Floodgate management
The effective management of floodgates depends on their location and the issues that are linked to the structure. As with other fish passage barriers, if a floodgate is on a natural watercourse, and it is no longer required and no longer serves its original purpose, removal rather than management is the preferred option. Where floodgates are present on constructed drains, fish passage is likely to be less important. Here managing water quality impacts such as acid sulfate drainage will be more important and will guide floodgate management.
Solutions
To improve fish passage and water quality, the controlled opening of floodgates during non-flood periods is generally the best solution. A number of simple, yet effective engineering - or structural - solutions have been designed to achieve this. These structural solutions also have to be considered alongside social and economic factors.
Structural solutions
A structural solution includes any physical modification that allows for regulated amounts of water to flush the system during non-flood periods. Different designs of managed floodgates are available including:
- A winch system lifts the whole floodgate clear of the water column, allowing for maximum flushing. This type needs someone to operate the winch on a regular basis to achieve the best results.
- A sluice gate is comprised of a hole cut into the main floodgate with a sliding metal panel that can be adjusted to cover more or less of the aperture. This requires less operational intervention than a winch.
- A tidally-operated floodgate also has an aperture with a panel, but the panel is opened and closed by a float mechanism on the downstream side of the floodgate. As the water level rises (due to floods or high tide) the panel is closed shut. When water levels fall to acceptable levels, the float opens the panel over the aperture allowing for controlled flushing again. Tidal gates do not require regular, active intervention once they have been installed.
- Smart gates. This design automatically opens and closes floodgates (or a small aperture in the floodgate) when sensors identify changes in water quality.
Smart gate
Economic solutions
These include providing incentive payments for the installation of floodgate modifications, stewardship payments for alternative land use and possible removal of structures where they are no longer required.
Social change solutions
Better results are achieved by working closely with local Councils and farmers. When the benefits of active floodgate management are recognised by the stakeholders for themselves, the adoption of improved management techniques accelerates.
Monitoring and communication
This includes comprehensive monitoring programs of fish stocks, water quality and vegetation. Communication involves distribution of educational material such as brochures, posters, broadcast quality video, and websites.
Benefits
The benefits of opening floodgates during non-flood times are many and varied and include:
Enhanced water quality
A return to natural flows through managed floodgates leads to rapid and sustained improvements in water quality including higher dissolved oxygen levels, moderated temperature fluctuations and less acidity.
Improved fish passage and habitat
Opened floodgate systems have been shown to have a greater fish and prawn diversity and far higher numbers of individuals, than unmanaged sites. This is due to both improved fish passage and enhanced habitat values. Click for more information on rehabilitating the habitat of Eastern King Prawns.
Reference: "Boys, C.A., Kroon, F.J., Glasby, T.M. and Wilkinson, K. (2012) Improved fish and crustacean passage in tidal creeks following floodgate remediation. J. App. Ecol., 49: 223–233This graph shows the change in abundance of juvenile Yellowfin Bream resulting from active floodgate management in the Clarence River. Fish populations around managed gates, unmanaged gates and natural creeks (no gates) were compared before and after gates began to be managed. Active floodgate management involved opening floodgates during non-flood periods to allow for fish passage and tidal flushing. During the study this could be for as short as 5 minutes at a time to as much as 300 minutes.
Better agricultural production
Active floodgate management enhances production for both farmers who graze stock and those who grow crops. Instream weeds are controlled naturally and wetlands once again become important drought fodder refuges for stock.
Acid sulfate soil management
Through the relatively simple tool of opening floodgates in non-flood periods, acid scalds can be kept moist to reduce further acid production and lead to a whole suite of flow-on benefits. At some sites however, where the volume or extent of acid generation is high, effective management will require the use of alternate methods such as the construction of low weirs that continue to block fish passage but prevent acid generation and release into adjacent waterways. Click here for more information on Acid Sulfate Soils.
The steps to managing a floodgate
Some new and existing floodgates will require a permit from DPIRD Fisheries, see also Council and Developer Toolkit for permit information.
Most floodgate management projects involve the following steps:
- Identify a potential site
- Contact the manager / owner of the floodgate (usually the local council or a landholder)
- Discuss the proposal with all landholders upstream of the floodgate. Assess the suitability of the site in terms of any areas of low elevation, salinity levels, tidal range, soil permeability, etc.
- Conduct any necessary "before" monitoring such as water quality, fish monitoring, vegetation photo-points, etc.
- Select the appropriate style of floodgate modification
- Install the floodgate modification
- Monitor water levels, particularly at any areas of low elevation, and conduct any complementary "after" monitoring.
- Adjust the modification to allow in more or less water as necessary
- Usually a written agreement is signed by all stakeholders to clarify the aims and responsibilities of those involved.
- Some new and existing floodgates will require a permit from NSW DPIRD Fisheries.
Restoring the balance: Guidelines for managing floodgate and drainage systems on coastal floodplains
Guidelines for managing floodgates and drainage systems have been developed from the results of a Fisheries Research and Development Corporation (FRDC) project managed by DPIRD Fisheries.
In the field, different opening regimes of floodgates (particularly one-way flap valves), and different gate structure types were investigated to determine how effective they were in allowing movement of juvenile fish and invertebrates past the structures and providing improvements in water quality.
Results showed that the numbers and biomass of estuarine fish differed significantly and consistently between drainage systems with and without floodgates.
An increase in opening frequency of floodgates resulted in a significant increase in the number and diversity of marine species. However, improvements to fish passage rapidly disappeared if floodgates were not opened for prolonged periods of time. It was recommended that continued, active management of floodgates (i.e. frequent and regular opening) should be maintained once it has commenced.
Interestingly, juvenile fish and prawns moved into drainage systems with opened floodgates, regardless of whether the system was a modified natural creek or a man-made drain.
Opening of floodgates resulted in significant improvements in water quality in managed drainage systems, including reduced total phosphorus, phosphate, and total aluminium concentrations, but not total nitrogen. To improve water quality, it was strongly suggested that, in addition to opening floodgates, best land management practices should be implemented to reduce nutrient input (e.g. reduced and/or more effective and efficient use of fertilizers, fencing off water courses, and rehabilitation of riparian vegetation).
In addition to improvements in water quality and fish passage, opening of floodgates also resulted in significant improvements in habitat quality in managed drainage systems, including the disappearance of waterlilies, grasses and rushes, most likely due to tidal influx.
In the laboratory, researchers tested whether juvenile fish and prawn species would actively avoid acidic water - a common feature of drained floodplain wetlands. They found that juvenile Australian Bass, Snapper, Yellowfin Bream and School Prawns all showed avoidance of acidified water.
Researchers concluded that active avoidance behaviour may seriously affect migration patterns in areas with acid sulfate run-off. This would reduce the capacity of habitats near or downstream of acid discharge points to act as spawning or nursery areas.
Active reduction of acid sulfate run-off through improved floodgate management can therefore not only benefit land management activities, but also have a positive impact on commercial and recreational fishing activities.
Click here to read Restoring the balance: Guidelines for managing floodgate and drainage systems on coastal floodplains
Floodgate management in NSW
NSW DPIRD Fisheries has been involved in assessing and assisting to improve management of floodgates since 1999. At this time the North Coast Floodgate project aimed to improve coastal floodplain management practices through trials of floodgate modifications located in rivers between the Manning and the Tweed.
Following devastating fish kills in the Richmond, Clarence and Macleay rivers after major flooding in February and March 2001, active floodgate management was identified as a tool to assist in reducing the frequency, severity and duration of fish kills. In response, the NSW Government funded structural modifications to floodgates and improved floodgate management regimes in 57 subcatchments. This led to over 660 kilometres of habitat being restored to a more natural flow regime with improved fish passage.
Landholders reported seeing large schools of mullet, prawns and other species in areas previously devoid of fish for decades. Extensive water quality monitoring showed rapid improvements to dissolved oxygen levels, pH values and more moderate fluctuations in water temperature following floodgate openings. Tidal flushing was also shown to reduce the accumulation of toxic drain sediments, which were implicated in the deoxygenation processes that led to the 2001 fish kills.
A screen shot of an online floodgate mapping tool.More recently the Marine Estate Management Strategy’s Coastal Floodplain Prioritisation Study project has assessed coastal floodplain systems and infrastructure (e.g. floodgates, levees and drains) in 7 priority catchments across New South Wales - the Tweed, Richmond, Clarence, Macleay, Hastings, Manning and Shoalhaven River estuaries.
To assist decision makers improve water quality, habitat and biodiversity, the project researched ways to identify sources of water pollution from coastal floodplains and how to reduce the risk. It also researched how vulnerable floodgate infrastructure and floodplains are to sea level rise.
Data collected, including floodgate locations and details, has been collated into reports for each of the catchments. The reports and an online mapping tool can be accessed through the Coastal Floodplain Prioritisation Study webpage (under “More information” on that page).