A clearer understanding of the Brisbane River

Why did the Brisbane River change from blue to brown? A University of Queensland PhD student’s research could provide the answer.

Luis Neumann has been researching the sediment processes in the river that are responsible for the turbidity or “dirtiness” of the water. Supervised by lecturer in environmental engineering Dr Tony Howes, Mr Neumann’s innovative thesis is unique in that his research methods border on the fields of chemical and environmental engineering.

Using equations known as population balances to calculate changes to the population of cohesive sediments in the river, he has demonstrated that the sediment does not have enough time to settle, giving the river a dirty appearance.

“Today’s large tidal intrusion means the tidal velocities are much higher than in the past. The larger the tidal velocity, the higher the bottom shear stress that erodes the river bottom and the higher the turbulence that keeps the particles in suspension.”

He said about 80 years ago swimming carnivals were held on the river as the tidal intrusion was much smaller and therefore the river looked blue and much cleaner.

Mr Neumann is examining the process known as flocculation through which suspended solids in the river collide and stick together to form flocs. He said in the past researchers had assumed values for parameters affecting flocculation whereas he had been able to quantify the influence of different parameters.

“We looked at what researchers were doing in chemical engineering with flocculation using population balances and applied this to river sediments,” he said.

He said the aim of the research was to develop models using population balances to predict particle size distribution and settling velocities of solids or flocs in the river system, taking into account different salinities and tidal cycles.

“I demonstrated that shear, the force caused by velocity differences, was the dominating parameter and salinity was not as important as people thought. These findings will lead to better management of the river because it will provide a greater understanding of the river system,” he said.

Mr Neumann said the size and shape of the flocs affected sediment transport and light penetration, which was largely responsible for the colour of the river.

“The light penetration depends on how the particles scatter light; the smaller the particle, the higher the scatter of light. Smaller particles will make the water look dirtier than the same mass of large particles,” he said.

In September Mr Neumann was presented with the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management’s (Coastal CRC) Best Student Achievement Award for his research. His PhD is part of two projects funded by the Coastal CRC that aim to understand the mechanisms of turbidity in the river.

Mr Neumann’s research will also be applied to the Coastal CRC’s Contaminant Pathways project on Rockhampton’s Fitzroy Estuary.

Media: For more information, contact Luis Neumann (telephone 07 3365 8398, email luisn@cheque.uq.edu.au) or Dr Tony Howes (telephone 07 3365 4262, email tonyh@cheque.uq.edu.au) or Chris Saxby at UQ Communications (telephone 07 3365 2479, email c.saxby@uq.edu.au).