Katrina's Surge, Part 3

A Weather Underground 16 part series about Hurricane Katrina, by Margie Kieper.

For the remainder of the month, we're traveling the coastline destroyed by Hurricane Katrina's record storm surge. This is something that has never been shown on the news or talked about, either in the overall, or in detail. What you'll be seeing here is what people on the Gulf Coast have been calling the “Invisible Coastline” for almost a year now.

Today's journey takes us along a 20-mile stretch through the center of Plaquemines Parish, where Katrina made official landfall: Triumph, Buras, and Empire, up through Tropical Blend, Port Sulphur, and Happy Jack. This area was almost completely destroyed by Katrina, and one year later, progress is slow.

Empire LA

Image courtesy of Google Maps

We're covering a lot of cities today, and to keep track of that, and where we've been, I've marked up this Mapquest image that shows all of the communities:

Mapquest mid-Plaquemines

Image courtesy of Mapquest

The strange thing is, if you thought that yesterday's images of miles and miles of devastation were shocking and extreme, today we're covering about twice as many communities, about three times as many square miles, and you will be able to see from the aerial images, even before zooming in, the devastation from wind and surge in this area was even more complete, and more shocking, than further south in Plaquemines Parish. Homes in most of these communities were completely mashed into a pile of lumber by the strong current of the surge, and most trees were downed by the wind. What remained in the flooded “ring levees” of these communities was a toxic soup of oily briny water filled with pieces of what used to be buildings, along with vehicles, boats, etc. In yesterday's images we saw homes side by side, but flooded to the roof. Today we'll see the remains of homes all pushed together and piled near the levee, nothing more left than a tangled pile of two by fours and siding, with all the personal effects of those previous owners floating or blown or somewhere in that mess.

It's very sad, especially for people along the coast, to view this, and the intent is not to satisfy some shock-news Pavlovian need to sensationalize the impact of hurricane Katrina from some new angle. But it is so important that the rest of the country understand what really happened, and how abandoned in many ways that people felt, especially by media coverage. Miles and miles of land, and the infrastructure on that land, that supported communities, home to many people for generations – gone. This in many ways was much like the effects of the tsunami of winter 2004. That was a very common analogy, as was the idea that it looked like you might imagine someplace to look after an atomic bomb had exploded there. These were the only analogies that came to mind to convey the intensity of the annihilation. There weren't analogies that were extreme enough. There weren't enough adjectives. But it never made the news. Even overseas correspondents did not note the story. BBC News had on the order of 25 correspondents in the US covering the hurricane, I believe – at least 22 or 23 of them in New Orleans.

New Orleans was 80% flooded from levee breaks, those caused by the surge, which in turn were caused by problems with the levee design and maintenance over the years. In some alternative universe: preventable.

Whether or not this was the case with Plaquemines as well, that entire parish was flooded more than 90%; the extent of the flooding is seen in by the yellow area in the map below -- the area that did not flood is represented by light green. But I bet dollars to doughnuts that someone reading this blog is finding out about this for the very first time.

FEMA Plaquemines Flood Inundation

Image courtesy of FEMA - link (PDF Link)

Continuing along Plaquemines Parish north of Boothville, no matter what community we look at, every ring levee is completely flooded, as can be seen in these images of Gulftown, Triumph, and Fort Jackson:

NOAA Katrina Images Gulftown

Image courtesy of NOAA -- link

NOAA Katrina Images Triumph

Image courtesy of NOAA -- link

NOAA Katrina Images Fort Jackson

Image courtesy of NOAA -- link

But once we get to Buras, Sunrise, Empire, and Tropical Blend, the area where Katrina made landfall, the debris field becomes more extensive and the remains of buildings more mangled than before. These images are repetitious, but understand that each one represents several miles of communities that were almost completely annihilated (note: I modified the order of these images so they are running from north to south).

NOAA Katrina Images Tropical Blend

Image courtesy of NOAA -- link

NOAA Katrina Images north of Empire

Image courtesy of NOAA -- link

NOAA Katrina Images Empire

Image courtesy of NOAA -- link

NOAA Katrina Images Sunrise

Image courtesy of NOAA -- link

NOAA Katrina Images north of Buras

Image courtesy of NOAA -- link

NOAA Katrina Images Buras

Image courtesy of NOAA -- link

Some details from these images. The first three are in the Buras area, the fourth is from Empire, and the fifth from Tropical Blend. It all looks alike, for miles and miles:

NOAA Katrina Images Buras - Detail 1

Image courtesy of NOAA

NOAA Katrina Images Buras - Detail 2

Image courtesy of NOAA

NOAA Katrina Images Buras - Detail 3

Image courtesy of NOAA

NOAA Katrina Images Empire - Detail

Image courtesy of NOAA

NOAA Katrina Images Tropical Blend - Detail

Image courtesy of NOAA

These aerial images from Plaquemines Parish web site show Triumph, Buras, and a freighter that was washed up on the levee in Buras:

Plaquemines flooding Triumph

Image courtesy of Plaquemines Parish Government

Plaquemines flooding Buras

Image courtesy of Plaquemines Parish Government

Buras Freighter on Levee

Image courtesy of Plaquemines Parish Government

This is the Buras collapsed water tower and the destroyed Empire Civic Center:

Buras water tower

Image courtesy of Plaquemines Parish Government

Empire civic center

Image courtesy of Plaquemines Parish Government

A tangle of ships were washed up against the Empire overpass, including two large trawlers. Here's an aerial view, a view from the overpass, and a ground-level view of a NOAA Survey crew, showing just how massive these are:

Ships blocking Hwy 23 overpass

Image courtesy of Plaquemines Parish Government

Ships view from overpass

Image courtesy of Plaquemines Parish Government

Ships view from ground level

Image courtesy of USGS

I don't have an exact location in Plaquemines for this USGS photo of a truck in a tree, but there's a good chance it wasn't unique:

Plaquemines Truck in Tree

Image courtesy of USGS

Storm Surge – the Basics

I deliberately haven't talked too much about numbers so far; otherwise you'll be hitting the snooze button by midweek. Surge heights were certainly significant in Plaquemines Parish – as I've said, the entire parish was basically underwater (and, surprisingly, quite a large portion of Jefferson Parish as well…all the way up to Jean Lafitte). The highest surge was on the Mississippi coastline, for reasons that will be explained, and that is where we'll really get into the numbers. Deep in the IPET report, the numbers did show that levees in some areas were overtopped by three feet of surge or more, so levees there fell far short of the height needed.

Storm surge doesn't exist, for the most part, when a hurricane is in deep water, either at sea or in this case in the Gulf of Mexico (GOM). In fact the only change in the height of the water is the amount that water is raised above the normal sea level by the low barometric pressure in the eye. This pressure gradient drops off very, very quickly, so this additional height is mainly in the area of the eye and not much further out.

Hang in here, now, even if you're not technical, because you're going to be able to understand this explanation of storm surge.

Now what is interesting is that many people, including many meteorologists (and you'll find a lot of diagrams on the web aren't exactly correct), think that the bulk of the surge comes from this raised “dome” of water due to the lower barometric pressure. But there will be very few divers (or oceanographers) who accept this notion. This is the reason why: when you dive to the depth of "one atmosphere," you dive about 33 feet, or, 10 meters. And what this is, is the depth at which the pressure on you is doubled; that is, the pressure you are used to at sea level, one atmosphere, the weight of the entire atmosphere pressing on you, plus a second equal amount of pressure, another atmosphere, exerted on you by the weight of the 33 feet of water.

The normal barometric pressure for the GOM is somewhere in the neighborhood of 1010 to 1015 mbar (this is one of the areas I can probably stand to be corrected, just a tad, not being a meteorologist), or we can even round this off to say, around 1000 mbar. A powerful storm such as Katrina had a low pressure of somewhere between 915 and 930 mbar near landfall. So, roughly speaking, that's about a tenth of one atmosphere. And so, logically, that should only displace about one-tenth of 33 feet. And it did; the amount of water displaced upward in this "dome" under Katrina's eye was on the order of about three feet, or one meter. To put this in perspective, there were waves as high as 100 feet in the region near Katrina's eye (while in deep water), in the area of strongest winds (the “radius of maximum winds,” or RMW). Katrina actually set a record for the highest waves ever recorded, and we'll see more about that when we get towards the end of this journey, and look at the information recorded on a buoy south of Dauphin Island, AL.

Some will claim that this dome of water really is responsible for most of the surge, that it somehow migrates to the front of the storm as it approaches land, and piles up in front. That's also not true. The bulk of the storm surge, in Katrina's case, came from an entirely different process at work in the tropical cyclone.

Surge doesn't exist in deep water, but the energy that will become surge once approaching land, does exist. Because this is the case, computer models can correctly predict the surge that will occur. And this was true with Katrina. We'll talk about this model and about the surge predictions, and how well this information was disseminated both to emergency planners and to the general public, in some of the later blog entries.

What happens is that the strongest winds in the RMW drive energy into the water, towards the center of the storm, and over time this creates in effect a rotating cylinder of water underneath the hurricane. This is pictured in this image below, borrowed from a hurricanetrack.com web page that can no longer be linked to from their home page, but which is a great write up and is still out there on the web.

How storm surge is created

Image courtesy of HurricaneTrack

Now for our purposes we can treat this as a rotating cylinder, but the actual shape of the rotating water underneath the hurricane is much more complex, and it is not symmetrical about the storm at all, but is mostly concentrated in the strong northeast quad, as is the RMW (for north-moving storms). But one thing that is important is that as water is driven into the center of the cylinder by the winds, it can sink and escape out the bottom of the cylinder, and so does not build up -- as long as the hurricane is in deep water.

Once a storm gets to major hurricane strength (Category 3-5), the depth of the rotating water goes down several hundred feet. And the width of this rotating water depends on how big the eyewall is, and the associated RMW. Remembering that the area is a function of the square of the radius, storms with a much larger eye will be able to set a much larger area of water rotating underneath them. And this is what happened with Katrina, and why the surge was so large and so widespread along the coastline.

Once a hurricane approaches land, if the depth of the water becomes shallow, then the water that is pushed into the eye cannot escape out the bottom any longer, but builds up inside the cylinder. The cylinder of rotating water tries to spread out, but it cannot, easily, because of friction with the shallow ocean bottom. Conservation of angular momentum will then cause the water to rise a bit against the force of gravity.

You can see by the GulfBase.org image below, that shows the depth of water in the GOM, that the northern GOM coastline is shallow, and so, prone to buildup of surge.

Gulf of Mexico depth

Image courtesy of Gulfbase.org

At landfall, additional surge is caused directly by the action of the wind, if it is onshore. Trying to factor out how much of the surge is caused by water that was brought up and carried along the shelf, and how much was caused by wind-driven waves, is not easy, because it is not additive or linear. However it appears the bulk of Katrina's surge was brought up onto the shelf. Some analyses of the surge have been done, varying different factors such as the strength of the storm after coming up onto the shelf, and these studies will be released as technical papers and presented at conferences or submitted to journals.

Finally -- the term storm surge is used only for the part of the inundation that is directly caused by the storm. It is more correct to refer to the entire surge inundation at landfall as storm tide. This would include not only the storm surge, but high tide, and wave runup at landfall. In the area of the northern GOM, high tide is usually only one to two feet, and so is not a huge factor in differentiating between storm surge and storm tide. When surveying for surge heights after a storm, comparisons of the water marks from both inside and outside are valuable in subtracting wave runup. However for this blog series, storm surge will be used interchangeably with storm tide unless specified.

Hurricane Katrina Storm Surge:

Weather Underground Storm Surge Articles

Storm Surge Safety Actions

  • Minimize the distance you must travel to reach a safe location; the further you drive the higher the likelihood of encountering traffic congestion and other problems on the roadways.

  • Select the nearest possible evacuation destination, preferably within your local area, and map out your route. Do not get on the road without a planned route, or a place to go.

  • Choose the home of the closest friend or relative outside a designated evacuation zone and discuss your plan with them before hurricane season.

  • You may also choose a hotel/motel outside of the vulnerable area.

  • If neither of these options is available, consider the closest possible public shelter, preferably within your local area.

  • Use the evacuation routes designated by authorities and, if possible, become familiar with your route by driving it before an evacuation order is issued.

  • Contact your local emergency management office to register or get information regarding anyone in your household whom may require special assistance in order to evacuate.

  • Prepare a separate pet plan, most public shelters do not accept pets.

  • Prepare your home prior to leaving by boarding up doors and windows, securing or moving indoors all yard objects, and turning off all utilities.

  • Before leaving, fill your car with gas and withdraw extra money from the ATM.

  • Take all prescription medicines and special medical items, such as glasses and diapers.

  • If your family evacuation plan includes an RV, boat or trailer, leave early. Do not wait until the evacuation order or exodus is well underway to start your trip.

  • If you live in an evacuation zone and are ordered to evacuate by state or local officials, do so as quickly as possible. Do not wait or delay your departure, to do so will only increase your chances of being stuck in traffic, or even worse, not being able to get out at all.

  • Expect traffic congestion and delays during evacuations. Expect and plan for significantly longer travel times than normal to reach your family's intended destination.

  • Stay tuned to a local radio or television station and listen carefully for any advisories or specific instructions from local officials. Monitor your NOAA Weather Radio.

Source: NOAA