Rain Gardens

It is usually best to use a loamy sand as the filter media for bioretention systems, with an effective particle diameter of around 0.45 mm and a hydraulic conductivity of 200 mm/hr.

Ideally the filter media used in bioretention systems and raingardens should be tested from NATA accredited laboratory and accurate figures based on locally-sourced filter media should be used where available and justification provided in the report

The vertical structure of the bioretention media particularly the transition layer, which consists of medium to coarse sand below the filter media - is important to prevent filter media from washing into the drainage layer and the drainage pipe. The drainage layer, which typically consists of fine 5 mm gravel conveys filtered water to and holds the perforated collection pipe.

Filter Depth (metres): The recommended bioretention filter depth is between 0.4 m and 1 m. The depth depends on the available depth based on the inlet and outlet levels and the plant species being used. For particularly flat sites where streetscape bioretention pods are used, it may be possible to limit the filter depth to 0.3 m. For any filter media depth more than 0.8 m, the planting of deep-rooted plants such as trees is necessary. If a filter media depth of more than 0.8 m is proposed, obtain expert advice from a landscape architect or ecologist study that provides adequate justification for plant selection. Such advice should be lodged with the water cycle management plan.

The base of the bioretention system should be within soil and not recessed into rock, and should also be located at least 0.5 m above the seasonal high groundwater table to ensure that the media is not periodically saturated by groundwater. Where parts of a bioretention system are recessed into rock or lined with concrete (some raingardens), they must have a sufficient media depth for the proposed deep-rooted vegetation and must incorporate an underdrain.

Lining: On steep (more than 25%) or constrained sites (for example, where the basin is located close to building footings), bioretention systems should be lined and/or should incorporate a suitably designed retaining wall (Water by Design, 2014)

Vegetation Properties: Plant types have a significant impact on reducing nutrient loads. Root morphology and associated physiochemical processes are key factors (Read et al., 2009). Bioretention systems perform best with deep-rooting plants

Choosing plants for a bioretention system is critical to ensure best performance in terms of removing nutrients and maintaining saturated hydraulic conductivity. The best deep rooting plants to remove nutrients include Carex sp, Melaleuca ericfolia, M, inacana, M lateritia, Juncus sp., Goodenia ovata, Baume and Ficinia (Payne et al., 2015). Grass and turf are not effective plants for removing nutrients from the full filter media depth, and this should be shown as such in the Bioretention node. Plant selection depends on a number of factors including habitat, amenity, microclimate, safety and biodiversity.

Submerged Zone with Carbon Present: Submerged zones improve the potential for microbial denitrification, and provide more permanent storage of water for plants, which can be important during dry periods. Where practical, a submerged zone should be included below the underdrain. The submerged zone typically has a depth of between 0.2 m and 0.4 m, and consists of medium to coarse sand, or fine gravel, combined with a carbon source (usually 5% by volume of hardwood chips) to provide a permanent pool of water to support plants and to enhance microbial nitrogen removal.

Mulch: Organic mulch is not considered suitable due to risk of floating and clogging outlets and removal via wind. Gravel mulch can be useful to decrease the ponding depth for safety reasons and prevent scouring, but it may increase stress on plants due to heat retention and impede removal of accumulated sediment. Using a high planting density and care during seedling establishment is recommended to quickly develop high plant cover. Use of trees to shade the surface can also reduce drying.

Optimising a bioretention system is firstly about ensuring that the size is large enough to treat a significant proportion of the flow, but that it also drains quickly enough so that it is ready to capture the next event. Sizing a bioretention system is important and ideally should be around 2% of the upstream impervious catchment area. Anything greater than this 2% is going to be somewhat inefficient. Hydraulic conductivity should be set to half of the hydraulic conductivity for the filter media proposed to be used.

The extended detention depth (EDD) for bioretention systems within lots or road reserves should be between 0.15 m and 0.30 m.

Where bioretention systems are located downstream of a large source area and unsealed area, pre-treatment, such as a buffer strip or gross pollutant trap, should be installed to reduce the risk of clogging filter media and to prolong the life of the system.

Supporting resources:

• Building and development - WaterNSW 
• Developments in Sydney's Drinking Water Catchments

• NorBE Assessment Guideline January 2026

For best practice WaterNSW recommend instructions sheets for Rain Gardens from Melbourne Water: Raingardens

Rain gardens can be both practical and beautiful. When selecting plants, aim for a diverse mix of species that can cope with changing conditions — from periods of heavy rainfall and temporary waterlogging through to longer dry periods.

Using a variety of plants helps create a healthier, more resilient rain garden by reducing the risk of large-scale plant loss, limiting weed invasion and providing habitat for local wildlife.

Choose plants that are suited to the Wingecarribee Shire environment, including species that can tolerate cooler Southern Highlands conditions, occasional frost, wet periods and dry spells. Native plants are generally a great choice because they are adapted to local conditions, require less maintenance once established and provide benefits for local biodiversity.

Suggested plants for Wingecarribee Shire rain gardens

The following native plants may be suitable for different areas of a rain garden:

Plants suited to wetter areas (lowest points where water collects)

• Carex appressa (Tall Sedge) – a hardy sedge that tolerates wet conditions and helps stabilise soil.
• Lomandra longifolia (Lomandra) – a resilient grass-like plant that tolerates both wet and dry conditions.
• Juncus usitatus (Common Rush) – suited to wetter areas and helps provide structure.
• Baumea articulata (Jointed Twig-rush) – suitable for permanently or seasonally wet areas.

Plants suited to the edges and transition areas

• Dianella caerulea (Blue Flax Lily) – attractive foliage plant that tolerates a range of conditions.
• Westringia fruticosa (Coastal Rosemary) – hardy native shrub suitable for drier edges.
• Grevillea species – provide flowers and habitat while tolerating well-drained conditions.
• Correa species (Native Fuchsia) – hardy shrubs that provide colour and support local fauna.

Groundcovers and lower-growing plants

• Myoporum parvifolium (Creeping Boobialla) – useful groundcover for stabilising soil.
• Viola hederacea (Native Violet) – suitable for shaded, moist areas.
• Dichondra repens (Kidney Weed) – a low-growing native groundcover that can suit moist areas.

Designing your rain garden

Be creative with your design and consider using a mix of plant heights, textures and flowering periods to create an attractive garden feature.

Avoid planting a single species across the entire rain garden. A diverse planting approach creates a more balanced ecosystem, reduces the likelihood of plant die-off and helps prevent weeds from becoming established.

Your local nursery can help recommend plants that suit your specific location. When selecting plants, look for species that can tolerate both heavy rainfall and extended dry periods, and consider local native plants wherever possible.