Recommendations for improving the design, amenity and performance of privately owned, small-scale biobasins

Sylvie Chell , Ruby Naomi Michael, Matthew Moore, Jonas Larsen

Due to spatial constraints in cities, it is increasingly challenging to integrate appropriate water sensitive urban design (WSUD) solutions. There has been a shift from larger, precinct-scale bioretention systems owned by local governments to smaller scale ‘biobasins’ managed by private landholders. This shift has brought about unique challenges in regulation, design, maintenance, and performance, while also intensifying the pressure to deliver high-quality amenity outcomes. This research triangulates: literature, site analysis of small, privately owned biobasins, and practitioner interviews in Southeast Queensland, Australia to inform the best practice design for functional biobasins that also contribute to public amenity value.
Fourteen biobasins on private land were critically evaluated through site visits, review of design and maintenance documentation and historical site images with an emphasis on the long-term health of the designed plant communities. About one third of the biobasins (n=4/14) had a deep (>1m) drop from the surrounding ground level to the filter media surface, restricting access for maintenance and leading to poor amenity outcomes.
Half (n=7/14) of the biobasins were fenced and three of the fenced basins further severely restricted maintenance by having no gate. Visual amenity of biobasins was low as exposed hardscapes limited the visibility of plants and basins failed to establish healthy target vegetation coverage. A lack of adequate maintenance access was strongly linked to low amenity biobasin outcomes with all basins that lacked gates showing weed intrusion and a low average amenity rating of 1.33/5. Practitioner interviews revealed concerns about herbicide-based interventions and site revisits after a minimum of 6 months confirmed the suspected lack of maintenance. As-designed species richness was low with an average of three species per biobasin and a minimum of one (monoculture). A higher plant species richness may lead to faster and sustained green coverage establishment and may reduce the required maintenance efforts and expenses. This study found the linear design process to be a major barrier to achieving good biobasin outcomes on private land. Expertise on plant requirements needs to be included early in the design process and independent actors should be enabled to collaborate across disciplines.
We recommend decreasing the depth and encourage pro-active maintenance regimes similar to publicly owned bioretention basins. Until the design barriers are remedied to make the systems easier to maintain and access, we are unlikely to see bioretention management plans practically implemented or high-quality amenity outcomes.