Reconnecting to the sea for coastal wetlands restoration

Dr Nathan Waltham from James Cook University’s Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER) led the JCU research team on the study, in collaboration with the CSIRO.

He said the building of bunds (embankments) many decades ago was intended to close off freshwater sources from the sea and provide ponded pasture for cattle.

But while they provide habitat for many freshwater species, most of these areas are now heavily infested with freshwater aquatic weeds, support few fish and suffer from poor water quality.

“Widespread use of herbicides to control weeds has resulted in environmental issues and has led to the search for more natural control methods such as using saline water.

“Our study involved the proposed removal of a bund separating the Mungalla wetland in Queensland from the sea and allowing the salt water to do the job for us instead of using chemicals,” said Dr Waltham.

He said in light of recent major advancements in computational methods, hydrodynamic modelling could reliably predict what would happen if the bund was taken down.

“We obviously wanted to assess what was likely to happen in advance of committing to extensive earthworks, so we carried out simulations of 11 different scenarios under different tide, onshore wind, low atmospheric pressure and sea level rise conditions,” said Dr Waltham.

He said the simulations pointed to favourable outcomes and the bund wall was removed, resulting in improvement in water quality, which has helped control freshwater weeds.

“Few wetland restoration projects in the past have gone as far as reintroducing seawater, mainly because of uncertainties in the extent, frequency and duration of seawater ingress after removing man-made obstructions.”

He said the study showed simulations can now be used to determine the potential impact of various interventions before committing to any action.

“For example, the removal or height adjustment of tidal barriers, the dredging of silted streams, the removal of weeds from choked streams and the reintroduction of tidal flows can now all be modelled beforehand,” said Dr Waltham.

He said the restriction of tidal flushing onto coastal wetlands and intersecting creeks by roads, pipelines, floodgates and culverts has negatively impacted wetland productivity and detrimentally impacted birds, fish and plants.

“What is most interesting here is that under current sea levels the tides will inundate the wetland now several times each year, less during above average rainfall wet seasons.  However, in the future under sea level rise more large tides are expected, which makes this research even more critical for restoration and management of sensitive coastal wetland,” said Dr Waltham.

He said there has been increasing effort in recent decades to rehabilitate coastal wetlands by removing man-made barriers.

“These efforts will only increase following the UN’s recent declaration of a decade on ecosystem restoration – and now we have the tools to make it much more effective, which is really exciting given the opportunities of blue carbon that these kinds of restoration work offer us as we face climate change and sequestration of carbon from the atmosphere.”

The research project was funded by the Australian Government’s National Environment Science Program Tropical Water Quality Hub.

Link to pictures of the Mungalla wetlands here.