DREDGING of the Hawkesbury River was once a common activity in years gone by, but more recently it has ceased.
Dredging, or digging up the bottom of the river, can be used for many purposes: flood mitigation, deepening a river for navigation purposes, as well as excavating sand for construction.
In the past, there have been calls to dredge the Hawkesbury River. Flood mitigation is one of the reasons often cited.
The Gazette interviewed three academics on the topic of dredging, and in particular, whether it was a good way to mitigate the damage of floods.
The general consensus from the academics was that dredging was a viable method of flood mitigation, although there were a number of consequences that would arise from doing so.
One of the academics, who has experience with the river, was more bullish on the prospects of dredging than the others, who both made it clear they had never studied the Hawkesbury River, and were speaking in general terms.
All, however, agreed, that you should be careful what you wish for when dredging, because the consequences of dredging could be more harmful than the original problem.
Who did we speak to?
The Gazette spoke to three academics.
The first was Water, Environmental and Sustainability Professor Basant Maheshwari, from Western Sydney University’s School of Science & Health based at its Hawkesbury Campus.
Over the last 15 years, Prof Maheshwari has worked on water, environment and sustainability aspects of urban, peri-urban and rural landscapes. His research focused on understanding how water, landscape and people interact and how we impact on water and food security. He has also spent much time working on the health of the Hawkesbury River.
Prof Maheshwari is on Hawkesbury Council committees regarding the river.
The second was associate professor Gavin Birch from the University of Sydney.
Prof Birch is an environmental chemist and toxicologist, who deals with contaminated estuary systems. He specialises in Sydney Harbour and has also written a paper on the toxicology make-up of the Hawkesbury River, released in 1997.
Prof Birch made it clear he had not worked closely on the Hawkesbury River for many years, and was speaking generally about the nature of dredging.
The third was the Australian National University’s Associate Professor Jamie Pittock.
Dr Pittock said his research has focused on conservation of freshwater biodiversity.
He too spoke in general terms about river dredging.
So what is dredging exactly?
WSU’s Prof Maheshwari described it like this.
“Basically dredging is the excavation and removal of material from river bed,” he said.
“It is commonly used to mine sand from river, improve the navigable depths, create more capacity of river channel to handle floods.”
Prof Maheshwari went further, saying dredging caused changes in a river system, and not always for the better.
“River dredging is a significant operation if we consider the complex aquatic life that exist in the river water and hidden in the river material that is to be excavated,” Prof Maheshwari said.
“Dredging will bring some changes in the river environment. Some of those changes may be short-term and some could be longer-term but eventually at some point a natural equilibrium will come after dredging.
“Some periodic dredging may be necessary but it should done with caution and at an appropriate time while monitoring the effects of dredging on water quality and other changes in the aquatic environment. We should not take this operation lightly.”
Dredging should be done with caution
University of Sydney’s Prof Birch said people should be careful what they wished for when deciding to dredge.
“One of the reasons people put forward for not dredging is that if one disturbs contaminated sediments in the river contaminants may be released into the overlying water,” he said.
“The opinions [on dredging] range from 'let the system settle into some natural rhythm' and then some people who say it has been anthropogenically influenced and silting has not been natural.”
Prof Birch said one of the main problems for dredging was that it could disturb contaminants such as herbicides and fertilizers, which had settled on the bottom of the river.
“You don't want to go disturbing contaminated sediment,” he said.
“That [Hawkesbury] river is a massive river and if you want to go and dredge a significant part of the river for recreational purposes, then you will have to deal with a lot of sand and silt. What will you do with it?
“That sediment is not just all sand, it has a lot of fine material and it would have to be washed out and then the mud would have to go back into the river and it would increase the turbidity and smother sea grasses and other benthic [on the bottom of the river] fauna and flora.”
Dr Pittock said dredging was an activity that should be avoided it at all possible.
“Usually, dredging is environmentally damaging,” he said.
“Rivers, their beds, banks and floodplains represent a balance between human activities in the catchment, water flows and sediment supply.
“Taking away sediment with dredging would normally upset this balance and result in such impacts as greater bank erosion and changes in the river channel location.
“This may simply trigger demands for more and more interventions, such as infrastructure to stabilise river banks.”
Aquatic life could be harmed
Prof Maheshwari said dredging would almost certainly affect aquatic life in some ways, and those could be harmful.
“Dredging could disrupt [the] fishing and prawn industry in the Hawkesbury-Nepean river system,” he said.
“Dredging can increase the inland reach of tidal water flow upstream and may affect river water quality and marine life.”
He said a thorough investigation was the best way to determine any such effects before dredging.
“The marine environment in the river channel is [a] quite complex combination of natural features and interactions that happen with underwater life,” he said.
“Soon after dredging, water quality, particularly temperature, turbidity, dissolved oxygen, pH, nutrients and conductivity of [the] river may change and can affect fish and other aquatic organisms in [the] river at least for a short period.
“It can also affect [the] surrounding habitat for wildlife and other animals. These changes can affect fishing and other industries that depend on maintaining river water quality.
“Because of the complexity of [the] marine environment, we sometimes cannot predict the effects of dredging but in [the] short-term the dredging will affect marine life. Some detailed investigations of environmental effects is essential before any dredging is commenced with an objective assessment of [the] potential pros and cons.”
What about for flood mitigation?
Flooding is a very serious issue in the Hawkesbury.
There are a number of reasons why the Hawkesbury River floods. One such reason, although certainly not the main culprit, is that the river has become visibly shallower in many parts over years.
One potential solution, therefore, is flood mitigation through dredging. The argument in favour is the river would be deeper and capable of carrying more water.
Prof Birch said it was certainly possible to dredge for such a reason, and said it happened across the world in various scales
“Whether you dredge for flood mitigation depends on the circumstances,” he said.
“The size, the energy of the system, and also if you dredge one part it destabilises other parts of the river.
“You would have to do quite a substantial amount of hydropologlical modelling to make sure the dredging just doesn't impact other parts of the river.”
He warned, however, that even localised dredging in select parts of the river could have big effects downstream.
“You might just create another problem further downstream from where you do the dredging. It should involve a more holistic approach to increase water velocity and volume flow.”
Prof Maheshwari said dredging was a legitimate flood mitigation method.
“Dredging does improve the river channel capacity and can improve the travel of water and so it can help as a flood mitigation strategy in the Hawkesbury – Nepean catchment,” he said.
Dr Pittock, however, felt differently.
“Dredging is a very poor way of managing flood risk,” he said.
“Usually the dredged channel fills with sediment again fairly quickly. Faster water flows in the dredged channel cause other problems, like river bank erosion.”
Dr Pittock said dredging for flood mitigation had further consequences, outside of the river system itself.
“The illusion of safety induced by dredging may encourage governments to approve new development on flood prone lands,” he said.
“In much of Europe, the United States and China, more investment is being made in restoring floodplains to manage floods.
“This involves relocating vulnerable infrastructure and changing to flood tolerant land uses such as grazing land, parks for recreation and nature conservation.
“Well managed floodplains catch flood peaks and slowly release the water without harming urban areas.
“Restoring floodplains usually has many additional benefits, for example, filtering water to improve its quality, increasing fish populations, restoring habitat for threatened species and providing recreational opportunities.”
The conclusion from the academics is that dredging is possible, but the pros and cons should be seriously weighed before undertaking any such action.
“Dredging is a complex phenomenon in rivers and should be done with caution to not affect marine life irreversibly,” Prof Maheshwari said.
“Any large scale dredging should be avoided but some periodic and controlled dredging may be required after several years to remove any on-going deposition of sediments and other materials in the river bed. If it is done based on precautionary principals, it can benefit flood mitigation, river maintenance and local economy.”
Prof Birch said a study of any potential impacts, along with a qualitative scientific assessment was essential.
“Many rivers in the world are dredged continuously where they keep rivers open for harbour activities, Newcastle for example, so it can be done but there needs to be a good reason for it and good science behind it to make sure there is no adverse effects,” he said.
Dr Pittock was firm in his opinion that dredging was a less than desirable option.
“A better, longer term solution is to better manage catchment lands to reduce erosion to more natural levels, reducing sediment influx into rivers,” he said.