Meteorologists tell us they are redrawing their precipitation maps to accommodate “unprecedented” 500- to 1,000-year totals, while politicians proclaim the arrival of a once-in-500-year-storm. The fact is, the math behind these numbers also tells us a 500-year event at a given place has about a 10 percent chance of happening over a 50-year period.
Harvey is extreme by any measure, but we don’t really know how rare Harvey is because we simply don’t have long enough records, nor enough knowledge of how storms will behave in the future to pin down a number. Undeterred, we extrapolate and get what Mark Twain called “wholesale returns of conjecture out of such a trifling investment of fact.”
Our inability to understand and effectively communicate flood hazards is especially well-established. For decades, researchers have asked: Do the public and policymakers understand the 100-year flood concept? Do we understand what flood plain maps are telling us (and not telling us)?
The answer is a resounding no. The 100-year flood, the flood that has a 1 percent chance of being exceeded in any given year, simply does not register in our consciousness. Just the sound of it — a 100-year flood, longer than a lifetime — lulls us into believing such events are rare and immutable acts of God. But to paraphrase the geographer Gilbert White: Floods are acts of God — flood losses are largely acts of people. Harvey is no exception.
Cities in the United States have tried to regulate development in “100-year floodplains” and provide maps of flood hazard zones to the public for several decades. Yet most people are still surprised, if not astonished, to learn that the 100-year flood at a given location has more than a 1 in 4 chance of occurring within the term of a 30-year mortgage. For most of us, this 26 percent chance our home will be flooded before we have a chance to pay it off is troubling if not unacceptable.
This 26 percent is the risk if one lives at the upper edge of the 100-year flood “fringe,” the regulated outer zone of a flood plain. Structures at lower elevations are at even greater risk. To reduce the likelihood of flooding to a more tolerable 1 in 10 chance over a 30-year mortgage, structures need to be above the height of the 285-year flood. That is, if floods behave in the future as they did in the past records used to estimate these probabilities.
Will flooding behave in the future as it has in the past? Most of us intuitively grasp that intense rainfall interacts with increases in impervious surfaces such as roads, sidewalks, parking lots and rooftops to amplify the volume and speed of storm runoff. We are less inclined to think about where the water goes from there — how flood mitigation measures like ponds and detention basins can become less effective over time or fail if not properly maintained, or how the ability of streams and rivers to carry runoff changes due to sediment movement and other natural processes. It’s not just the rain that can change. It’s the rain, the urban footprint and the drainage systems all changing together.
Engineers and hydrologists are trained to look to the past to understand the future. They analyze records of past events (where the information exists) to assign probabilities to future events. This approach will always be necessary and useful; however, it generally presumes that the future will continue to behave within the same range of variability observed in the past.
In the context of warmer seas, mounting urbanization and intensifying rainfall, solely relying on the past becomes questionable at best. Asserting that rain and floods will continue to behave as they have in the past is akin to building a taller water tower that increases the pressure on a town’s pipes but believing that the flows from showers and faucets will remain the same.
Flood hazards are moving targets that often reflect a mix of circumstances. A 50-year rainstorm may produce a greater than 100-year flood if it falls on soaked or burned ground, melts a snowpack or coincides with a coastal storm surge or dam failure. Yet even the most up-to-date insurance maps portray the edges of flood hazard zones as a bright line in the sand — you’re either in or out. This clarity is an illusion. Basic engineering analysis reveals that there is substantial uncertainty as to where the lines should be drawn. But flood plain maps still depict it as a precise line, regardless of whether the potential error is 10 feet or 1,000 feet.
What can we do? Let’s start talking about flood risks over time frames we truly care about — for example, over the term of a mortgage, a lifetime or other planning horizon of tangible concern.
When one realizes their home has a 1-in-4 chance of experiencing a 100-year flood in 30 years, or a 15 percent chance of a 500-year flood over an 80-year lifetime, then perceptions shift and denial becomes more conspicuous. We need to invest in better flood hazard maps and update them to transparently show the uncertainty in flood levels due to model inaccuracies and potential changes in weather, urbanization and drainage. Simply knowing whether the margin of error in mapped flood extents is a big or little number would help.
Ultimately, improved communication of natural hazards must translate to vision and action if we are to avoid repeating the mistakes of the past. To some extent, our language and misconceptions around floods have enabled a status quo in which policy decisions are driven by election cycles and business cycles rather than by analysis of the potential costs of underinvesting in flood resilience over the long run.
The future is unknowable, but investments in hybrid systems of traditional “gray” and natural “green” infrastructure that work together along with nonstructural measures such as insurance reform, zoning, buyout and relocation can improve outcomes across a wide range of future scenarios. Building such redundancy and resilience into flood mitigation projects should boost their benefit to cost ratios, not decrease them as is the case when we focus on a single, oversimplified version of the future.
Regardless of which solution one favors, they all hinge on better communication. Crises open windows of opportunity for doing things better — windows that all too often close before we get unstuck from the status quo. Let’s hope Harvey and Irma will inspire many communities to rethink how they talk about floods, and to begin preparing in earnest for when their window opens.