Assessing sea level-rise risks to coastal floodplains in the Kakadu Region, northern Australia, using a tidally driven hydrodynamic model

The low-lying coastal floodplains of the Kakadu Region in tropical northern Australia encompass World Heritage Kakadu National Park and are highly vulnerable to future sea level-rise (SLR) and extreme weather events, yet there are no modelling tools to assess potential impacts of saltwater inundation (SWI) on freshwater ecosystems and to evaluate future management options. A tidally driven hydrodynamic model was developed to simulate the frequency and extent of SWI in the Kakadu Region for the following four mean SLR scenarios: 0 m (present-day, 2013); 0.14 m (2030); 0.70 m (2070); and 1.1 m (2100). Simulations were undertaken at 60-m spatial resolution using October dry-season tides, and a digital elevation model (0.10-m vertical resolution) constructed from LiDAR point cloud data was used to resolve coastal and river-system terrains. Model outputs (maximum extent and frequency of SWI) were used to assess potential loss of freshwater floodplains for each scenario at a park-wide scale and for three case-study areas that differ in tidal influence. Results show little loss by 2030 (+3%), a possible threshold effect by 2070 (-42%) and ameliorating after 2100 (-65%). Although freshwater floodplains further from the coast showed least exposure to simulated SLR, indicating potential refuge areas, all floodplains on Kakadu will be exposed to SWI by 2132 (+117 years).