Hurricane Katrina is often referred to as a "category 3" hurricane. I want to make the case that this label is misleading.
Precisely
10 years ago this morning, Hurricane Katrina made landfall in Louisiana and
Mississippi as a category-3 hurricane on the Saffir-Simpson Scale. The first
landfall along the Northern Gulf Coast occurred just after 6AM CDT near Buras,
Louisiana, where maximum sustained winds were estimated at 110 kts (127 MPH)
(Knabb et al. 2011). Katrina then tracked across shallow water and wetlands
before making a final landfall near the Louisiana/ Mississippi border, with
maximum sustained winds around 105 kts (121 MPH) (Knabb et al. 2011).
Although Katrina is often referred to as a "category-3" hurricane, referring to its maximum sustained wind speed at landfall, it generated a massive storm surge that flooded approximately 80% of metro New Orleans.
Image: http://i.dailymail.co.uk/i/pix/2012/08/28/article-2193403-05488FCE0000044D-642_964x642.jpg.
Both
of these intensities are classified as category-3 winds on the Saffir-Simpson
Scale.
However,
Katrina generated a 27.8 ft (8.47 m) storm surge at Pass Christian, Mississippi
(Knabb et al. 2011), which is the highest surge in the history of the Western
Hemisphere (Needham et al. 2015). The surge in St. Bernard Parish, Louisiana,
topped out at 18.7 ft (Knabb et al. 2011), which is the highest surge level in
the modern history of Louisiana (Needham et al. 2013). This storm surge was
responsible for flooding approximately 80% of metro New Orleans (Kates et al.
2006) and led to a catastrophe in which more than 1800 lives were lost
(McTaggart-Cowan et al. 2008). The storm surge directly caused between 600-700
of these deaths (Boyd 2011). Katrina’s price tag totaled more than $100 billion
(Blake et al. 2011), making it the most costly natural disaster in U.S. history
(Kessler et al. 2006; Baade et al. 2007).
Katrina's rapidly rising flood waters trapped thousands of people on roofs and attics in the metro New Orleans area. These flood waters directly claimed more than 600 fatalities.
Image: https://media.epactnetwork.com/wp-content/uploads/2013/11/04depa450.jpg.
So how
did a “category-3” hurricane generate such a terrible catastrophe?
Fortunately,
since Hurricane Katrina, much research has investigated the complex nature of
storm surge, helping us understand the complexities of this hazard. The bottom
line is that many variables contribute to storm surge height, not the maximum
sustained wind speed of a hurricane alone.
Hurricane
size, or the area of strong winds, jumped to the forefront, as a variable that
was not understood very well preceding Hurricane Katrina. The importance of cyclone
size became clear as Katrina generated a higher storm surge than Hurricane
Camille of 1969, although Camille produced stronger winds. Katrina’s large size
played an important role in this process, as the diameter of hurricane force winds
extended over 210 miles of open water and tropical storm force winds extended
over 460 miles of open water as the sun set on Katrina the night before
landfall (Knabb et al. 2011). This tremendously large wind field displaced a
massive amount of water that piled up on the Louisiana and Mississippi coasts.
As the sun set on Hurricane Katrina on Sunday, August 28, 2005, the storm was centered 130 miles S of the Mouth of the Mississippi River. Maximum sustained winds exceeded 160MPH, the diameter of hurricane force winds extended for 210 miles and tropical storm force winds extended across a diameter of 460 miles. Image: NOAA.
Some
notable papers in the past 10 years that investigated the role of hurricane
size for generating storm surge include Irish et al. (2008); Nielsen (2009);
and Dietrich et al. (2011).
Several
hurricanes following Katrina also taught us about the importance of hurricane
size for generating storm surge. In 2008, Hurricane Ike made landfall along the
Texas Coast as a category-2 hurricane, but generated a storm surge of 17.5 ft
(5.33 m) in Chambers County, Texas. (Berg 2010). Ike also inundated much of
South Louisiana with a storm surge that exceeded 11.5 ft (3.5 m) and extended
inland for more than 33 miles (55 km) (Federal Emergency Management Agency
2008). In 2012, Hurricane Isaac made landfall as a category-1 hurricane in
South Louisiana, but generated a storm surge that exceeded 14 ft (4.3 m) (McCallum
et al. 2012). Later that year, Hurricane Sandy approached the coastline
of New Jersey as a category-1 hurricane, but generated a massive storm surge that
inundated New Jersey and New York with seawater, inflicting a price tag of more
than $50 billion and contributing to 147 fatalities (Blake et al. 2013).
In 2012, Hurricane Isaac's large size enabled it to inundate Long Beach, Mississippi (above), although the storm was a category-1 hurricane that made landfall south of New Orleans.
Image:
http://www.baynews9.com/content/dam/news/images/2012/08/Long-Beach-MS-082912.jpg.
Hurricanes
Ike, Isaac and Sandy were not officially classified as “major” hurricanes,
because they made landfall with wind speeds less than the category-3 threshold.
However, they collectively generated more than $60 billion in damage (Blake et al. 2011), primarily
from large, destructive storm surges. All three of these hurricanes were
geographically large, enabling them to generate massive surges.
Hurricane
forward speed is another variable that influences surge timing and height. This
role of this variable was investigated by (Rego and Li 2009). Hurricane Isaac
provides a classic example of the increased surge potential from slow-moving
hurricanes. Isaac’s forward motion became nearly stationary along the South
Louisiana coast, enabling the storm to push water into Southeast Louisiana,
Lake Pontchartrain and Mississippi for an extended period of time.
Other
variables that generate storm surge include bathymetry and coastal shape. The
influence of bathymetry, or offshore water depth, is a bit counter-intuitive
because storm surges tend to reach higher levels in areas with shallow
bathymetry (Needham and Keim 2011). If you’ve visited coastal Mississippi you
know that the water is so shallow that you can walk offshore 150 feet (~45
meters) and the water will not even be as deep as your knee in some places. The
influence of harbors and bays are also counterintuitive, as they typically
provide a safe haven for boats and ships, but during a hurricane-generated
storm surge actually enhance surge heights (Needham and Keim 2011).
A 25.1-ft (7.65-m) rod does not quite reach the height of tree bark
removal on East Ship Island, Mississippi, following Hurricane Katrina.
The tree bark removal at this site extended to 8.2 m (26.9 ft),
indicating an extraordinarily high storm surge at this location. Katrina's surge in Mississippi was the highest surge level in the modern history of the Western Hemisphere. Photo
provided by Hermann Fritz.
Two overlooked variables for storm surge generation are the
roles of pre-landfall wind speed and size. Jordan and Clayson (2008) first
investigated this topic, and several years later I wrote companion papers with
Dr. Barry Keim that provided the first data-driven analysis of this phenomenon
for the U.S. Gulf Coast region (Needham and Keim 2014a; Needham and Keim 2014b).
These analyses quantified the improved correlation of storm surge heights with
pre-landfall wind speeds and size, compared to winds and size at landfall.
The
graphic below shows the correlation between storm surge height and maximum
sustained winds at landfall and at 3-hour increments preceding landfall. The
blue bars represent the correlation using an array, or list, of actual wind
speeds. The red bars indicate the improved correlation levels when that array
of wind speeds is raised to the optimal exponential power. This graphic is
adapted from Needham and Keim (2014a).
Needham and Keim (2014a) found that storm surge heights correlate better with
wind speeds 18 hours before landfall than any other time period. Graphic
adapted from Needham and Keim (2014a).
This
graphic shows that storm surge heights and winds have the poorest correlation
at landfall, but that correlation dramatically improves as we use the wind
speed before landfall. The relationship is optimal at 18 hours before landfall,
when the R-squared value approaches 0.70, if the array of wind speeds is raised
to the power of 2.2. This analysis reveals that the relationship between surge
heights and wind speed is quite non-linear; we found that doubling the strength
of pre-landfall winds increases the surge potential by a factor of around 4.60.
Hurricane
Katrina was a very large and intense tropical cyclone 18 hours before its final
landfall. The maximum sustained winds at that time exceeded 170 MPH (Elsner and
Jagger 2013), which would have placed this storm on the threshold of a
category-6 hurricane, if the Saffir-Simpson scale were a continuous scale. In
fact, Hurricane Katrina’s pre-landfall wind speed was comparable to Camille’s.
Katrina’s wind at this time was only about 11 kts, or 13 mph, weaker than
Camille’s (at the same time increment) (Elsner and Jagger 2013), but Katrina’s
larger size enabled it to generate a higher surge.
Map of Hurricane Katrina's hourly position and intensity, as well as the height of storm surge and storm tide observations. Katrina was a large, cat-5 hurricane before striking Louisiana and Mississippi. Storm surge and storm tide heights exceeded 16-ft (4.88 m) along the entire Mississippi Coast and portions of SE Louisiana.
Map archive at: http://surge.srcc.lsu.edu/historical_maps.html.
So
we must be careful when we refer to Katrina as a “cat-3” hurricane. Katrina’s storm surge caused much more damage
and loss of life than its strong winds, and levees are built to protect from
water, not wind. To call Katrina a “cat-3” storm uses only one metric- the
maximum sustained wind speed at landfall, to classify this cyclone. This label
is misleading, as the public is aware that the Saffir-Simpson Scale ranges from
1 to 5 and a “cat-3” sounds like an “average” hurricane to the public. This
label may be misused to imply that the flood protection around New Orleans
could not withstand a strike from an “average” (cat-3) hurricane, which is very
misleading.
In
reality, Katrina was a large hurricane, with maximum sustained winds that would
have approached category-6, if the Saffir-Simpson Scale were a continuous
scale. And those wind speeds peaked around 18 hours before final landfall, the
time that correlates best for generating catastrophic surges. Taking into
account its large size, as well as the shallow bathymetry and presence of bays
(like Bay St. Louis) near it’s final landfall, we should not be surprised that
Hurricane Katrina generated the largest storm surge in the modern history of
the Western Hemisphere.
Perhaps
we should say Katrina struck the Northern Gulf Coast with cat-3 winds and a
cat-5 surge? It’s a controversial statement, especially after much effort to disassociate Saffir-Simpson wind speed categories from storm surge heights. Yet, in practice, Katrina is regularly called a "cat-3" hurricane, and on the ground, many people still refer to flood protection as "cat-4" or "cat-3" levees. Calling Katrina a cat-3 (at landfall) wind event and cat-5 surge event, would at least communicate the notion that these storms are more complex than just one number on a scale of 1-5. Either that, or perhaps we should just say it was a hurricane that made landfall as a cat-3, but produced the highest surge in the history of our hemisphere.
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