OzSET: integration and publication of the Australian Surface Elevation Table dataset

A useful measure of resilience is the extent to which coastal wetland sediments can accrete vertically at a rate matching that of sea-level rise. This requires monitoring vertical accretion, subsidence and elevation gain across a range of coastal settings.

The Surface Elevation Table-Marker Horizon (SET-MH) technique and associated instrumentation measures the vertical accretion of coastal wetland sediments. This allows the assessment of whether wetlands are keeping pace with sea level rise (measured at tide gauges) or subsiding relative to local sea level rise and thus vulnerable to permanent inundation and loss. This knowledge can underpin spatial models of the resilience of coastal wetlands to sea level rise, and facilitate coastal planning, estimates of blue carbon sequestration, and potential changes in other ecosystem services.

The global network of SET monitoring stations is concentrated in the North America, Europe and Australia, though SETs are found in Asia, Oceania, Central America and the Caribbean, South Africa, South America, India. Australia has an extensive network of surface elevation tables in coastal wetlands. The majority were installed in 2000–2001, funded by state and local government agencies seeking better information on the resilience of coastal sedimentary environments to sea-level rise. They have been maintained by Marine and Coastal Hub partners including Macquarie University, University of Wollongong, University of Queensland, University of Adelaide, Charles Darwin University and Edith Cowan University.  The Australian network is one of the largest, but has hitherto developed without national coordination, and the data gathered have not been readily available to the research community or research users.

This project collated Australian SET data and provided an initial analysis of accretion and surface elevation trends.

Approach and findings

The OzSET project identified and collated existing data for 268 SETs across four states and the Northern Territory. The network is clustered near major populations centres of Brisbane, Sydney, Newcastle, Wollongong, Melbourne, Perth and Darwin. SET installations are mostly in mangrove forests, but also cover a range of tidal marsh and tidal forest ecosystems. Core data collated for each SET-MH installation include:

• rate of sediment accretion: sediment accumulation above the baseline for each measurement period, and the linear trend through time;
• rate of elevation gain: elevation in relation to the benchmark (vertical position at installation) for each measurement period, and the linear trend through time;
• rate of upper level subsidence: difference between the rate of sediment accretion and the rate of elevation gain; and
• elevation deficit: difference between rate of sea-level rise and the rate of elevation gain.

Mangroves were found to have higher rates of accretion and elevation gain than all categories of tidal marsh, a result attributable to their lower position within the tidal frame (promoting higher rates of accretion) and higher biomass (with potentially higher rates of root growth). While Australian tidal marshes show an increase in elevation over time, this elevation gain did not match the rate of water level increase at more than 80% of SET stations. High rates of accretion did not translate into high rates of elevation gain, because the rate of upper level subsidence increased with rate of accretion. This association between accretion and subsidence has been noted for tidal marshes in global syntheses, but was particularly strong in the Australian network, with 87% of variability in upper subsidence explained by the accretion rate.

Outcomes

The coordinated SET-MH platform provides a national resource for research and management agencies to monitor the resilience of coastal wetlands with sea level rise and other climate change impacts. These data can be used to validate remotely sensed products, underpin modelling of the impacts of sea level rise on wetland distribution and monitor variation in soil accretion and carbon sequestration over time.

In addition, information on shoreline trends in the vicinity of SET-MH installations will be accessed from Digital Earth Australia Coastlines, a continental dataset that includes annual shorelines and rates of coastal change along the entire Australian coastline from 1988 to the present. The comparison of data will provide insight into processes influencing shoreline change.

Next steps

Further development of the Australian SET network should focus on under-represented wetland types (specifically tidal forests) and geographic locations (Darwin Harbour and the Daintree being the only sites in the top end). Long-term datasets are required to clarify the strength of feedbacks between sea-level rise and marsh accretion, and while some of the Australian sites have records spanning two decades, many are new and ongoing effort in monitoring is required.

The Australian SET network is well placed to guide the development of landscape-scale sea-level rise visualisation tools. Products developed in Australia and publicly available currently lack distribution and accretion models for coastal wetlands. As a result, the resilience of coastal wetlands to sea-level rise scenarios is poorly represented, and landward translation not currently included in tools such as Coast Risk Australia.

The sea level rise visualisation tool released in 2021 by the US National Oceanographic and Atmospheric Administration (NOAA) has both functionalities, allowing users to model the distribution of tidal wetland types under IPCC sea-level scenarios. The NOAA tool’s marsh migration model (https://coast.noaa.gov/slr/#) is driven by accretion models derived from the United States SET network, and illustrations the potential for further development and application of the Australian network.

Project location

National