Water and sediment movement in the Alligator Rivers estuaries

This project was focused on the coastal and estuarine regions of the Alligator Rivers, which are wholly located within the world heritage area of Kakadu National Park. The pulse of large tides drives huge volumes of water and sediment in and out of the Alligator Rivers estuaries twice each day. The dynamic movement of water and sediment has a significant impact on the aquatic biodiversity of the region by creating different types of habitat. It also strongly influences water quality and light penetration, which in turn affects the growth of plants including algae.

This project created hydrodynamic, sediment transport, water quality and tide models for the Alligator Rivers estuaries. These models can be used to predict the impacts of a range of coastal change, land use and climate change scenarios. The models have already been used by other teams undertaking ecological research in the region.

Climate change is predicted to result in sea level rise and more intense rainfall and cyclone activity which will increase the risk of saltwater moving into areas within Kakadu National Park that are currently fresh water systems with high levels of biodiversity.  Salt water intrusion will change the salinity of these systems which could have potentially disastrous effects on biodiversity, tourism and cultural values.

The Alligator Rivers are also the focus of other ecological and social research which will benefit from a better understanding of water and sediment flows and how these will change under various climate change scenarios.

Detailed hydrodynamic models were created based on extensive field measurements. The research team surveyed the bathymetry (bottom of the estuary and channels) and measured tides and water velocity during both the wet and dry seasons. The team then used LIDAR remote sensing to survey the adjoining floodplain and the tributary channels, which carry water between the estuary and floodplain. These models were compared against observations researchers took in the field. They were then used to run a number of potential future scenarios, such as elevated sea levels, to predict the effects on the estuary, coastal floodplains and freshwater wetlands.

This project created hydrodynamic, sediment transport, water quality and tide models for the Alligator Rivers estuaries.  These models are particularly relevant to research looking at estuarine plant and animal species, as well as the management of water quality. They show that saltwater intrusion depends on the rate of sea level rise, as well as how stable the channel banks are — which is also partly dependent on sediment transport.

The models can be used as a base on which to develop strategies in areas where managers choose to undertake actions to mitigate the effects of saltwater intrusion. Understanding where and how salt water travels into floodplains is vital to planning any development. The models can also be used for other applications such as combining with fish data to look at fish passage, or the movement and fate of herbicides used in the control of weeds on floodplains.

This project was focused on the coastal and estuarine regions of the Alligator Rivers, which are wholly located within the world heritage area of Kakadu National Park.

The South Alligator is the only river system in Australia that is completely contained within a national park. The two Alligator river systems are only minimally disturbed by outside human activities and they encompass a wide range of features such as gorges, wetlands, escarpments, coastal floodplains and estuaries. This made it an ideal location to research natural macro tidal estuarine processes and how they interact with the adjacent freshwater wetlands and riverine systems.