Reducing Acid Water Impacts in Anna Bay

Client: NSW Department of Primary Industries (DPI)
Year: 2008-2009
Project Reference: 08087
WRL Technical Report: Anna Bay Drainage Study: Reducing Acid Water Impacts via Water Control Structures and Tidal Restoration (2009/12)

Anna Bay is located approximately 60 km north of Newcastle on the mid-north coast of New South Wales at Port Stephens. The catchment is low-lying and an agricultural drainage network has been constructed to facilitate removal of water from the catchment. Currently floodgates on the end of the drainage system prevent tidal intrusion.

Anna Bay drainage system affected by acid sulphate soils.

Acid sulphate soils occur naturally throughout the catchment. In natural drainage conditions these soils do not have an environmental impact, but due to the extensive drainage network, the soils have oxidised and acidic by-products are now polluting the surface waters. The Water Research Laboratory has been working with NSW DPI to determine an acid sulphate soil management strategy that will limit acid drainage without impacting catchment/floodplain drainage.

Two remediation techniques have been proposed for the site: a low-set weir to maintain a higher surface and groundwater table, and a modified floodgate that allows tidal water to intrude and neutralise acidic water.

A numerical (i.e. computer) model was developed, calibrated and applied to the site, to assess the impact of the two proposed remediation actions. The model was developed using the DHI software MIKE Flood. Several scenarios were simulated in the model to determine the impact of each remediation action on the site. Further on-ground and modelling work is underway.

This ArcGIS image highlights the drainage network and the flooding likely to occur immediately after a large flood. MIKE Flood modelling was undertaken to determine how the water would drain under a range of scenarios.