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Roni Deitz

Project Engineer with Arcadis, passionate about improving the way we think, plan, and communicate the risks associated with climate change.

Created: Thursday, February 7, 2019 | Updated: Tuesday, March 19, 2019

Communicating flood risk to homeowners is essential to resilience

The way we talk about flood risk is broken. It’s hindering our resilience.

Over the last few years there are unfortunately plenty of examples to prove this point — whether it’s homeowners standing with two feet of water in their living room after a large storm event, without flood insurance and unaware of their proximity or elevation in relation to the banks of a river; residents unnecessarily evacuating ahead of a hurricane, causing lengthy traffic jams; residents not evacuating ahead of a hurricane and scrambling when fuel and water become scarce; or, neighborhoods settled behind a levee or floodwall caught off-guard when their homes and businesses flood simply because the storm was larger than the levee or floodwall was designed to withstand. Each of these examples is proof that we are falling short in our ability to communicate risk.

As we recover from this past hurricane season, now is the time to acknowledge that the language we use is inadequate for communicating risk.

We must acknowledge that flood risk is dynamic and therefore the way we think about it, communicate it, and plan for it, must be dynamic too; and that community resilience begins with dialogue.

Flooded houses and cars during Hurricane Harvey. Flood risks are going to be at the forefront of hurricane preparedness. PC: Luke Sharrett | Bloomberg | Getty Images
Flooded houses and cars during Hurricane Harvey. Flood risks are going to be at the forefront of hurricane preparedness. PC: Luke Sharrett | Bloomberg | Getty Images

It’s Time to Retire “The 100-Year Flood”

A conversation about how we communicate flood risk requires taking a closer look at a term that has become central to the way we speak about storm events: the 100-year flood. After Hurricane Harvey, Dan Zarrilli, the Chief Resilience Officer for New York City, took to Twitter, urging followers to retire the term “100-year storm.”

100 year storm needs to have communication of a flood risk. PC: Twitter.
100 year storm needs to have communication of a flood risk. PC: Twitter.

Hydrologists use the term “100-year flood” to describe a flood event that statistically has a 1 in 100 (1%) chance of occurring in any given year. While the term is correct, it’s confusing to residents and business owners. To most people, the “100-year flood” sounds like a storm that has a chance of occurring once in 100 years, and, if it just occurred, will not happen again for a long time. But the reality is that there is no guarantee that if a 100-year flood happens this year that it won’t happen next year, or even that it won’t happen again in the next month. Further complicating things is that what constitutes the 100-year flood is changing because of climate change and urban development (I highly recommend this clip, where Last Week Tonight with John Oliver explains how climate change and aggressive development can exacerbate flood risk).

When we start to think about the 100-year flood as a flood that has a 1% chance of occurring in any given year, compounded over 30 years (i.e., the duration of a typical home mortgage) the probability of a 100-year flood event happening is 26%. The difference between actual risk and a homeowner’s perception of risk could determine whether a homeowner purchases flood insurance or not.

If we’re not thinking about and communicating the ways in which our surroundings are also changing, regions are left exposed to greater flood risk. This means greater risk to individual lives, homes and community assets — like the uninsured home that wasn’t initially in the floodplain but now suddenly needs to rebuild after a large storm.

What Are We Protecting Against?

When designers, engineers and policymakers set out to design levees, floodwalls, pumps and other infrastructure solutions to “protect” communities from flood risk, one of the first questions that must be answered is “what level of protection are we designing to?” In countries like the Netherlands, that level of protection is often the 10,000-year flood; in the United States, it’s frequently the 100-year or 500-year flood.

For example, during Hurricane Sandy, Coney Island Beach performed as intended — it protected residents from flooding — until the storm surge was higher than the height of the beach. At that point, floodwaters breached the beach and homes and businesses were inundated. In situations like these when the system performs exactly as it was designed to, yet homes, businesses, and roadways still flood, many residents become confused by the term “flood protection.”

Understanding what happened during Hurricane Sandy, what could happen in the future, and communicating risk and opportunities to further mitigate risk was a large part of community outreach and engagement of the Arcadis-led Coney Island Resiliency Study.

It’s confusing to residents because there’s a tough underlying message to communicate: it is impossible to protect neighborhoods and communities from every possible storm. No amount of planning, design or spectacular engineering could have completely protected the Greater Houston region from the 50+ inches of rain that fell during Hurricane Harvey.

Instead of complete protection to every possible storm, resilient infrastructure offers risk reduction. That’s why, when it comes to floods, we need to talk about risk reduction instead of protection.

Shifting the Flood Conversation

Resilience begins with robust discussions between designers, engineers, policymakers, and residents about flood risk, and using the appropriate language to educate and accurately convey risk. These conversations need to be ongoing and rely on the best science and data available.

As we refine our resilience lexicon, two simple switches I recommend are 1) Replacing the term “100-Year Flood” with “1% Annual-Chance-Flood” and 2) Replacing the term “Flood Protection” with “Flood Risk Reduction.” These two swaps alone may be sufficient to begin garnering the public support needed to change how we think about building and our response ahead of a disaster.

Similarly, a few topics to begin the dialogue include:

  • What risk is my community exposed to?
  • How does this risk change over time?
  • What level of risk is my region exposed to today and in 50 years?
  • How do we continue to build in redundancy should one layer of risk reduction measures become overwhelmed?
  • What can individuals or neighborhoods do to further address their residual risk from flood hazards?

The more we talk, understand and educate one another about the risks associated with urban flooding, the better our communities can prepare, respond and become more resilient in response to an ever-changing and unpredictable climate.