Media specification for stormwater bioretention devices

Elizabeth A Fassman, Robyn Simcock, Simon Wang
Auckland Council Research, Investigations and Monitoring Unit, RIMU
Publication date:

Bioretention devices including rain gardens are a Low Impact Design (LID) device primarily used to treat pollutants found in stormwater runoff. The filter media (or fill media) used in bioretention is a critical component to ensure effective bioretention performance. The current design advice in Auckland is a generic “sandy loam, loamy sand, loam, or a loam/sand mix (35-60% sand), with a maximum of 25% clay content” (ARC 2003).

An investigation has been carried out to assess combinations of materials readily available in the Auckland region which create consistent physical and chemical properties to satisfy hydraulic and water quality objectives for stormwater management. The bioretention media should have hydraulic conductivity low enough ensure adequate contact time for pollutant removal mechanisms to be effective, while keeping hydraulic conductivity high enough to minimize (untreated) overflow from water quality volume events. Chemical properties of the media influences the ability to support plant life and influences effluent water quality with potential to balance high hydraulic conductivity/low pollutant-to-media contact time.

The investigation process included:

  • establishing physical characteristics and performance criteria;
  • investigation into available materials;
  • particle size distribution (PSD) testing and analysis;
  • compaction assessment;
  • hydraulic conductivity testing;
  • chemical analysis of materials;
  • water quality testing.

Auckland Council technical report TR2013/011

Prepared by Auckland UniServices

Last updated: 2018-07-10