Australia’s coastal floodplains have many kilometres of constructed drains and modified watercourses with floodgates. While floodgates help prevent tides and small floods from overtopping the land, they create a stagnant environment in the drain. This can favour the development of poor water quality in the drain.
Water quality varies a lot between drains, and even within the same drain over time. Some drains have acceptable water quality, while others have very poor water quality from acidity or low dissolved oxygen. Drains in areas with acid sulfate soils may intercept acid groundwater and can export this acid along with toxic metals to the estuary.
Controlled opening of floodgates can improve water quality by allowing river water into drains (Figure 1). River water can help neutralise and dilute acidity and increase and stabilise dissolved oxygen levels. While saline water generally has a greater capacity to neutralise acid, it needs careful management (see Managing salt water when opening floodgates).
As a general rule of thumb, the improvement in drain water quality is related to how often the gates are open. It is best to have regular openings (i.e. every day) even if they are small, using automated devices like tidal floodgates (see Figure 2). Irregular or infrequent openings provide limited benefit, as water quality can quickly decline again once floodgates are closed.
There are many different types of floodgate opening designs and devices available. Some are operated manually, while others close automatically when water levels reach a pre-set height (Figure 2). Automatic devices are preferred as they provide:
Each floodgate device has advantages and disadvantages and it is important to choose a design that suits both the drain and your objectives (see Floodgates and retention structures: choosing the right design for more information). A more detailed review of different floodgate devices and designs is available at the website listed under 'Further information'.
Before opening floodgates it is important to think about what you are trying to achieve. There are some risks associated with opening floodgates. These risks need to be understood and appreciated. However, they can be easily managed. Proper site assessment is essential and will help you predict any risks, allowing you to open floodgates with confidence.
One risk is leaving the floodgate open during a rise in river levels, causing local flooding. This is only an issue with manual opening devices and is very simply avoided by using automatic devices that close as water levels rise (i.e. tidal floodgates).
Another important risk for agriculture is overtopping of land with saline water. This is also best avoided by using floodgate opening devices that automatically close when water levels reach a set height (Figure 2). Monitoring river salinity levels can also be done with inexpensive hand-held meters. Seepage of salt water into adjacent groundwater may be an issue at some sites with very permeable soils (see Managing salt water when opening floodgates).
Assessing the drain before opening the floodgates is vital. Various state and local government organisations have expertise in this kind of assessment and can be contacted for assistance. Important things to consider in any assessment include:
Monitoring water quality and water levels in the drain is important and can contribute useful information to your drain management.
For further information, advice and assistance on floodgate and drain management contact NSW DPI.
Citation: Johnston S (2004) Floodgate and drain management on coastal floodplains leaflet 1: Managing floodgates to improve drain water quality. (NSW Agriculture: Wollongbar)