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WHAT CAN HELENE’S ‘BIBLICAL’ FLOODING TEACH  
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Carl Smith
October 3, 2024
Governing
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_ Historic rainfall that devastated the Southeast was generated by
conditions that still exist. What lessons can local governments in
other parts of the country take from Helene? _

A submerged home in Austell, Ga., part of metropolitan Atlanta., John
Spink/TNS

 

In Brief:

* 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
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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
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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
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Higher temperatures mean warmer oceans, and warmer ocean water means
more evaporation
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The conditions that set the stage for “biblical flooding
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in the Southeast still exist.

Floods are already the most common and widespread
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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?

[BC-WEA-HELENE-NC-2-RA]
 
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
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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
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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
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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.”

[Carl Smith] [[link removed]]

Carl Smith [[link removed]]
Carl Smith is a senior staff writer for _Governing_ and covers a broad
range of issues affecting states and localities. He can be reached at
[email protected] or on Twitter at @governingwriter.

* Climate Change
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* Hurricane Helene; Asheville
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