This is Eureka, Missouri, a city that floods
repeatedly.
We’re talking catastrophic floods, twice
in less than two years.
“Yet more rain and dangerous flooding are
affecting parts of the Midwest and South tonight.”
“Take you across the Meramec River, notorious
for flooding very quickly.”
“Towns like Eureka, and others, well they’re
in for a rough few days.”
But, just 12 miles downstream is a town called
Valley Park.
It stayed pretty dry thanks to a levee.
A giant artificial embankment that surrounds
the city, designed to keep water out.
Valley Park got their levee  largely because
the city was willing and able to pay for it.
It kept their own town dry but here’s what
flooding looked like in neighboring towns,
including Eureka, that didn’t have levees.
“The valley Park levee withstood the water.”
“That levee holding could be part of the
problem.”
“They suspected the same levee that protected
Valley Park may have been to blame for devastating
flooding in Fenton, Eureka, and Arnold.”
Can a levee protect one town while making
flooding worse for others -- especially towns
that can't afford a levee?
A fluid mechanics lab, a 13-foot long model
of a river, and some adorable tiny houses
will help us find out.
People love levees.
There’s about 100,000 miles of these embankments
across the US.
Sometimes concrete, sometimes earthen for
centuries, we’ve built them between humans
and rivers.
We even write songs about them.
But it turns out, even when levees are effective,
they can still be devastating.
As far back as 1852, levees came with a very
important warning.
Charles Ellet Jr. - a famed US civil engineer
- cautioned that levees confine rivers and
cause them to "rise higher and flow faster."
And relying on levees “encourages a false
security.”
But he was largely ignored.
Levees became the default for flood control
- mostly funded and constructed by local entities.
Some riverside communities that could afford
it, built taller levees for more protection,
while less fortunate neighbors dealt with
the devastating effects.
To show you what we mean we went to the banks
of the Mississippi River.
To a fluid mechanics lab, where a team of
engineers from the University of Minnesota
built us this landscape model to test flood
scenarios.
Overhead, a scanner collects 3D data to measure
exactly what’s happening.
It’s a generic model of a river with no
levees.
That means, when the water level increases,
it overflows.
And spreads across the floodplain, often creating
important wetland habitat that’s home to
a variety of species.
Putting in levees cuts rivers off from this
land, destroying floodplains and wetlands.
It allows people to convert these areas into
farmland or build houses on them.
And while levees protect these communities
from flooding, they constrict the river into
a narrow channel, making the water flow faster
and higher.
That creates a bottleneck leading to additional
flooding upstream.
If all the levees are the same height, both
sides should be about equally protected from
the average flood.
And if the river rises so much that the water
overtops the levee, then both sides should
flood pretty equally.
But let’s say, people on one side of the
river lobby for higher levees.
Now, instead of both sides flooding, only
one floods.
The side with the lower levees is at a clear
disadvantage.
So what can people in flood zones do?
We can't just pick up and move major cities.
We need levees to protect places like these.
There is an alternative.
We could build levees farther back, so rivers
could still expand and create wetlands.
These “setback levees” ease flooding on
both sides, rather than protecting one city
at the expense of another.
This approach is common in other parts of
the world - like Holland for example - but
not in the U.S.
Here, we tend to build levees right next to
rivers.
Some communities that can afford to build
higher levees do so at the risk of others
with little oversight.
We do have the Army Corps of Engineers -- a
federal agency tasked with regulating a fraction
of all levees.
They have to ensure -- at least on paper -- that
federal levees won't dramatically raise local
flood levels.
But those engineering predictions don’t
always match reality.
For instance, the Army Corps designed Valley
Park’s levee in the early 90s with data
and software from that era.
Engineers estimated the levee’s impact on
neighboring areas would be minimal.
But by the time it was completed in 2005,
the region had grown significantly.
More people had built alongside the river,
increasing the risk of flooding - which wasn’t
considered in the original plan.
While we don’t know the precise impact of
Valley Park’s levee on neighboring towns
just yet, since the levee was built, the region
outside Valley Park has suffered two of the
worst floods in its history.
The Army Corps says they’ve done nothing
wrong, and that the levee meets all state
and federal laws.
But there’s a growing body of research that
shows levees push flooding onto surrounding
communities that have lower levees or no
levees at all.
Researchers measured water levels around 13
levees in the Midwest and found they all increased
flooding -- some by over five feet.
As the climate changes and cities push for
higher levees, flooding is only expected to
get worse.
Especially along the Mississippi River, which
is almost entirely lined with levees.
Some of these embankments have been substantially
raised since their completion, against federal
rules, making flooding worse across the river
and upstream.
So, yes, we need some levees.
But the system for regulating them is broken.
Even though the science overwhelmingly  shows
that constructing higher levees makes flooding
worse in the long-term.
We keep building them taller, passing our
problems upstream.
