OUTCOME BASED SIZING OF STORMWATER TREATMENT SYSTEMS

M. Murdock & M. Groves (WSP)

ABSTRACT

Stormwater treatment devices or Best Management Practices (BMPs) can provide both water quantity and water quality benefits. These systems are typically sized using either a water quality volume (WQV) or a flow rate (WQF). Although methods for calculating the WQV & WQF can vary across regions and councils within Aotearoa, the same volumes or flow rates are typically applied across a range of devices for each locale, regardless of their function. For example, the same WQV will be used to size a raingarden as a wetland. This paper explores the validity of this approach with a focus on achieving a consistent outcome (in terms of run-off treated), rather than a consistent input. The analysis challenges the status quo by promoting outcome-focused thinking when it comes to stormwater device sizing, based on a more comprehensive understanding of system function.

The intended outcome adopted for this analysis was for a benchmark percentage of annual run-off, on average, to be treated (a commonly used outcome metric). To do this, Long Time Series (LTS) rainfall was analysed through each BMP and the input (WQV or WQF) iteratively changed, using automation, until the target outcome was achieved across all systems assessed, including varying maintenance conditions and bypass configurations. The results illustrate why there is potentially value in using device specific inputs to achieve a set outcome (as opposed to fixed inputs across a range of system types) and how factors such as maintenance condition or bypass arrangement can influence the results. The methodology used in this study could be replicated for any location, or system, and used to optimise stormwater treatment sizing across Aotearoa.