Permeability of Raingardens – Field Measurements and Observations in Auckland

The aim of this research was to assess the hydraulic performance of raingardens over time. Six Auckland raingardens were selected for permeability testing using a double-ring infiltrometer, based on the availability of historical data. Additionally, the state of maintenance and factors affecting the performance were assessed.

Raingardens, or bioretention cells (BRCs), are a popular Water Sensitive Design practice for at-source pollution control and have been used in Auckland for the last decade and a half. The Proposed Auckland Unitary Plan should result in a large-scale implementation of this practice, particularly in greenfield sites and along high contaminant-generating roads. To date, most BRCs in Auckland have been designed to be consistent with TP10 (Auckland Regional Council 2003) regulations. This specifies a minimum permeability of 12.5 mm/hr with no upper permeability limit, and a clay content up to 25%. However, post-construction monitoring of long-term permeability has been conducted at very few sites. Infiltration rate is a primary determinant of performance, as it controls the volume of stormwater treated. Permeability can also influence the effectiveness of some contaminant removal processes.

Half the BRCs had infiltration rates above the minimum permeability requirements. Prolonged periods of ponding, bypass flows, and variations in permeability over time were observed. Plant health and weed composition varied. Maintenance did not conform with guidelines at most sites, evidenced by, for example, blocked inlets and the absence of a mulch layer. The research shows that, whilst BRCs can be both an effective stormwater treatment device and provide aesthetic amenity, proactive monitoring and maintenance is needed to ensure that this outcome is realised.