The media used in bioretention influence hydraulic, chemical and plant growth performance. Resilient media have characteristics that resist or buffer changes to maintain design performance over the life of a device. We followed the performance of Wynyard Quarter raingardens installed between 2007 and 2013. Three contrasting media were used; two are soil-like with >20% fines and organic contents (Auckland Council ‘TP10’ media); one is 85% v/v sand with low fines and organic matter (Melbourne ‘FAWB-type’ medium).
Infiltration rates generally increased as plant cover increased. The initial infiltration rate was influenced by each media’s response to compaction at installation. The ‘TP10’ media with infiltration closest to design after 1 to 4 years were rejected at installation in favour of the ‘FAWB-type’ medium with high infiltration rates that likely exacerbated drought-related mortality, despite high water storage, due to uneven water distribution across individual raingardens.
The organic content of raingardens may be moving towards equilibrium now a dense plant cover is maintained. Organic matter is probably increasing in the FAWB-specified media, and may be decreasing in media with high initial organic matter contents. The FAWB-type media is poorly buffered. Further, the low nitrogen content lead to some groundcover and tree species exhibiting signs of nitrogen stress.
The resilience of rain garden media infiltration rates to compaction and changes in organic content, and the impact of sustained plant growth on infiltration and permeability should be considered. Adequate organic contents are needed to support acceptable plant growth in environments where storm water nutrient inputs are low. Alternatively, mulches can be selected to provide nutrition and organic matter.