What’s been missing from the discussion is the idea of resilience, defined as how easily and quickly a system can recover from extreme weather events, whether these events involve too much water or too little. Resilience matters, because despite all the effort put into understanding climate change, there remains a great deal of uncertainty as to what the world will be like in 50 or 100 years. This uncertainty means we must be flexible in preparing to handle what comes next.

Why it’s no longer business as usual

The uncertainty surrounding climate change is different from the concept of risk that engineers are familiar with.

An understanding of risk underlies how engineers create cost-effective solutions. We design solutions knowing that, based on historic records, there is the potential (e.g., once in 5 years, once in 25 years, etc.) for an event to occur that may exceed the design capacity. We work with clients and regulators to make sure the risk associated with the event is proportional to the impact of failure and cost of replacement.

What’s challenging about factoring in uncertainty in climate change is that the goal posts we’ve been using (such as the 1:5-year flood or the 1:25-year flood) have shifted and will continue to shift in the coming decades. Because of the future uncertainty – both what effect climate change will have and how humanity will address it – we cannot determine where the goal posts will be or what size or type of weather events we should plan for.

While there are climate change models that try to predict what will happen, each of the available models runs a range of different emissions scenarios, including factors like the level of carbon dioxide in the atmosphere between now and 2100. Projected temperatures and precipitation vary widely across models and scenarios.

Each model and scenario produce a different projection for temperature and precipitation, and there’s no way to say for sure which projection will be the closest match to the future conditions. That’s the uncertainty in future climate.

In response to that uncertainty, we could take the most conservative approach — design for the maximum of all projections and install huge culverts over what are usually small streams, or place large armoring on a slope to protect the roadway from being eroded by floodwater. However, this sort of over-engineering would be cost-prohibitive if applied in every case. Furthermore, broad-brush mitigation steps will never satisfy all possible futures.

We need a better way to meet the challenges of protecting people and the environment from the uncertain impacts of climate change.

Why resilience matters in a world of climate change

While we can’t address all possible futures resulting from climate change, we can supplement our design process with another approach. This new approach involves accepting that more extreme events may happen and focusing on building ways to support recovery after an event has occurred. Engineers call this “resiliency” – the ability for a system to recover after a damaging event.

As an example, repair crews that go out to a roadway that has been washed out by a flood are concerned about not just the roadway surface and emergency access it provides, but also any sewer pipes, water-supply pipes, underground cabling and other infrastructure that may have been impacted by the floodwater. These elements take time to repair, which has larger societal and environmental impacts.

Resiliency, in this case, is about increasing the speed at which the functions provided by the washed-out road can be restored. This might involve taking steps to have material on hand to repair the road, as well as having trained personnel to make the repairs. Resiliency can also be by design, such as by burying services deeper so they are less likely to be damaged, or using modular, replaceable sections over the crossing to speed repair.

Going further, resiliency requires that money be set aside in maintenance budgets for emergency repairs, and it requires the willingness to replace existing culverts with larger culverts or more resilient systems if it is deemed appropriate.

Resiliency is evaluated through four aspects:

• Societal: How quickly can services be restored, and homes made livable again?
• Environmental: Did the event damage the environment through erosion, habitat loss, or spills, and how easily can it be cleaned up?
• Financial: Is there enough money set aside to quickly invest in new or repaired infrastructure and cleanup?
• Economic: How quickly can economic vitality be restored so that services are available, and productivity retained?

Resiliency applied to the supply of water

As well as dealing with the problem of too much water, resiliency principles help deal with the likelihood that climate change will bring increasing instances of water scarcity. WSP professionals help communities institute Low Impact Development (LID) measures that can help mitigate uncertainty in groundwater scarcity.

LID measures address the concern that when an area gets built up, the precipitation that formerly percolated into the groundwater now flows quickly off hard surfaces like roofs and roads. From there, this runoff may be funneled directly into a watercourse – potentially causing erosion. Fast diversion of runoff also reduces groundwater replenishment, which may in turn reduce the flow of water to private and public wells as well as streams and wetlands.

LID best practice includes creating many small, local projects such as rainwater infiltration trenches that augment the natural infiltration processes that were present pre-development, to remove contaminants and funnel precipitation into the groundwater.

LID installations are resistant to high-flow events, so they can continue their task of restoring groundwater after extreme events. They can also be added retroactively to existing developments to mitigate uncertainty with respect to the capacity of downstream infrastructure. Most importantly, the augmented infiltration helps ensure that water wells, wetlands and watercourses have enough water even in dry periods.

Public policy and regulations can be used to encourage the use of LID measures in future property developments and retrofitting at existing properties. Legal structures can also make it possible for cities to survive dry periods through steps such as prohibiting lawn watering and managing water use during and after dry periods, so that a city’s water supply can recover more quickly.

Resilience is becoming an increasingly important part of preparing for climate change uncertainty. Incorporating resilience into planning acknowledges that while the future is a mystery, there are steps we can and should take to be ready for the impacts of change.