Standing water may be unsightly, have smells, and be a vector for insects
Surface ponding is designed to drain over a 24 hour period to prevent stagnation. Average ponding depths are 200-300 mm which can be screened by vegetation and/or rocks.
Rubbish and sediment can build up in practices and affect performance
Inlet systems and overflow structures can be designed to trap and separate floatables and sediments. Erosion within the device should also be prevented through energy dissipators at inlets. Regardless of these approaches, bioretention systems will require periodic maintenance to ensure the best operation.
Residency time for detention of stormwater volumes must be balanced with loading rates for water quality treatment
Residency time can be controlled through a restricted outlet or an adjustable level control.
Maintenance requirements are seen as being greater for bioretention practices than for catchpits
Bioretention practices provide a very effective return on investment for water quality treatment and volume reduction, including relative maintenance costs.
Apart from general litter removal, which is applicable to most landscape areas, bioretention practices are generally designed to self maintain. The application of mulch is the key requirement until plants establish.
Periodic monitoring of bioretention practices during storm events will identify potential operation or clogging issues.
Groundwater levels may be affected by bioretention, which may affect stability of adjacent slopes and structures
Increase drainage rates and/or line the bioretention practice to prevent saturation of adjacent soils. In some cases, bioretention practices may not be suitable for use near steep slopes.
Space is not available for bioretention practices
Bioretention can be retrofitted and can be any shape to fit within spatial constraints.
Planter boxes may be used inside foyers of buildings, or may be cantilevered on the outside of buildings.
Tree pits can occur in footpaths, roadways or courtyards. Their encapsulated forms allow cantilevering structures above them, which can be paved and may be designed to sustain the weight of people and vehicles.
Conflicts with existing infrastructure
Infrastructure can be protected behind root guards or concrete gaskets. Additionally, tree roots can be confined and guided to specific locations through root ‘cell’ technology.
The use of retaining structures, check dams and terracing may address slope issues. Planter boxes can be stepped down a slope, with the drainage pipe from a previous planter box entering the upper soil horizons of the downhill planter.
On-site soils may be unsuitable for use in the raingarden media
In general, Waitemata Group clay soils are not suitable for use in raingarden media. There are manufactured commercial mixes of bioretention media available, or a custom media may be developed as described in Auckland Council Technical Report TR2013/011 Media Specification for Stormwater Bioretention Devices.
Plant health may be affected by contaminants in stormwater
This can be addressed by species selection and appropriate soil media. In many circumstances tree growth rates will benefit from the increased nutrients associated with stormwater runoff. Contaminants will often be stored in organelles within plant cells, so effects on a plant’s biological health are limited. In many instances, microbial processes within the root zone will transform pollutants into innocuous forms.
Safety issues for bioretention practices
There are specific examples where raised and sunken tree pits in New Zealand have led to serious public safety issues due to tripping and falls. Bioretention design should incorporate visual cues to their presence, appropriate tree guards, cantilevered or continuous pavers that prevent tripping, or seating walls or similar to prevent conflicts.
Bioretention in roadways should consider pedestrian crossing points and opportunities for cyclists to leave the road to avoid erratic drivers.
Bioretention systems should be inspected every two years. Any issues identified during inspection should be addressed.
Correctly maintained bioretention systems have a design life of 25 years. If maintenance is not carried out, system life can reduce due to blockage and clogging. However, lack of maintenance does not ensure device failure.