||Potential mitigating design response
Sites that are constantly wet or receive continuous or very frequent flows. This includes sites:
- In retarding basins
- In flow paths
- Downstream of wetlands
- Catchments more than 10 hectares
- On floodplains*
- Areas with too much shade
(less than 8 hours sunlight per day)
|Moss or algae can form thick surface biofilms in continuously wetted or shaded systems, which reduce the rate of infiltration into the filter media and cause clogging.
Assets fully connected to larger catchments or downstream of regional wetlands are at
risk of excessive wetting due being exposed to almost continuous inflows.
Regular wetting and drying of a bioretention system is necessary. Continuous inflows,
damp flow paths and floodplains may inhibit this. They are also more likely to backwater drainage layers.
| Low flow diversions may reduce problems caused by continuous inflow but may also compromise overall
pollutant reduction performance.
*Floodplain sites may be considered as long as it can be shown the site will not experience frequent interflows (shallow groundwater) and has free
drainage into the waterway.
Downstream of oversized sediment ponds.
Bioretention overflowing in series.
|Upstream sediment ponds must not be oversized for their catchment as these may reduce the frequency of flows into the bioretention and lead to long dry periods with inadequate soil moisture to sustain planting.
A bioretention downstream of another bioretention asset receiving only overflows may experience long dry periods between inflow events.
|Over-sized sediment ponds are not acceptable under any circumstances.
Bioretention assets in series should each have an independent impervious catchment. to ensure each bioretention relatively frequent inflows.
Modelling to track soil moisture patterns (durations, spells) may be needed to verify design in uncertain circumstances.
The media should be specified to increase soil moisture retention capacity, a submerged zone of at least 450 mm adopted and more drought resilient plants chosen for any assets with overflows in series.
|Site without appropriately
measures including gross pollutant traps (GPTs) and
|Correct treatment train sequencing is important.
GPTs are required to ensure that litter and debris does not smother vegetation or increase the difficulty and cost of litter removal.
GPTs or sediment ponds/traps are required to ensure coarse sediment does not block the surface of the filter media. These must have adequate storage capacity with respect to expected sediment loads and clean-out frequency.
|Requirements for upstream primary treatment are catchment specific. Cleaner catchments may need only smaller-scale primary treatment elements.
|Sites subject to velocities>1m/s for the 1 percent AEP (1 in
100 year ARI).
|Velocities >1m/s are likely to scour the surface of bioretention systems and damage vegetation.
||High flows to be diverted or the bioretention system protected by ‘feedback control’ through backwatering of the inlet to prevent inflow after extended detention is filled.
|Where dwarf galaxia habitat needs to be protected.
||The absence of permanent water in a bioretention system prevents the distribution of species that are a threat to
|Consider using a wetland instead or establishing complementary dwarf galaxia habitat such as wetlands or ponds.
|Sites with insufficient drainage outfall depth or no option to provide an overflow weir and high flow bypass.
||A frequently back-watered drainage layer will not support drainage of the filter media – which may adversely impact plant health; and may cause blockages in the pipes within the drainage layer.
|| Designs for potentially vulnerable sites need to be informed by an understanding of downstream water level pattern (for example: dry weather flows for all seasons.
Frequent event water levels and associated spells).
A design for a submerged zone is compatible with a shallower drainage outfall.
|Sites with tidal influence or shallow saline groundwater.
||Saline water compromises the biological function of the system.
||Designs for potentially vulnerable sites need to be informed by an understanding of observed tidal patterns.
As above, a submerged zone may help to raise the outlet pipe above the tidal influence.
|Sites subject to toxic runoff.
||When the system is at risk of being exposed to toxic substances such as herbicides, solvents or industrial
contaminants, its biological function will be compromised.
|Structural separation must be used to mitigate the impact of industrial activity (and associated harmful toxicants) on the stormwater system.
|Asset cannot be accessed for maintenance.
||Regular maintenance is vital to ensure optimal function of the system and asset longevity.
||Design to minimise maintenance requirements, for example: upstream litter and sediment control, tree planting and dense planting to control weeds.
| Sites with acid sulphate soils.
||Acid sulphate soils are harmful when exposed to air, such as through drainage or excavation.
|| Activities with the potential to disturb acid sulphate soils must be managed carefully to avoid serious environmental harm.
|The quality of inflow is impacted by upstream construction phase activities.
||High sediment loads running off developing catchments clog the filter media requiring it to be reset. Bioretention assets need to be protected from sediment runoff until at least 90 percent of construction and building works are completed.
||The bioretention asset may be constructed without planting and covered with a protective surface that will be removed (together with accumulated sediment) once the high-risk development activity threshold has passed.