Adopting a ‘hard’ solution to keep invasive fish out and our wetlands full at Mount Burr Swamp
At NGT we love ‘soft’ engineering solutions. Our overarching philosophy is to work with nature to achieve rehabilitation solutions that integrate with the environment, mimic natural conditions, and make the most of the existing materials, processes and landforms. For this reason, we really love sandbags, especially for testing ideas and trialing solutions that are complementary with this philosophy. Their portability means we can carefully hand sculpture a wetland sill, observe how the wetland behaves and easily modify or adjust in real time where necessary. Sandbags are relatively cheap, offer a low risk solution and, after more then 15 years using them for many great outcomes, demonstrate an approach which stands the test of time. For many sites we have been able to eventually bury the sandbags (like this example at Long Swamp), to more permanently consolidate the work. However, while soft-engineering is our default, situations sometimes call for ‘harder’, more highly engineered solutions.
Recently we announced new funding for consolidation of previous works at our Mount Burr Swamp Restoration Reserve. Crucially, this included the need to construct of a fish barrier to prevent a highly invasive feral fish species (Mosquitofish – Gambusia holbrooki) making its way into what has become a thriving and biodiverse ecosystem since it was restored. These fish are really good at moving around but, fortunately for us, aren’t great jumpers. So the premise was that if we could establish a hard barrier and maintain a decent vertical drop, we should be able to stop them if (or when) they attempt to make their way into the drains downstream of the Mount Burr Swamp Restoration Reserve wetlands. Thanks to designs and lots of logistical support by South East Water Conservation and Drainage Board engineer Ryan Domleo, and oversight and construction by Jeffrey Briggs (who previously worked at the SEWCDB for 40 years, but we enticed out of retirement!) we now have something in place to ensure these little ferals will never crash our wetland party.
I have included some images below to give you an idea of how it came together. The lowest point of the weir is the same height as the sandbags we had installed and tested previously. You will notice that there is an upright post in the middle of the weir. Combined with slots on the left and right hand side, this allows us to insert boards to hold the water level higher, to provide some additional flexibility to manage water levels in the wetlands closest to the structure. After watching how everything behaved in the locally wet winter of 2023 (with the original sandbag weir in place) we know that under these types of conditions we may still want to be able to pass water out at lower level. But then, as the season progresses, when rainfall starts to drop off and evaporation takes over, we’ll have the ability to hold the water lever higher and make sure we keep the wetlands as full as possible going into the summer months. So aside from keeping the feral fish out, this new structure also enhances our capacity to maximise wetland habitat for threatened species like bittern, brolga, growling grass frogs, little galaxiids and Yarra pygmy perch during those crucial drier months. And or course the wetland dependent birds will really appreciate the additional habitat on offer.
Yes we love soft engineering whenever possible, but hard engineering solutions like this are sometimes required, and have an important place in our toolkit too!
The project at Mt Burr Swamp is being delivered by Nature Glenelg Trust, via by the Shorebird and Wetland Habitat program,
which is funded by the Australian Government and administered by the South Australian Government.
