Our vulnerable water systems

This is a part of the Climate action feature

By Professor Iain White - 1 August 2018, Build 167

Research into stormwater and wastewater (sewage) systems is setting the stage for big, generational decisions that will have to be made in the face of a changing climate.

NEW ZEALAND’S hottest summer on record is now only a memory, but now is a good time to reflect on how climate change may affect us.

A recent research project has been analysing our current knowledge of stormwater, wastewater and climate change and identifying the key areas to focus on to adapt our stormwater and wastewater systems for a changing climate.

The research was commissioned by the Deep South National Science Challenge, an interdisciplinary collaboration designed to enable New Zealanders to adapt, manage risk and thrive in a changing climate.

Findings from workshops

Workshops were held with people from a variety of backgrounds such as academia, industry and government. This spread of knowledge allowed different aspects of the research to be addressed, including climate science, the practicalities of managing infrastructure, the extent decision-making frameworks are fit for purpose and international perspectives on adaptation.

There were several key findings:

Climate change is happening and stormwater and wastewater systems are vulnerable, but it is not yet known how this will unfold over varying places, timescales and speeds.
The projected changes will challenge existing design and performance parameters, particularly those relating to rainfall intensity and distribution, drought severity and coastal inundation.
There is no good information on the cascading disruptive effects of infrastructure failure on health, transport, culturally significant locations, evacuation routes, recreational and commercial activities and waterway ecology.
Land uses and the ways we make decisions need to be adapted, and we need to manage uncertainty and complexity better over multiple scales and timeframes.

Need to increase resilience

The results have significant implications for central and local government, those concerned with stormwater and wastewater infrastructure and the planning sector. Better management of new development, how to retrofit the urban development to be more wastewater disposal sensitive and how to work across different scales and agencies in managing complex risk are all issues.

More generally, to increase the resilience of our wastewater systems, those involved with infrastructure provision need to make effective decisions under uncertain conditions, because even as the evidence base grows, there will always be gaps in our knowledge about extremes and the speed of change.

Significant assets at increasing risk

The scale of the sector is significant – stormwater infrastructure has a replacement value of around $8.6 billion whereas the wastewater network has a replacement value of $15.8 billion.

The assets include 24,000 km of public wastewater network, more than 3,000 treatment plants and over 17,000 km of stormwater network, although these figures should be treated with caution. The data covers replacement to current specifications, not those that may be required to maintain standards of operation in a changing climate where extremes are more commonplace.

Other information gaps were identified. For example, age and deterioration of assets is an issue, but there is no consistency of reporting, which makes national understanding and assessment difficult.

Overall, the picture is one of increasing exposure to risk. Approximately 6.6% of our total population and $52 billion worth of our built assets are located within higher-risk areas. Significantly, this is happening while parts of the country are experiencing growth pressures that will unevenly affect the performance of the current system.

Areas that are growing rapidly will need to tackle this issue more urgently. The political context is also difficult. Existing assets need reinvestment and possible expansion at a time when it is becoming more apparent that there is a squeeze on local government finances to build new infrastructure to enable and accommodate growth.

Climate change bringing drier and wetter

The warming of our climate is unequivocal, and we are already being affected. It is very likely that there will be an increase in rainfall in the west during winter and spring, while it will be drier in the east and north. In summer, it is likely the east will be wetter, while the west and central North Island will be drier.

Predicted increases in rainfall intensity will challenge current design parameters and increase the occurrence of infiltration of wastewater into stormwater.

Droughts are likely to increase in severity and frequency, particularly in already dry areas, and wastewater systems will require extra maintenance to protect existing service levels. Drought will also affect networks, disrupting gravity systems by slowing flow, and may also affect wastewater treatment processes, creating functional and safety concerns.

Sea level rise and storms

Sea levels in New Zealand have risen by up to 22 cm in the last century, consistent with global trends. They are almost certain to rise at a faster rate in the future and have a driving effect on what is understood to be a normal climate event.

For instance, analysis of the four main ports found that the kind of event currently expected to have a 1% AEP (annual exceedance probability or a 1-in-100-year event) will occur at least once a year after only a 0.30–0.45 m sea level rise relative to presentday mean sea level.

More broadly, this brings into question how we communicate risk in a changing climate, particularly the application of historical events when designing for the present and future.

There is also a predicted increase in the severity and frequency of coastal storms. New Zealand has observed a statistically significant increase in waves in the 99 percentile wave height. The most costly areas of our network are often located in low-lying areas or on the coast. When combined with storms and tides, this will increase sewage overflows, erosion and saltwater corrosion and cause infrastructure failure.

Adaptation and decision making

The research emphasised how adaptations should be considered part of an integrated approach and range from those applied to buildings or infrastructure assets through to modification in a community or an entire catchment. From a planning perspective, a distinction was made between different opportunities and constraints relating to the redevelopment of brownfield sites, new greenfield sites and retrofitting.

Building-scale approaches were seen as appropriate, particularly if assets are of high importance or their failure will result in cascading impacts. Measures include waterproof sealants, raising floor levels or rainwater harvesting. Integrated approaches adopted across multiple buildings can also manage water on a community level.

For example, water can be held back in safe storage areas, slowed to give the receiving infrastructure time to adjust or retained as a resource to reduce the demands on the system.

International examples suggest that decentralised approaches can minimise stormwater and wastewater flows at source and reduce demand on existing systems. However, these will have to be linked with parallel public education campaigns to be effective.

Move from fail-safe to safe-to-fail

Given the uncertainty of climate extremes and the lag in upgrading infrastructure, the research emphasised a change from thinking in terms of fail-safe to safe-to-fail. The former focuses on achieving stability, whereas the latter is more appropriate in a system subject to such uncertainty.

An example is considering where excess water can be redirected once design parameters are exceeded. This is essentially moving towards the multi-functional use of space, one that is beyond a development site perspective and instead a catchment approach to absorbing water.

To stay the same, things will have to change …

Overall, our systems have been designed to cope with a 20th century climate that has been assumed relatively stable. The future climate will not only be different, but will see more extremes and require our buildings, infrastructure, land use and decision making to adapt.

It will require increased data on the climatic risks to our stormwater and wastewater systems. This will allow better understanding of what is at risk and where and the consequences of failure, including the cascading indirect effects we may not yet be aware of.

Buildings, land uses and assets may have long lifetimes. Once decisions are made, they tend to be locked in for generations, meaning it is important to make informed decisions now even though information is imperfect.