Sea Level rise and potential mitigation of saline intrusion in Northern Australia

Australia’s northern coastline is home to many coastal freshwater wetlands of international significance. They are high in biodiversity, are a large carbon store and have economic significance to tourism and pastoral production. In recent decades they have been degraded and transformed to different degrees due to sea level rise, severe storms and buffalo activity. 
The Mary River estuary is modelled using 2D finite volume MIKE 21 software to assess its resilience to sea level rise and the effectiveness of engineering intervention to mitigate sea level rise of up to 0.8m. The Mary River was chosen for its long history of channel extension and widening leading to extensive saltwater intrusion. 
Coastal wetlands of the Northern Territory are very expansive and low-lying. The area between 0 and 5m above mean sea level is over 10,000km2 , and a significant amount of that land is below the spring high tide level. Small thresholds in elevation are then critical to the inundation of very large areas. The Mary River is extremely vulnerable to small rises in sea level (0.3m), while a 0.8m rise will create expansive salt water environments in previously freshwater wetlands. 
Submerged weirs were tested for their effectiveness in reducing the environmental consequences of sea level rise. Floodplain inundation and channel salinity levels are marginally affected by the simulation of submerged weirs in present day scenarios. As sea levels rise, weirs serve to increase both the floodplain inundation extent and salinity concentrations, relative to sea level rise scenarios without weirs. Weirs are very effective at reducing flow velocity peaks as wet season floods initiate and recede, due to dampening of the hydraulic potential. Velocities were reduced to levels below the critical entrainment velocity of 0.4ms-1 for the silt/sand sediment found in these areas. This would result in a reduction in channel widening and headwater extension, both key factors in sea level rise mitigation. Although weirs can reduce erosion potential, their negative properties related to inundation and increased salinity makes them a counterproductive sea level rise management tool. 
It is concluded that submerged weirs may not be effective in reducing the impacts of sea level rise on wetlands of the Northern Territory.