Investigating the hydrology and restoration potential of Lisdillon Lagoon, east coast Tasmania

Lisdillon Lagoon is located on the east coast of Tasmania, between Rocky Hill and Pontypool, south of Swansea. Like many coastal lagoons, Lisdillon has a history of hydrological changes, including the previous installation of a substantial levee bank that is impeding natural flows.

As long-term readers of the NGT newsletter may already appreciate, before any restoration work is initiated, we first like to take some time to better understand the changes have been made and their ongoing impact. Although this is currently an unfunded project, we’ve been able to undertake some initial voluntary research and baseline monitoring to gain some insights into the hydrology of Lisdillon Lagoon.

Lisdillon Lagoon is primarily fed by Lisdillon Rivulet, which flows off the ranges to the west and out to the ocean via the southern end of Lisdillon Beach, into Great Oyster Bay. The lagoon also receives minor freshwater runoff from surrounding land to the north.

A substantial embankment was built across the narrow southern opening between the lagoon and rivulet in the early 1990s, as part of an unrealised aquaculture venture. In fact, this was the very same venture which also resulted in significant drain construction and earthworks at The Grange, north-east of Swansea, and including the area now known as the TLC’s Long Point Reserve. Those aquaculture works (and much earlier levee banks constructed by convicts) at The Grange and Long Point were largely remediated by NGT in 2022 and 2023, as shown in this previous article.

Prior to the 1990s, there is also evidence of a minor causeway in the historic aerial imagery at Lisdillon Lagoon in the same location as the newer, much larger embankment. This appears to have been constructed by a previous landowner many decades earlier, possibly even as far back as the 1800s, and may have been used to facilitate dry-weather access from west to east. This ‘short cut’ would have saved a lot of time, by removing the need to travel the long way around the north of the lagoon.

It is likely that the modern, more substantial levee embankment has impacted the effective exchange and equalisation of both tidal and freshwater flows, and therefore water regime, water quality and saltmarsh condition and extent since that time. We anticipate that this impact would be continuing despite an adjacent landholder putting a minor cut in the levee prior to 2020, to allow for some exchange to occur once more.

The aerial photographic record sums up these changes very clearly, as shown below.

In 1949, the remnants of the old causeway crossing are more visible than in the image from 1988. In 1992, the newly constructed major embankment first appears in the same location, and then in the 2024 image, the minor cutting in the levee can also be seen.

Out on the ground today, erosion of this cutting that has occurred over the past 5 years is apparent, due to a concentration and increase in the velocity of flows at this artificially narrow opening. Indeed, we have witnessed the rapid flow of water through this cutting upon turn of the tide during field visits, as shown below.

Since one of the adjacent landholders contacted us in August 2023 to discuss potential future restoration opportunities, we have been able to set up and begin collecting baseline hydrological data from April 2024, both upstream and downstream of the levee. We were interested to see what impact the levee might be having on both water level and salinity.

The data gathered so far tells an interesting story, that – as we expected might be the case – indicates that the embankment is continuing to impact the Lagoon, even it its current form. For example, as shown in the charts below, note how salinity of water in the Lagoon (i.e. upstream of the embankment) is consistently much higher than that of the tidal Rivulet (i.e. on the downstream side of the embankment). While some natural concentration of salt in the Lagoon might be reasonably expected, it is (a) the magnitude of the difference shown here, and (b) the relatively close proximity of our monitoring sites, that indicates a clear link in this data to the constriction of flows caused by the levee.

Also note how the temporary dips in salinity seen above, coincide with rainfall events as shown in the graph below – June, July, September and December.

At Lisdillon Lagoon, the potential future hydrological restoration works required at this site appear to be more straightforward than at many of the other sites we’ve investigated over the years. Ideally, the levee would be removed in its entirety, with the embankment material simply placed back into the borrow pits nearby, where it originally came from. This in turn would enable the full natural exchange of flows (i.e. flows of original volumetric capacity) to occur between the Lagoon and Rivulet (and vice versa) – allowing for more effective equalisation of water level and salinity throughout the site.

We thank the local landholders for their patience, while we await an appropriate funding opportunity that will enable us to complete this important restoration project and contribute to the recovery of another significant area of wetland and saltmarsh habitat on the east coast of Tasmania.