OUR RAIN GARDEN SUCKS!? INVESTIGATING NATURE-BASED SOLUTIONS FOR VOLUME REDUCTION AND CLIMATE CHANGE RESILIENCE

M. Hannah, K. Jonathan & J. Watson (Stormwater 360), J. Lear & H. Jones (Pattle Delamore Partners Ltd)

ABSTRACT

Green spaces play a vital role in stormwater management, acting like giant sponges. They slow down the flow of rainwater and filter pollutants. Estimates suggest that urban vegetation can absorb up to a third of the water resulting from extreme rainfall events, showcasing the importance of green spaces in mitigating stormwater challenges, and embracing ‘sponge cities,’ a practice advocated by the Parliamentary Commissioner for the Environment, Simon Upton.

Central to achieving ‘sponge cities’ is volume reduction, important for peak flow reduction, contaminant removal, habitat protection and maintaining environmental flows. By harnessing volume reduction methods like infiltration, water use, and evapotranspiration (ET), we can increase the capacity of our stormwater systems without enlarging infrastructure. Infiltration is known and quantifiable, as is water demand and supply. ET is not so well understood.

Regulations recognise the importance of volume reduction but reflect the ET knowledge gap. Stormwater management area flow (SMAF) requirements in Auckland aim to reduce stormwater runoff from impervious areas, and Waikato Regional Council defines the objectives for infiltration devices as: volume reduction, contaminant removal, and low stream flow augmentation. Attention isn’t paid to volume reduction via ET potential in nature-based systems.

Pattle Delamore Partners Limited and Stormwater360 have undertaken a 24-month hydraulic investigation of a high-flow biofiltration system (HFBF)(Filterra®). The fast infiltration system (›2500mm/hr) was developed in New Zealand using local materials to have greater ET potential and enhance plant available water. The system comprises a concrete vault and does not allow infiltration, therefore any volume reduction is associated with ET. Flows were monitored with six pressure transducers and two V-notch weirs, at the inlet and outlet. Despite widespread assumption that HFBF systems aren’t capable of such, the key areas of investigation were peak flow reduction, detention time, and run-off reduction to better understand the potential hydrological mitigation.

The study showed peak flow reduction between 0% and 47% and the lag time between the inlet and outlet peaks, a detention time proxy, suggested a runoff detention time between ~12 and 22 minutes in the interquartile range. The results provide evidence to conclude that the HFBF is capable of peak flow reduction and detention; however, given the variability of the results, the true extent is currently unclear.

Runoff reduction results were more consistent and demonstrated the HFBF system achieved reduction in the final cumulative volume, averaging 12.96% across the observed events. Given that infiltration was intentionally prevented, ET was thought to be a significant driver of this phenomenon, which is in general accordance with various international literature asserting that ET provides 19-84% volume reduction for stormwater bioretention devices.

This study demonstrates ET as an important consideration in quantifying volume reduction for nature-based stormwater solutions, such as bioretention. Development of an industry wide design approach for volume reduction, which considers evapotranspiration and variables such as plant species, localised climate, and the available water capacity of the infiltration media, is warranted. Addressing ‘unknowns’ in volume reduction, such as ET, is paramount to adapt to climate change and design resilient, nature-based stormwater solutions.