Monday, September 1, 2008

Storm Surge

UPDATE (9/9/2008):
There is an awesome (silent) video from the BBC showing the storm surge as Ike made landfall in Cuba. It is one thing to read about it, quite another to see the spray from a breaking wave rising above a 5-story apartment building and a half meter ? high wave running down a street and smashing into a house. Also see picture number 6 on this photo page.

The live news coverage of Hurricane Gustav has included video from an NBC affiliate showing waves breaking over a levee on the Industrial Canal, an area where there were failures during Katrina. The MSNBC web site has a Reuters photo showing the area in a viewer that makes a link rather useless, but the BBC has an AP photo showing this area:

This is a great picture because it illustrates one of the main improvements made after Katrina. (It also shows that the surge is at treetop level in the canal.)

Analysis of the failures after Katrina by the American Society of Civil Engineers and the US Corps of Engineers showed that many levees failed even though the water level from the storm surge was below the top of the levee. Why?

The surge itself is not much more than a steady rise in the water level, although it can be as rapid as a flash flood on a river. This would be a static equilibrium problem familiar to any student of physics and calculus. [The force on the wall is found by multiplying the water pressure times the area of the wall. Since the water pressure increases with depth, you need to do an integral to add up the forces on the wall that try to push it sideways, or the torque that tries to rotate it about its base.] At least one wall failed during Katrina because its foundation was not strong enough to withstand those static loads even when the water was a foot or so below the top (as it is in this picture).

However, there are also dynamics at work here. The wind produces waves that pound on the wall and break over it. Water landing on the back side of the wall can erode the dirt holding the wall in place. Weaken that foundation, which resists the loads on the wall, and it can slide sideways or tip over, or both. Notice that the wall in this picture has a concrete footing, like a sidewalk, where the water will land if/when waves (or even the storm surge itself) go over the top of the wall. This dissipates the energy of the falling water, which can then run gently off down the sodded levee below rather than erode the base of the wall.

Imagine the difference between running a hose full blast on a sidewalk and running it on your garden or lawn for a day or two. The sidewalk will still be there, the garden will not. Now imagine it was a fire hose rather than a garden hose, and you will see the problem when Katrina drove 6 foot waves over a levee while the storm surge itself was still below the top of the levee.

PS -
Another picture from that same set shows the other side of a Weather Channel standup out in the storm. The secret to getting a steady shot of the reported being buffeted by the wind is to have a small camera man being held by what looks like a retired offensive lineman. I guess if you can bench press 400 pounds, you can hold onto anything in any wind!

Historical Context

Although this NOAA picture is not as clear as the news photo, this photo of a breach of a flood wall due to Katrina shows only grass, rather than a concrete roadway, on the back side of the wall. However, this wall was an outright engineering design failure of the foundation, not a result of overtopping. In the bottom half of the picture, you can still see the levee and floodwall that was pushed, intact, about 30 feet sideways into the Lakeview neighborhood by the water pressure. Metairie (on the left) was dry because water levels never reached the top of the wall.

I thought I should also point out that this famous picture does not show water pouring into the city as was frequently stated in the news. Anyone paying attention will notice that it shows water flowing back out of the city, into the canal! Water was flowing OUT of the flooded areas during the time they were desperately trying to close the breech in the levee with helicopters dropping giant sand bags, not in.

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