Evaluation of Rain Gardens in Auckland Region

The Auckland Regional Council (ARC) is in the process of reviewing its Stormwater Management Devices: Design Guidelines Manual, Technical Publication 10, Second Edition (TP10 2003). Rain gardens are components of ‘low impact design’ stormwater management systems that have been used in the Auckland region for more than 10 years. Rain gardens (also known as bioretention basins and biofiltration basins) are one of the preferred management options used in the Auckland region to treat stormwater. The ARC conducted a field evaluation of 41 rain gardens from 30 sites in the Auckland region in 2006. The evaluation included analysis of the rain gardens media depth, particle size distribution, and the organic matter content. Permeability of the media was determined using a falling head percometer. Variation in the design and construction of the rain gardens was evaluated by visual inspections and documented by photographs. Two rain gardens were assessed in 2007 and 2008 by measuring inlet and outlet flows and evaluating water quality monitoring data.