A drain cleaning project in the Northern Territory saw the equivalent of 2,800 box trailer loads of sediment removed from the Ludmilla Creek Trunk Drain, and formed part of larger flood mitigation works within the Ludmilla catchment to reduce the overflow of stormwater in the region.
The Ludmilla Creek Trunk Drain Reinstatement project removed 18,000 tonnes of sediment from the Ludmilla Creek Trunk Drain and 700 tonnes from the local drainage network in the Narrows, a massive amount that had built up over decades and was causing major problems for locals.
Built up sediment was contributing to localised flooding in the Narrows after small amounts of rainfall and even worse flooding during bigger storm events, as it would clog up the drain causing stormwater to overflow.
Graeme Finch, Senior Director, Infrastructure at the Northern Territory Department of Lands, Planning and the Environment, led the reinstatement project that enabled the local drainage network to resume its proper function and restored the drain to its original 1974 state.
Mr Finch said the project took three months to complete and was an important project due to changes in the way the creek operated.
“The trunk drain is the headwaters of a tidally influenced creek that has been significantly modified by roads and sewer infrastructure which has changed the natural flushing of the creek,” Mr Finch said.
“The drain has been cleared in the past but not to the extent of this project.”
BMD Urban was awarded the contract and works included clearing the accumulated sediment from the drain, treating the Potential Acid Sulphate Soil (PASS) with lime on a specifically constructed treatment lime pad, and disposing the treated material to landfill.
“BMD Urban used a range of 30 tonne excavators to dig the drain and load the sediment onto moxies to transport to the treatment pad,” Mr Finch said.
“The sediment was also mixed with lime by an excavator and loader until the potential acid sulphate was neutralised.”
Mr Finch said although a drain cleaning project might sound simple, there were several challenges that had to be overcome during the delivery of the project.
“All the material being removed was PASS which requires either treatment or disposal in a lined cell at a waste station.
“Disposal of waste in a lined cell in the Northern Territory is particularly costly and the lime needed to treat the PASS was not readily available and had to be hauled in from Mataranka, which is 400km from Darwin.
“Also, as a tidal mangrove environment, earthworks were particularly difficult so the contractor had to be innovative in how it managed the movements of machines (the moxies) along the length of the drain and how it supported the weight of excavators while digging out the sediment.”
In addition to the project group (including the Department of Lands, Planning and the Environment, the Department of Infrastructure, an environmental consultant and BMD Urban), a range of other stakeholders were also involved in the project, including the Northern Territory EPA, Department of Land Resource Management’s Weeds Branch, Darwin City Council, an independent third party environmental auditor and the Ludmilla Landcare community group.
Removing the sediment was a significant step in reinstating the drain to its optimal state, but Mr Finch said there are other stages of works still to be completed that will help reduce flood risk in the Ludmilla catchment.
“Sediment traps are being designed in detention basins upstream from the drain to limit the volume of sediment that enters the drain, with the second stage of works including an improved culvert under the sewer main that crosses the drain to improve natural flushing during high tides and flood events,” Mr Finch said.
“The second stage of works also include lining the base of the drain to provide a separation between the PASS below the base and any sediment that is deposited in the drain.
“After these measures are implemented, routine maintenance is all that should be required to maintain the functioning of the drain.”
The next stage of works at Ludmilla will also involve extending the drain to the culverts under Dick Ward Drive, increasing the natural flushing of sediment from the drain and reducing the need for further works to clear the drain of potential acid sulphate soil.
Mr Finch said the design work for the extension has been completed and all these works combined were contributing to mitigating flood risk and improving stormwater management across Darwin.
“The reinstatement of the drain is a fundamental part of the flood mitigation works within the Ludmilla catchment as without a functioning trunk drain, a major subcatchment can’t function as it is designed.”