba Creek and is adjacent to Doboy wetland and the village of Hemmant. Past industrial use and the construction of rail embankment and public utility services altered the area’s flood regime and subsequently reduced the effectiveness of its filtering process. Embankments had been constructed that separated the Oxbow waterway system from the natural tidal flows, and areas of the system have been used as waste treatment ponds for organic waste products from abattoir (slaughterhouse) operations.

Despite this degradation, the local community, and Brisbane City Council, and catchment groups, such as the Bulimba Creek Catchment Coordinating Committee (B4C) and Brisbane Region Environment Committee (BREC), recognised the potential for the floodplain and the function it provides in the protection of the Moreton Bay area. These groups raised concerns that the impacts from the PoBM would cause further degradation to the area, knowing that stormwater runoff from highways contains high concentrations of pollutants that are harmful to receiving watercourses, and that the PoBM crossed or passed near a number of waterways that form a unique matrix of freshwater, brackish and estuarine habitats, including Bulimba Creek and its oxbow (Figure 1). In response, the alliance identified the opportunities and constraints for the oxbow and developed a rehabilitation strategy to ”bring back to life” the degraded habitat.

Figure 1: Port of Brisbane Motorway Crossing the Bulimba Creek Floodplain and Oxbow

Rehabilitation Strategy

The rehabilitation strategy was driven by a project commitment to rehabilitate off-road areas, including the oxbow, to a point where there was zero negative impact. Our investigations identified the existing natural and physical environment in the study area and provided the following recommendations for rehabilitation:

• Remove existing bunds (embankments) and re-open the Oxbow, returning it to a tidal system similar to its natural river flow regime and allowing natural processes to flush accumulated sediments
• Remove dumped gravel within the Oxbow
• Remove surrounding landfill that promotes weed growth
• Maintain existing freshwater flows and increase flows where possible during flood events to help flush salts from the tidal zone of the floodplain. Additional freshwater flows through the wetland will be achieved by “recycling” stormwater run-off from the Port of Brisbane Motorway. A series of stormwater treatment devices, such as wetlands, buffer strips and sedimentation basins, will reduce pollutants in the run-off before it enters the natural wetland system.

While the first three measures were straight forward, the fourth one was not. In addition, there was a requirement to not increase flood levels in the Bulimba Creek flood plain as a result of the project. Typical treatment levels that have been adopted on similar projects would not meet the high water quality objectives set for the PoBM project, or could not be achieved at the minimal additional cost we were limited to, so the design team had to approach stormwater management in new and innovative ways.

Common Ground: Stakeholder Involvement

During the early design phase of the motorway, PB’s team expanded negotiations with stakeholders to include the Oxbow. The most successful (and valuable) aspect of the consultation process was the involvement of B4C and BREC in the flood modeling design meetings. The detailed two-dimensional hydraulic model with easily interpretable graphical output provided the basis for meaningful consultation between the Department of Main Roads (DMR), the B4C, the BREC and the PBM Alliance. The final modeling indicated enhancement to the freshwater wetland area by encouraging freshwater flood flows to pass through the wetland. Increased flushing will improve the health of the wetland. The final design represented a break through in being a cost -effective solution that met environmental and hydraulic objectives, and also the zero allowable flood afflux limits.

Some examples of innovative stormwater treatments our team specified for the PoBM include:

• Providing water quality treatment devices that will capture a 20,000 litres (5,200 gallons) of spill along the entire length of the motorway, thereby preventing the discharge of major chemical spills into the natural environment.
• Using linear, permanently inundated wetlands that collect pavement runoff and transport it to receiving waterways via underflow weirs, thereby reducing maintenance problems in low-lying, flat channels.
• Replacing grass with rock mulch (riprap) in most vegetated swales, thereby providing a physical barrier that prevented scouring of the channel, minimised maintenance and improved the water quality by filtering during low-flow events.

Safeguarding and Enhancing Environmental Quality

The water quality treatment practises PB designed for this project have been recognised by Brisbane City Council as Best Practice, and either PB or the project were awarded the following:

• 2002 Queensland Stormwater Industry Association Award in the Design and Planning Category (PB)
• Ministers Grand prize in the Healthy Waterways Awards 2002 (project)
• National winner in the 2003 Case Earth Awards for Environment (project).