COULD SOMEONE PLEASE TURN DOWN THE VOLUME? Unpacking the myths, misunderstandings and missed opportunities with stormwater retention

ABSTRACT

The perilous state of our urban waterways has been well documented over the past decades. Our understanding of the unique indigenous biodiversity they support and the adverse impacts of the cocktail of contaminants in stormwater discharges has prompted action by councils, communities, and developers across the country. Increasingly new developments are integrating basic water sensitive design measures to mitigate water quality impacts through the use of green infrastructure such as bioretention, constructed wetlands and swales. Or understanding of the current and future threats of flooding and intense rainfall has also prompted action with most councils now requiring flood attenuation either at individual lot scale or through integrated landscape solutions to hold back infrequent peak flows and release them slowly to match predevelopment rates. This evolving practice around stormwater quality and peak flood flows is encouraging for the potential to one day see meaningful improvements in the ecological and cultural value of these long neglected waterways and to enable communities to again connect with and appreciate our urban centers within the context of a catchment.

Unfortunately, a significantly less developed approach to managing stormwater volumes jeopardizes the long term success of these well intended actions. An ongoing lack of clear policy, guidance and industry capability around how to manage stormwater runoff from frequent small rainfall events continues to threaten steam health. Conversion of land for urban development without management of stormwater volumes results in a fundamental shift in the rainfall runoff relationships in our once thriving urban waterways. This shift results in persistent flashy flows, repeated riparian instability, incision of streambeds and highly dynamic sediment mobilisation which, when combined with the impacts of elevated stormwater temperatures, fluctuating dissolved oxygen and discharge of contaminants results in what is often termed ‘urban stream syndrome’. The resultant loss of endemic and indigenous biodiversity means that our urban streams are now only home to a very limited palette of the most tolerant species and the aspirations and memories of mahinga kai are seemingly lost alongside the community connections with waterways that are now conduits for exotic weeds and showcases of instream engineered retaining structures. The ongoing cost of these structures is rapidly escalating as councils and property owners are forced to attempt to engineer their way out of a situation that is often seen as a conflict with these once benign waterways that are now seen as unpredictable and menacing. When we cast our eyes forward and consider the future climate we soon see an increased frequency of high intensity rainfall and extended dry periods. These conditions will only exacerbate the current urban stream conditions and therefore increase the future cost to provide resilience to communities which co-exist with urban streams.

But this current condition is not a foregone conclusion, and the missing part of the urban stormwater puzzle is key to the protection and enhancement of our urban waterways and the alignment with the principles of Te Mana o te Wai. Management of the volume of stormwater discharged to the environment is fundamental to mimic the natural hydrology of urban streams and enable them to be resilient to existing and future conditions whilst sustaining a thriving biodiversity that is unique to Aotearoa. In a natural catchment, the role of vegetation and shallow soils is critical to the resulting hydrologic response with the most influence being in small frequent rainfall events. The interception of shallow rainfall depths, and the corresponding evapotranspiration, is a key component of any natural water balance with excess able to be infiltrated and only a small proportion of rainfall ever discharging to streams as surface flow. This is clearly observed in natural streams, without direct stormwater discharges, where even in moderately large rainfall events the flowrate in the streams remains near constant and only respond to the large but infrequent peak storms. The volume of annual rainfall intercepted and assimilated in natural catchments is significant with estimates varying from 20% – 50% of the mean annual rainfall which reflects the reality that the vast majority of rainfall is from small events with less than 10 mm depth. Therefore, to respect and protect our urban waterways from the impacts of development we need to urgently address the current shortcomings with regards to the retention of stormwater within urban developments and collectively raise the bar to mimic as best we can the natural hydrological cycle alongside efforts to reduce contaminants and provide flood resilience. The simple term ‘Sponge City’ has recently become more prevalent as communities struggle to comprehend the complex challenges they face with their relationship with stormwater. It is noted that the management of stormwater volumes through meaningful retention is perhaps the most obvious exemplar of the potential to reflect the analogy of a sponge in practice.

Whilst some councils have been requiring retention of stormwater for a number of years many are either silent or are subject to policy and rules which are vague or lacking in efficacy. In many instances there is active push back by land developers, civil engineers, planners and council officers to efforts to integrate meaningful retention of stormwater through measures such as rainwater reuse, stormwater harvesting, green roofs and shallow infiltration. This push back is often based on a general human reluctance for change but at the core is a mix of mis understandings and perpetuated myths around the importance of urban hydrology and the clear opportunities to implement efficient and effective measures to avoid or minimise adverse impacts.

This presentation will therefore unpack the misunderstandings, myths and opportunities to implement stormwater retention to mimic natural volumes and in turn support stable and thriving urban waterways. Drawing on international research/practice, local case studies, and variable policy options this talk will drill into the perceived complexity and challenge many of the myths which currently permeate the development industry. Through providing a better understand of the issues with urban hydrology and outlining readily implemented solutions the talk will provide practitioners with an evidence base to support the changes we need to see if we are serious about the health and wellbeing of our freshwater streams and the communities they support. This includes a better understanding of the instream processes which destabilize many urban streams, an understanding of a water balance approach to urban stormwater volumes and how less ambiguous policy and rules are needed to appropriately inform future practice.

At this time of increasing development pressure/intensification, changing rainfall patterns, growing awareness of the benefits of human connection with nature and a need to achieve positive outcomes without excessive costs the need for clear and consistent practice is more critical than ever. If we are unable to turn down the volume of urban stormwater we will continue to be witness to the downward spiral in stream health and the lost opportunities that this will bring. On the other hand, if we are able to turn down the volume perhaps we can see the mauri of these once celebrated waterways increase and avoid the cost to future generations from their demise.