What is a Bioretention Basin?

Bioretention Basin Integrated Into a Local Streetscape on the left and a Car Park on the right are integral requirements under Gold Coast Planning Scheme Policies

Generally, bioretention basins are vegetated areas where overland stormwater runoff is filtered through a filter media layer such as sandy loam as it percolates downwards. As it percolates down, it is then collected into slotted or perforated subsurface drains that flow to downstream pits or waterways or underground rainwater tanks for reuse.

All bioretention basins operate by filtering stormwater runoff through densely planted surface vegetation. This vegetation slows the stormwater runoff to allow the water to percolate through the sandy loam filter media. As the stormwater percolates the pollutants picked up by the overland flow of the stormwater are retained through fine filtration, adsorption and some biological uptake in the filter media.

Contrary to what ill-informed property managers and owners believe the vegetation in a bioretention system is an essential functional element of the system. The vegetation enables a substrate for biofilm growth within the upper layer of the filter media. The vegetation also facilitates the transfer of oxygen to the filter media. It enhances soil microbial communities which enhance the biological transformation of pollutants contained in the layers of filter media.

Bioretention Basin Maintenance & Management Requirements

As the vegetation plays a crucial role in maintaining the porosity of the filter media of a bioretention basin. Therefore strong healthy growth of the vegetation is essential to the performance of the bioretention basin. Intensive maintenance of the vegetation is required during the plant establishment periods when weed removal and replanting of the vegetation will be required.

The hydraulics of stormwater inflow systems and overflow pits require rigorous monitoring, as these will be prone to scouring and debris buildup, especially during the summer months when we have major storm events.

Debris buildups can easily block inlets or outlets plus can be unattractive in high visibility areas that are otherwise meticulously maintained. As part of any maintenance program inspection and removal of the debris, buildup needs to be undertaken regularly and removed from the site. If you have sediment forebays, they need to be inspected every quarter, with all of the accumulated sediment wholly removed from the site.

Typical Bioretention Basin Maintenace We Undertake

Typical maintenance of every bioretention basin we will undertake will include:

• inspection of the bioretention basin to identify any areas of increased sediment and debris deposition

• examination for any scouring from stormwater overland flows

• checking for any erosion of the batters from lateral stormwater inflows,

• inspection for any damage from vehicles and clogging of the bioretention basin

• inspection of inflow systems

• inspection of overflow pits and the subsurface drains to identify and clear any scouring, litter and debris buildup and blockages

• removal of sediment where it is building up in and around the vegetation

• inspection of the sediment forebay and removal of accumulated sediment, debris and litter

• repairing any damage to the retention basin caused by scouring, rill erosion or vehicle damage

• inspection of the bioretention basin surface and tilling or removal of the surface layer, if there is evidence of clogging

• removal of invasive weeds without the use of herbicides

• removal of any plants that have died or been vandalised and replacement with plants of equivalent size and species as detailed in the original plant schedule

• pruning to remove any dead or diseased vegetation to stimulate growth

From experience, we have learnt maintenance should only occur after a reasonably rain-free period. This is because the filter media needs to be dry in the bioretention basin to enable us to check the subsurface drains correctly and avoid surface substrate damage.

We always recommend inspections after any significant rainstorm events so that we can check for scouring and debris buildup.

If you fail to undertake bioretention basin maintenance regularly, you risk being required by the council to reconstruct the bioretention basin entirely. Generally, this will be necessary if the bioretention basin fails to drain satisfactorily after tilling of the surface.