Major autumn wetland restoration works completed at Sloping Main

Setting the scene: the importance of background research at a complex site

A couple of years ago (in 2023), NGT were invited by the Tasmanian Land Conservancy (TLC) to take a detailed look at the significant saltmarsh area (Burdens Marsh / Prices Flats) within their recently acquired Sloping Main Reserve, on turrakana / the Tasman Peninsula. This followed on from another successful restoration project we previously worked on together at TLC’s very first Reserve, Long Point, on the edge of the Moulting Lagoon Ramsar site.

One of the most important tasks was piecing together the history of the site and carefully documenting the many hydrological changes that have occurred over the past two centuries – a process that started when the first artificial drains through the marsh were hand-dug by convicts in the 1820s.

This information was assembled into a detailed timeline of change, outlining how and when specific wetlands in the reserve were impacted by past modifications, and a description of their likely impact on the hydrology and ecology of the site.

Most importantly, the report which NGT completed in July 2024 (Eco-hydrological Assessment and Restoration Plan for Burdens Marsh, cover shown right), allowed us to articulate a plan for working with the TLC to reverse these past impacts and set the site on a long-term trajectory of ecological recovery and adaption, while protecting and enhancing existing biodiversity values.

If you are keen to dive right into the details and learn more about this fascinating background story, please take a look at the report in this newsletter article from last year, or you can click on the image of the report to open it as a pdf document. Of the dozens of wetlands we have investigated over the years, this has to be one of the most interesting yet, so I promise you won’t be disappointed!

Sometimes when we write restoration plans, especially for larger sites, it might take several years for those plans to come to fruition as we patiently await the right funding opportunity to come along. Luckily in this case, we didn’t have to wait long to get started!

Thanks to the support of the TLC, Purryburry Trust, and a recent grant from the NRE Tas Threatened Species Partnership Grant Program (supported by the Tasmanian Government), the restoration plan was ready to be implemented in autumn 2025, just several months after it was finalised!

The restoration works phase: turning plans into action on the ground

Fast forward to today, and we’re now able to look back and review an epic 6-week period of major remedial earthworks across the Sloping Main Reserve that recently occurred in March and April 2025.

The visual snapshot below is presented in sequential order to align with the different elements of the restoration plan:

1. Tidal weir and levee removal

This task was required to restore the natural waterway and landform that connects the saltmarsh to the ocean, due to the weir and levee impeding the movement of freshwater and tidal flows (noting that, depending on conditions, it could naturally function as both an outlet creek and a tidal inlet).

The image shown (from the plan) outlines the work that was required to restore the form and function of the waterway, namely:
– removal of the tidal gate/weir (constructed c. 1990) across the natural creekline
– removal of the levee bank, and redistribution (red arrows) of the levee bank material into the void that was created at the time of its construction (c. 1990).
– the dashed black lines show the natural contours required to recreate the original bank profile and allow future accommodation of variable flow volumes in either direction.

Now that these works are completed (as shown below, looking in the opposite direction), the natural watercourse between the saltmarsh and the sea will once again be able to naturally breach, erode, open and close, as the prevailing climatic conditions dictate, without human intervention. The mouth of the watercourse, where it reaches the sea and (subject to flows) breaches the beach sand berm, is shown at the top right of the image below.

To round out this part of the story, here are a couple of other before and after perspectives shared by the TLC on their Instagram page…

2. Saltmarsh landform rehabilitation

This task was required to remediate the footprint of artificial drains, embankments, causeways and dams throughout the main saltmarsh area that, in turn, will restore natural inundation patterns to this vast wetland.

By distance and area, this task comprised the largest component of the recent remedial works, with NGT staff supervising two medium-sized excavators working full-time on the TLC Reserve, 5 days a week, for a whole month!

The method that was adopted for drain remediation, as outlined in our 2024 report, was to seek to achieve continuous backfilling – carefully returning previously excavated spoil material into the channels and reinstating the natural surface level of the saltmarsh. This method maximises the area available for saltmarsh regeneration and in doing so also minimises the possibility of any ongoing residual impact from the drain or embankment on the micro-topography (the surface profile) of the saltmarsh. In saltmarsh, very minor changes in elevation have a big bearing on where water moves (especially during tidal flows) and sits, and therefore determines what vegetation can grow where – so yes, micro-topography matters!

Because the design of drains and embankments across the saltmarsh varies, the landform remediation method was adapted to suit. Shown below are three common scenarios we encountered (please click on image to enlarge).

Firstly, here is an example of Scenario A – a remediated drain with a single spoil bank at ground level (photos: TLC)…

And this is a before and after oblique aerial view of another (very long!) remediated drain with single spoil bank through saltmarsh habitat (photos: Lachlan Farrington)…

Next up – to illustrate Scenario B – here is a short video recorded by NGT’s Bryan Haywood (who narrates the recording), which shows a good example of a drain with dual spoil banks (situated on either side of the drain) before and after remediation works…

And here is an oblique aerial view of one of the main arterial drains through the saltmarsh, again with dual spoil banks, shown both before and after remediation works…

In Scenario C, what we encountered was essentially the reverse of the previous example, where in this case material from either side was used to build up a causeway to improve vehicle access across the saltmarsh. Here is a nice oblique aerial perspective of one of the remediated causeways…

3. Restoration of freshwater wetlands to the east of the saltmarsh area, including Mungaratya / Turners Lagoon.

During the eco-hydrological assessment undertaken by NGT, it was fascinating to discover that the Sloping Main Reserve also contains a series of important freshwater wetlands to the east of the saltmarsh habitat that provided the main initial focus of the project.

This chain of wetlands, as shown below, includes four shallow wetlands, and one deep freshwater marsh, Mungaratya / Turners Lagoon. All of these wetlands except for one (the wetland outlined in yellow below) had been artificially drained over the past 60 years.

Having identified the opportunity to reinstate these wetlands as part of the restoration plan for the Sloping Main Reserve, we’re pleased to report that these wetlands have also benefitted from the recent remediation works.

The three drained shallower wetlands have had their natural outlet sill levels reinstated through drain backfilling, as illustrated by the example shown of the northern-most drained wetland (image taken while the works were underway).

The final, much deeper freshwater wetland, Mungaratya / Turners Lagoon requires a slightly different approach, because the historic height (sill level) of the western bank of the wetland (which controls water depth) is not able to be precisely determined from the information at our disposal. While we can estimate a level from surrounding natural ground, the artificial drainage works of the 1960s were actually superimposed right on top of the natural outlet location, so we need to test and monitor (and if necessary adjust) the restored trial sill level before locking it in permanently.

As shown below, while we were able to backfill the shallow drain across the bed of the lagoon (left), the outlet drain now has a geofabric lined spillway in place and set to a conservative minimum level (right), which we will be able to fine-tune during flows via by placing an adjustable geofabric sandbag trial structure within the spillway. This will allow us to set and test a restored full supply level for the lagoon when it next fills, before we eventually backfill the drain and make this change permanent in the coming years.

Autumn 2025 works: the bigger picture

If you are not yet entirely familiar with NGT’s wetland restoration work, we realise it can come as a bit of a surprise to see heavy machinery being used in this way (albeit very carefully) within a newly purchased private conservation reserve. However, it is incredibly important to remember that this type of landform remediation work – as shown below – is (a) an activity that only involves temporarily re-disturbing previously disturbed ground, and (b) is also the only way to fully and permanently reverse past hydrological impacts to the landscape of this scale and magnitude.

With these once-off works now completed, the restored hydrological regime will drive a positive trajectory of ecological recovery, and, as a TLC permanent reserve, those outcomes will be perpetually protected.

In a nutshell, ecological restoration is about playing the long game: taking the time to understand the drivers of ecological condition at a site carrying legacy impacts, then reversing past damage, and working with nature to drive spontaneous recovery.

We look forward to sharing news of the site with you in the future as this evolving good news story unfolds!

Acknowledgements

This project has benefited greatly from the involvement and support of a wide range of local people in the community, and volunteers of the TLC, who continue to contribute in various ways (including pulling down fencing wire at short notice!). A huge thanks to you all!

We would also like to specifically recognise and thank the previous owner, John Price, whose intimate knowledge of the property through his family’s connection over the past 100 years has been invaluable. There is certainly no better bloke to catch up with for a cuppa to chat about the history of Sloping / Slopen Main!

This project is being delivered by NGT in partnership with the Tasmanian Land Conservancy, with support from the Purryburry Trust and a grant from the NRE Tas Threatened Species Partnership Grant Program supported by the Tasmanian Government.