Vegetated drains

Vegetated drains — Construction and operation

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• Overview
• Key considerations
• Planning and design
• Construction and operation
• Links and references

Approvals

Legislation and approvals may be required for the construction of a treatment system. Contact your local government before any construction is undertaken to understand requirements.

For example:

• As the hydrology of the area may be altered (e.g. introduction of more flow through the vegetation or redirecting flows), the sensitivity of the existing vegetation to periodic or extended wetting should be considered as should the potential impacts on any downstream wetlands and waterways.
• A new drain involving earthworks in the vicinity of an area mapped as a wetland protection area will likely trigger an approval process.

Construction

Vegetated drains usually involve modifying or managing existing drains and therefore should require minimal, if any earthworks. New drains or significantly altered drains (i.e. to provide lateral benches) will require earthworks to flatten the batters or construct benches.

Drains will generally be constructed at the same time as the farm or block is being established, involving earthworks. The earthworks should avoid disturbing acid sulfate soils and shallow bedrock. To minimise sediment loss, construction should occur in the dry season and groundcover established as soon as possible so soil is not left bare.

Topsoil should be replaced in the drain and batters following construction to provide a suitable substrate for vegetation growth.

Establishment

Vegetated drains need to be well vegetated using a mix of native grasses, sedges and rushes. Plants with a spreading/rhizomatous habit are ideal to stabilise the batters, while clumping reeds and sedges tend to grow taller and are better suited to the base of the drain[1]. Introduced pasture grass weeds such as hymenachne and paragrass can readily dominate drains and grow across the water surface. They can impact treatment performance if they restrict flow, causing water to stagnate. Weeds like these will need to be controlled in drains, prior to establishing desirable vegetation.

The most cost-effective approach is to seed the system with a mix of native species, or if suitable vegetation exists nearby on farm, allow the vegetation to recruit or colonise naturally. Natural recruitment runs the risk of erosion, increased weediness and increased maintenance and may need to be supplemented with seed.

Use local guidelines if they are available to assist with plant selection or contact your local Landcare or NRM group. Naturally vegetated systems such as waterways, wetlands and riparian zones are a good reference from which to create a species template. See plant guides.

Irrigation and weed management may be needed until the vegetation is fully established. The risk of not achieving the desired vegetation density is poor treatment performance, potential for erosion and the colonisation of weeds. Therefore, it is important that the vegetation in the drain is established successfully. Establish vegetation cover before the wet season to avoid young plants drowning or being damaged during flood events.

A vegetated drain can be constructed and established within a few months or it can take two to three years to establish vegetation depending on site conditions[3].

Operation

Covered in maintenance (below)

Monitoring and maintenance

Ensuring adequate, dense vegetation growth in most of the drain (or in nodes/sections of the drain) is the key maintenance objective[1]. Vegetated drains should be inspected every 6 months or after major rain events, to check vegetation cover is intact and ensure there is no erosion or sediment build-up.

Typical maintenance will involve:

• Weed management to prevent undesirable species taking over the drain. Weeds should be controlled by slashing or selective herbicide, to avoid killing desirable species. Monitoring of drains in the Wet Tropics indicate that patches of paragrass can provide conditions suitable for denitrification[2].
• Replanting/reseeding of desired species to achieve required vegetation density.
• Mowing/slashing of grass batters to maintain good cover and growth (grass clippings should not be left within the drain to prevent it washing away into wetlands or waterways where it can cause water quality problems by reducing dissolved oxygen levels).
• Irrigating vegetation during establishment, as required.
• Selectively removing sediment when build-up starts to impede even flows through the drain. This should be done carefully to only remove the accumulated sediment without removing the carbon-rich soil within the drain which is essential for denitrification[4]. Sediment removal should occur during the dry season to allow re-establishment of vegetation before the next wet season. Removed sediment should be disposed of away from any drainage lines and can be beneficially reused in the block.
• Selective filling any scoured channels or eroded areas that will cause channeling of flows or sediment loss.

If farm best management practices are not adopted in the catchment, large amounts of sediment may be exported, quickly filling drains with sediment. This will require frequent sediment removal to maintain drain capacity and reduce treatment performance, therefore preventing soil loss through best management practices is a priority.

Vegetated drains can be monitored to assess if they are effectively removing pollutants. Guidance on designing a monitoring program for assessing nitrogen removal by treatment wetlands and vegetated drains is provided in Monitoring guidelines to quantify nitrogen removal in vegetated water treatment systems.

Lifepsan/replacement time

Vegetated drains should last indefinitely if adequately maintained and farm best management practices adopted.

References

1. ^ a b Department of Employment, EDI (2011), Wetland Management Handbook: Farm Management Systems (FMS) guidelines for managing wetlands in intensive agriculture.. [online], Queensland Wetlands Program, Brisbane. Available at: https://wetlandinfo.des.qld.gov.au/resources/static/pdf/resources/reports/fms/fms_025_handbook_web.pdf.
2. ^ Kavehei, E & Adame, F (2021), Vegetated drains for water quality improvement in the Wet Tropics, vol. ARI report No. 2021/Australian Rivers Institute, Griffith University, Brisbane.
3. ^ Soana, E, Balestrini, R, Vincenzi, F, Bartoli, M & Castaldelli, G (June 2017), 'Mitigation of nitrogen pollution in vegetated ditches fed by nitrate-rich spring waters', Agriculture, Ecosystems & Environment. [online], vol. 243, pp. 74-82. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0167880917301597 [Accessed 26 May 2022].
4. ^ State of Queensland, Soil conservation waterways – construction and management. [online], vol. Science Notes Land series L270, Brisbane. Available at: https://www.publications.qld.gov.au/dataset/science-notes-soils/resource/5bfb8622-f091-4dd1-a712-d45b841cc72e.

Last updated: 24 May 2022