Hurricane Katrina Storm Surge

[Paranoid]

Hi all. I was looking though the Hurricane tutorial in the extreme weather section and something occured to me. Why was Katrina's storm surge so high in Lousiana, reaching up to 10m in Bay St Louis?

What puzzles me is how could Katrina, a Category 4/3 by the time it got near enough to effect this area, produce such a large storm surge? A category 4/3 storm should have produced a 4-5.5 m storm surge, so how could Katrina produce a 10m storm surge at that strength?

[Somerset Squall]

Hi all. I was looking though the Hurricane tutorial in the extreme weather section and something occured to me. Why was Katrina's storm surge so high in Lousiana, reaching up to 10m in Bay St Louis?

What puzzles me is how could Katrina, a Category 4/3 by the time it got near enough to effect this area, produce such a large storm surge? A category 4/3 storm should have produced a 4-5.5 m storm surge, so how could Katrina produce a 10m storm surge at that strength?

I'm not sure. Would it be the very low pressure? Just a guess really. Does anyone know?

[P.K.]

The surge built up while it was a cat 5. It doesn't fall off as quickly as the wind speed can.

[kold weather]

Also I think the size of the storm also helped it to whip up a massive surge as it did have a large radius of high winds.

[Nick F]

It seems to me that it was a combination of sustained winds of 140 mph (a strong category 4 hurricane) during landfall, the close proximity of the hurricane's eye possessing a minimum central pressure of 920 mb (third lowest on record) at landfall - causing water to rise, and maybe more importantly the shape of the coastline helping funnel the surge with the shallow water depths that are prevelant along the gulf coasts:

Katrina's biggest storm surge occured just East of the Eye across the Bay St Louis area, where water depths are shallow and the bay area helped the surge to be funneled and increase in height:

Here's a good article covering how the differences in shape/configuration of coastlines can have on the effects of storm surges:

http://www.wunderground.com/blog/ncforecas...p;tstamp=200605

To help understand this extraordinary discrepancy, we must realize that coastlines such as found in the Carolina's and the vast majority of the Gulf coastal areas all the way from Texas to Florida's west coast, have long and gently sloping shelves that are accompanied by very shallow water depths.

This had a dramatic effect on the U.S. record storm surge of 27-30 feet thrust ashore in late August of this past year (2005) with hurricane Katrina as this region along the Misissippi coastline is characterized by this dangerous geographical feature to include the shape of its coastline that in the bay areas around Waveland and Bay St. Louis for example help to funnel in an even larger storm surge as well (this same principle applies to the big bend region of the Florida panhandle as well).

As a result, these areas are subject to much higher and destructive storm surges. An interesting foot note is that they are also generally associated with smaller waves as well.

Coastlines that are less affected by storm surges due to their shape/configuration:

If the storms eye moves ashore where the ocean bottom has a narrow shelf where it drops steeply from the coastline and subsequently produces deep water in close proximity to the shoreline, it typically produces a smaller storm surge. However, this geographical set-up generally causes wave heights to be greater.

For example, this is basically the situation found around the southeast Florida coastline that in part helped reduce Hurricane Andrews storm surge damage (although it still reached 17.6 feet nonetheless coming ashore as a powerful 165 mph category five hurricane).

And even better example of this phenomenon would be Hurricane Iniki's relatively minimal 6 foot storm surge as it barreled ashore as a 140 mph 938 mb category four in Kauai, Hawaii as it struck the island head on perpendicular to the shoreline.

In stark contrast, Hurricane Hugo plowed ashore just north of Charleston, S.C. at exactly the same intensity as Iniki in late September 1989 and produced one of the U.S. largest storm surges on record and the East coast largest in history with a height of 20 feet.

[P.K.]

It has to be remembered that the SS scale is purely a wind scale, and not a surge scale. The surge values are there as an indication of general surge values but by no means will it follow them due to all the factors detailed above.