The Gellibrand River estuary is approximately 8.7km’s in length and flows through a variety of different landscape types before it enters the ocean directly into the Twelve Apostles Marine National Park.

The Gellibrand River flows out to Bass Straight near the town of Princetown in south western Victoria. The Gellibrand River estuary is classified as an Intermittently Closed and Open Lake and Lagoon (ICOLL) estuary, a type of wave-dominated estuary. This estuary closes intermittently due to the formation of a sand berm at the estuary entrance.

When the estuary entrance is closed, the build-up of water behind the sand berm can cause inundation of floodplain areas. The limited flushing and turnover of the water in closed estuaries can lead to a decline in water quality conditions and adverse ecological impacts.

At times when the estuary mouth is closed, there is a threat of flooding and damage to infrastructure and agricultural land, there can also be a threat to aquatic fauna due to adverse water quality outcomes. Artificial estuary openings are undertaken to alleviate these adverse outcomes.  However, artificial estuary openings can also contribute to adverse water quality outcomes, depending on the tidal exchange, inflows from the catchment and draining of water from the floodplain.

Alluvium led the study is to provide information and recommendations relating to flow in the Gellibrand River that will be used to inform the decision-making process when considering an artificial opening of the estuary during low flow conditions. This investigation used a data-driven approach to understand the complex processes that are occurring in the Gellibrand River estuary during estuary closure periods and openings.