Floodgates

Floodgates were first installed in Australia in the late 19th century. However, the majority of these structures were installed after major flood events in the 1950s and 1970s. They are designed to prevent inundation of low-lying floodplains by high tides or flood events. Floodgates were usually constructed from steel. Standard floodgates operate passively as one-way valves. Water is prevented from flowing upstream by the hinged flap and yet can flow out and downstream when water level allow.

In tidal reaches, waterway crossings (especially those over agricultural drains) commonly incorporate floodgates that restrict fish passage between flood events. Between floods, floodgates are generally maintained in the closed position thus ensuring a complete blockage to fish migration between estuaries and tidal tributaries."

Impacts of floodgates

Floodgates have been shown to have a range of unintended impacts. These include:

• Poor water quality: Floodgates prevent regular flushing which leads to stagnant conditions, sedimentation, algal blooms, exposure of acid sulfate soils, extreme temperature fluctuations and low dissolved oxygen levels, resulting in longer, more frequent and severe fish kills.
• Reduced fish passage: Floodgates act as a physical barrier to fish migration.
• Reduced agricultural production: The use of floodgates can cause excessive drainage of floodplains which leads to more aquatic weeds, less available drought fodder, reduced frost tolerance and a propensity for peat fires, meaning that agricultural production is reduced.
• Exposure of acid sulfate soils: Over drainage results in oxidation of acid-sulfate soil, causing release of deadly sulfuric acid and toxic metals – which can and have, led to fish kills. Longer term effects of acid sulfate soils on aquatic life include greater susceptibility to disease and stress, reduced reproductive fitness and avoidance behaviour by juvenile fish.

Floodgates can reduce estuarine fish stocks, as they deny juvenile fish and invertebrates access to habitat and food upstream of these structures. Recreationally and commercially important species of fish that can be affected include Australian Bass, Sea Mullet and School Prawns.

Floodgate audit in NSW

An audit of floodgate structures has been undertaken for the Manning River floodplain, south to the Victorian border, assessing a total of 521 floodgates and identifying 383 structures as obstructions to fish passage. Of these, 73 sites were classed as high priority for remediation. Remediation options included:

• Removal (for redundant structures)
• Basic management/maintenance
• Modification of current active management, and
• Active floodgate management

These recommendations form the basis for ongoing floodgate management in NSW.

Some links to more information are found below.

Fish passage database

NSW DPRID Fisheries continues to map and assess floodgates and other barriers to fish passage across the State in order to understand and improve fish habitat.

To view structures in your area of NSW visit the NSW Fisheries Spatial Data Portal.

The database includes the location of over 13,800 in stream structures within Key Fish Habitat. The data is fully downloadable as a shapefile from the NSW Fisheries Spatial Data Portal under a Creative Commons licence requiring credit be provided to DPIRD Fisheries.

The database is an ongoing initiative. As such, it will contain errors, omissions and outdated information. The accuracy will continue to improve as updates are made, feedback is gained, and new sites are added bi-annually or as required.

If you find a barrier to fish passage that is not showing on the spatial data portal (or even better - one that has been fixed), please report it by following this link and choosing "open in web browser".

If you are having difficulties doing this, you can always report a barrier to fish passage by getting in touch with your local fisheries manager at fish.passage@dpi.nsw.gov.au.