Resilient Rain Gardens: Selecting Fill Media and Mulch, and Influences of Urban Design

Rain gardens are Water Sensitive Design devices that use bioretention to retain, and reduce pollutants in, stormwater runoff. Resilient rain gardens consistently attenuate pollutants, volume, and peak flows from small rain events. Research projects investigated combinations of readily available materials in the Auckland region that have consistent physical and chemical properties suitable for bioretention. The mulch and filter (or fill) media used influence rain garden performance – mulches must not float and must have high permeability. Bioretention media must have permeability low enough to achieve adequate contact time (for effective pollutant removal), but high enough to minimize (untreated) overflow from water quality volume events, and avoid excessive ponding duration. Mulch and media chemistry influences effluent water quality and plant growth.

Results of two studies are summarised. The studies guide development of fit-for-purpose rain garden mulches and bioretention media in Auckland. Application at large Auckland rain gardens is reviewed. Although not part of the research, the potential effects of urban design priorities on rain garden resilience at these sites are discussed, including trading rain garden area for depth, increasing rain garden volume to support large trees, desiring lush growth and consistent landscape aesthetics, creating rain gardens ‘in series’, and ‘invisible’ inlets.