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Levees
(also known as dikes)
What is a levee?
BY MARSHALL BRAIN & ROBERT LAMB
In his poem "The
Dry Salvages," T.S. Eliot described the river as "a strong brown
god," a powerful force that, while often patient and nurturing, frequently
proves itself untamable and merciless.
The
greatest cities in human history have risen up on the banks of rivers and by
the seaside, but in doing so, their builders chose to live in close confines
with an unruly force.
Rivers
have surged to wash away whole communities or changed course to abandon
prosperous kingdoms to the dust.
Even
today, ocean storms threaten to decimate centuries' worth of human
endeavor.
Humans
have attempted to conquer the problems of changing water levels for thousands
of years.
One
of the oldest weapons they've wielded against the rivers and oceans is the levee, also known as a dike.
A
levee is simply a man-made embankment built to keep a river from overflowing
its banks or to prevent ocean waves from washing into undesired areas.
In
New Orleans, the levees attempt to perform dual duties: On one side of the
city, levees protect against floods from the Mississippi River, and on the
other side, they help to keep Lake Pontchartrain at bay.
In
parts of the Netherlands, dikes stop ocean waters from reclaiming thousands of
miles of land, much of which is either at or below sea level.
The
famous windmills of Holland pump water from behind the dikes and back out to
sea to keep the land dry.
There
are even parts of the Netherlands, called polders,
where the Dutch have created new dry land along the sea by diking and draining
it.
A
levee is typically little more than a mound of less permeable soil, like clay,
wider at the base and narrower at the top.
These
mounds run in a long strip, sometimes for many miles, along a river, lake or
ocean.
Levees
along the Mississippi River may range from 10 to 20 feet (3 to 7 meters) tall.
In
Holland, they can top 30 feet (10 meters).
But
there's no set height for levees. Their measurements vary according to the
storms the area receives, even if those storms occur only once every hundred or
thousand years.
Living
by the water provides humans with a number of advantages: fertile farm land,
transportation, trade and hydroelectric power.
Levees
allow humans to enjoy these assets without fear of flooding. But humans have a
bad habit of forgetting just how powerful their "strong brown god"
really is.
When the
Levee Breaks
While
strolling along the beach or enjoying a picnic by a riverbank, it's easy to
forget how powerful Earth’s waterways really are -- until floods and
storms jar us to remember.
In
1927, the Mississippi River swelled under heavy rains, charging through a line
of levees and flooding an area the size of Ireland.
In
1953, the North Sea broke through the Netherland's ancient system of dikes and
killed thousands.
In
2005, New Orleans made international news when Hurricane Katrina breached its levees. Much of the city
lies 10 feet (3 meters) below sea level.
Over
the course of the city's history, low-lying, boggy areas have been pumped dry
to create new land. Much of this reclaimed land has sunk as it dried out.
The
entire city now depends on the levees, along with massive pumping stations, to
keep the water out.
Hurricane
Katrina flooded 80 percent of the city, killing approximately 1,600 people and displacing
some 200,000 others [source: Dolfman et al., Gonzales].
How
could this happen? An investigation by the National Science Foundation pointed
to five major reasons:
1.
Insufficient planning: New Orleans' levee designs were based on an outdated
1965 study. Engineers built the levee system with the goal of creating a system
that could stand up to the worst storm possible in 200 years. Unfortunately,
the study greatly miscalculated how powerful potential storms could be.
2.
Riskier design: New Orleans' levees were built to sustain the city's
growth, unlike the levees in neighboring areas, which were built to provide
safety. As a result, New Orleans' levees were shorter and weaker.
3.
Safety compromised by bureaucracy: No central agency was in charge
of maintaining the levees. This task instead fell to several different private
firms and government agencies, leading to communication problems and the
breakdown of various upgrade projects.
4.
Poor maintenance: Levees require constant upkeep. As the land in New
Orleans sinks, so do the levees. Investigators also suspect that large trees
growing nearby undermined the levees.
5.
Insufficient funds: The U.S. Army Corps of Engineers, which oversees the
design and construction of levees, had been hit by budget cuts. This left the
agency with fewer experienced engineers.
As
New Orleans continues to rebuild from the disaster, some of these concerns are
finally being addressed.
The
Netherlands faced a similar situation following the country's 1953 flood. How
do its successes match up to New Orleans' failures
Dikes of the Netherlands
In
the decades that followed the deadly flood of 1953, Dutch engineers set out to
build a new kind of barrier against the sea.
They
steadily replaced the old dike system, which had been in place since the
medieval ages, and created 350 miles (563 km) of what many consider the safest
levee system in the world.
The
Dutch set the standard for levee construction by re-evaluating their entire
system in several key ways:
1.
Thinking long term: While the 1965 team of engineers in New Orleans
tried to build levees strong enough to withstand the strongest possible storm in
200 years, Dutch engineers designed a system strong enough to match the kind of
catastrophic storm that only occurs once in 10,000 years.
2.
Less reliance on solid barriers: Instead of constructing
increasingly bigger barriers like levees and floodwalls, Dutch engineers have
sought to create better ways of absorbing floodwaters in marsh plains and
specially constructed rivers. In some cases, this even involves setting dikes
farther back from the water.
3.
New textiles: The Dutch also developed tough, synthetic textiles to better
anchor earthen levees. These prevent soil movement and water penetration. The
New Orleans levee system began using this technology following Hurricane Katrina.
4.
Better monitoring systems: In addition to commanding more stringent,
centralized control and maintenance of their dikes, the Dutch also use
automated surveillance systems to keep an eye on how their levees are holding
up. They installed fiber-optic and electronic sensors in dike structures
to report changes back to a central monitoring station. Several other systems
monitor water pressure and water level.
Much
of the Dutch levee system relies on the understanding that levees require
regular maintenance, constant monitoring and a long-term appreciation for how
rivers, oceans and storms behave.
When
these are in place, communities can thrive safely alongside the beauty and
convenience of coastal and riverside areas.
It's
when we fail to remember this that rivers and oceans become destroyers.
Marshall Brain, Founder
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
About Robert Lamb
As a child, Robert Lamb dreamed of becoming a mad
scientist when he grew up. As this profession proved to be largely fictional,
however, he swallowed his heartbreak and turned his attention to the written
word instead. He earned his bachelor's degree in creative writing from the
University of Tennessee in Knoxville, which launched him on a career path
through high school English classrooms, small-town newsrooms and finally into
the offices of HowStuffWorks.
As a senior writer and podcaster
for Stuff to Blow
Your Mind, Robert now spends his days sifting through all the
scientific wonders that make the world so mad and amazing. He currently lives
in Atlanta with his lovely wife and their beautiful one-eyed cat. When he's not
researching the apocalypse or the miracle we call a space toilet, he enjoys
listening to electronic music, painting the odd miniature and writing fiction.
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