Restoring the balance - guidelines for managing floodgates and drainage systems on coastal floodplains

Abstract

Many coastal floodplains in Australia have an extensive network of floodgates, constructed drains and modified water courses. These are designed to mitigate the impacts of floods and large rainfall events. Floodgates prevent flood waters and tidal brackish water from inundating low areas of the floodplain. Constructed drains have converted prior wetlands into dryland farming areas. Whilst these developments have enhanced rural settlement and industries they have also caused unintended adverse impacts to fisheries, the ecology of estuaries and downstream water users. The expanded drainage network has increased the generation and export of acidity from acid sulfate soils. Drainage systems can rapidly transfer acidity and deoxygenated water from backswamp areas to creeks and estuaries after rain. Floodgates and constructed drains have also blocked fish movement to upstream habitat areas and provide conditions that are conducive to the formation of poor water quality, particularly water with low dissolved oxygen. These guidelines outline principles and strategies which can be employed to improve the environmental performance of coastal floodplain drainage systems, while retaining their benefits for agriculture. They have a particular focus on reducing drainage of acidity from areas with acid sulfate soils. The benefits, limitations and risks associated with management changes are described.