Water Sensitive Design Performance over the past 10 years: A Road Map to the Future

Over the past ten years, the percentage of people living in urban areas in New Zealand has increased from ~72% in 2008 to ~86% in 2018. Auckland’s population has increased by over 200,000. Our existing infrastructure cannot keep up with the current rate of intense urbanisation, a situation which will worsen with increased rainfall from climate change. New solutions for stormwater management must be resilient so urban centres can adapt and remain sustainable. Providing good stormwater management solutions for the future requires an understanding of the effectiveness of today’s existing systems.

The intensified land development across New Zealand has required new developments to manage at least a portion of new stormwater runoff on site so as to protect both stormwater infrastructure and natural waterways. Much of the increased stormwater runoff resulting from new developments has been managed through low impact or water sensitive design (WSD). Practitioners have designed raingardens, detention tanks, wetlands, permeable paving, swales and a myriad of other solutions to accomplish stormwater management over the past 10 years.

TO understand how WSD assets have performed over time 40 WSD assets installed within the last decade were revisited. The results of field inspections are presented herein. The current effectiveness of the WSD assets and reasons for failure are explored. Banded wetlands, raingardens, four types of permeable paving, stormwater detention tanks and vegetated swales were examined for condition and functionality.

Five raingardens which were installed as part of a residential subdivision that now lies within a mixed housing urban zone were revisited. It was found that after a decade, none remain. In a nearby residential area a more recent raingarden is operating near the design specifications but the property owner is finding it ugly and difficult to maintain.

In a residential development just inside the Residential Urban Zone, a comprehensive WSD increased property values and contributed to the subdivision becoming a sought after community. After 5 years most of the assets are intact and well maintained. However 3 km away, the banded wetlands, installed as a shared WSD asset within an industrial development, are in such poor condition that rehabilitation will be nearly as costly as the initial installation.

On the surface, maintenance appears to be the key to why some WSD succeed and others fail. A deeper look at the findings also indicates a more systemic failure than maintenance. The long-term success of a WSD depends on a number of factors including: regulatory framework, public awareness, engagement and maintainability. All parties involved throughout the lifetime of a WSD – the regulatory body the contactors who install the assets, the designers, the property developers and end users - all play a role in the successful implementation of WSD.