A performance monitoring framework for raingardens as decentralised stormwater treatment systems in the City of Sydney

Urban development has resulted in marked changes to the natural water cycle in the Sydney region, with a system of built and paved urban surfaces and a piped stormwater reticulation which short-circuits the terrestrial phase of the natural hydrologic cycle while adding contaminating substances to stormwater flows. Rainwater no longer falls on vegetated land where opportunities for stabilisation through filtration, oxidation and solar irradiation are presented, but after collecting road, roof, paved area and industrial contaminants, rainwater passes directly via channels and pipes to modified receiving waters where stabilisation of a range of chemical and bacterial substances is limited. In addition, the process of natural filtration by downward percolation into the aquifer is denied and physically-contaminating suspended matter hinders natural treatment processes in receiving waters.1,2 A further physical factor impacting seriously on stormwater quality is the removal of natural buffers to hydrological flow in urban settings where water rushing from pipes following heavy rainfall events gouges out sediments, collecting these by virtue of its velocity to deposit them at sites further downstream in receiving waters.3 This disengagement of liquid and solid components results in silting of rivers in urban settings, with characteristic pollution of downstream receiving waters, such as dams, lakes and marine reserves. Contamination of stormwater runoff is a well-recognised problem internationally, with potential for high bacterial and chemical loads during “first flush” events which follow dry periods when substances have had an opportunity to accumulate on urban surfaces.5,6 Storm events related to deforestation, urban heat islands and ultimately global climate change may exacerbate this phenomenon of sediment scouring and subsequent deposition. A range of interventions have been proposed or applied internationally as part of WSUD. These interventions primarily involve the limitation of initial contamination, strategic decoupling of sediment and velocity components through the provision of flow buffers and physical retention basins, and the introduction of an engineered terrestrial phase to provide some level of physical and natural stabilisation of stormwater. The construction of decentralised stormwater bioretention units, including raingardens, falls into the latter category of interventions, as applied by the City of Sydney as part of its commitment to the Sustainable Sydney 2030 Plan, and in terms of its Decentralised Water Master Plan.