• Hurricane Helene's destructive power was the consequence of record heat, ocean temperature and atmospheric moisture.
  
• Extreme rain, which caused the most widespread destruction, has impacted communities throughout the country in recent years.
• Planning to mitigate the worst consequences of future flooding depends on data that reflects a significantly different risk environment than historical norms.

The dimensions of Hurricane Helene’s impact on residents, infrastructure and the environment are still coming into view and won’t be fully understood for years. Winds as high as 140 mph devastated communities where Helene made landfall, but its most far-reaching impact was the 20 trillion gallons of rainfall it dropped on the Carolinas, Tennessee, Georgia and Florida.

It’s long been predicted that the amount of moisture in the atmosphere would increase with each degree of warming, says Daniel Swain, a UCLA scientist who studies relationships between climate change and extreme weather. This summer, he says, ocean and air temperatures were at or above previous records, and moisture in the global atmosphere reached new levels.

“It’s easier to achieve these seemingly impossible rain events, because there’s that much more water in the air to begin with,” Swain says. There have been six “shockingly devastating” flood events in the world this summer, he says, including catastrophic flooding in Central Europe just weeks before Helene.

Even after Helene extracted enough energy from the ocean to fuel a massive hurricane, Swain says, there was so much warm water beneath the surface of the Gulf of Mexico that it is still at record warmth. This is also true in parts of the Atlantic.

A warmer atmosphere can hold more moisture. Higher temperatures mean warmer oceans, and warmer ocean water means more evaporation. The conditions that set the stage for “biblical flooding” in the Southeast still exist.

Floods are already the most common and widespread weather-related disasters. What do public officials who oversee stormwater systems need to know about mitigating risks — and trying to prevent the worst-case scenario Helene brought to shore?

 

Flooding caused so much damage to North Carolina roads that early in the aftermath of Helene, the state's department of transportation issued a statement saying that residents should consider all roads in the Western part of the state closed.

(Travis Long/TNS)

Past Is Not Precedent

“Things are changing, whatever your beliefs are in terms of why they are changing,” says Steve Parrish, chief engineer and general manager for the Clark County, Nev., Regional Flood Control District (CCRFD). Historically, engineers have used past rainfall events to predict future demands on their systems, but “looking at the past doesn’t really apply to this new future,” Parrish says.

The CCRFD’s policy has been to design for a 100-year flood event, one with a 1 percent chance of happening in any given year based on past rainfall patterns. A bigger storm could overwhelm such a system, Parrish says, but balancing the added cost of more capacity with other needs is difficult, all the more so when the data that could be used to plan is still in flux.

So the Nevada flood control district hired Southern Illinois University in the hope it could help bring things into focus. “They ran something like 18 or 20 different models that showed different scenarios,” Parrish says. Some showed rainfall would drop a bit in the future, others that it would go up 50 percent. “We decided we were going to let the modeling get better, look at it again in the future and see if we need to change anything.”

Communities across the country are investing in flood risk data, comparing rainfall rates to existing infrastructure, says Sunny Simpkins, executive director of the National Association of Flood and Stormwater Management Agencies (NAFSMA). It’s one of NAFSMA’s main initiatives to support this work, which includes taking stock of land-use policies. Another major consideration is how decades of development, including new streets and other impermeable surfaces, have reframed assumptions behind systems that were built decades ago.

Updated, authoritative rainfall projections are essential to informed decisions about infrastructure investments and land use, Simpkins says. The Bipartisan Infrastructure Law directed the National Oceanic and Atmospheric Administration (NOAA) to update its precipitation estimates to account for climate change, and provided funding for this work. Estimates for jurisdictions throughout the country are expected to be available in 2027.

The Federal Emergency Management Agency (FEMA) is responsible for creating maps showing community flood risk. These can be based on historical rain patterns, not those of a warming world. A technical committee that advises FEMA recently recommended that the impacts of severe storms and sea-level rise be better represented in its maps.

A Bigger Picture

Engineer Andrew Sauer has helped design green infrastructure projects (those that use nature-based solutions such as rain gardens, permeable pavement or bioswales to capture rain) for a number of Midwest cities. He says that flood control efforts need to move further inland.

“The big missing piece is that we don’t have full system models of our watersheds,” Sauer says. “We have FEMA floodplain models, but those don’t represent the overall watershed.”

The characteristics of a watershed, defined by NOAA as “an area of land that channels rainfall, snowmelt and runoff into a common body of water,” influence flood behavior. They encompass factors such as topography, soil and vegetation, wetlands and impermeable surfaces in urban areas. (Watersheds typically cross multiple jurisdictions, Sauer says, and mapping them should occur at a regional level.)

Watersheds are dynamic, perhaps more so in a warming world. A forest fire can create a burn scar in place of vegetation that helped absorb rainfall, Parrish says. The 30 to 35 inches of rain that Helene brought to some communities is enough to cause creeks to carve new paths or reroute rivers. Bringing more green infrastructure strategies into watershed management could help reduce the volume of water that stormwater systems are being asked to handle, Sauer says. It’s too much to expect them to deal with the quantity of rain now being dumped by rainstorms.

“We don't have the capacity to deal with that volume," Sauer says, "And that's not going to change in 10 years."

Short Attention Span

“It might actually be true that we don't see more floods in a warming climate,” Swain says. “Maybe we even see fewer floods — but the very worst ones, the very biggest, the most destructive are likely to increase regardless of what happens with the more ordinary floods.” This could create a challenge from a communication and preparedness perspective, he says. Smaller floods serve as reminders to those who live in and manage floodplain regions to stay on top of mitigation.

Stormwater already tends to take a back seat to drinking water in planning discussions, says Sauer — after all, everyone wants to be able to turn on the tap or flush the toilet, but may not necessarily think about drainage systems with the same kind of urgency.

“We have events like we’ve had the last week, they make the news and then two weeks from now, no one wants to talk about stormwater,” Sauer says.

The current crisis may not remain front of mind for those not engaged in the day-to-day work of rebuilding, but the atmospheric and temperature conditions necessary to generate massive rainfall still exist. When and where may not be known but, Swain says, “there’s a 100 percent chance that this will happen again.”