Friday, September 19, 2008

Hurricane sedimentology


Hurricane Ike knocked me off the Internets for a while, but things are slowly getting back to normal. I haven't been so close to - that is, in the middle of - a hurricane before; I have to say it was quite an adrenaline rush to hear and, to a lesser degree, watch the wind going by our windows with gusts of (probably) more than 100 miles per hour. In Houston, the damage was largely restricted to fallen trees and a few broken windows; fortunately not too exciting (see a few post-Ike photos over here). However, things are very different as you get close to the coast. Along the East Texas coast, the storm surge (unusually large for a category 2 hurricane) has shifted the coastline a few tens of meters landward, deposited lots of washover fans toward the lagoonal sides of the barrier beaches, and destroyed a large number of homes in the meantime.

It is worth playing the before-and-after game with the aerial imagery shot by the NOAA's Remote Sensing Division and made available in Google Earth. Here are a few screenshots from the Bolivar Peninsula.

Before:



After:



The light-colored lobes in the upper part of the 'after' image are the washover fans that in places reach the lagoon. Even more interestingly, there are some beautiful little fans built by the water flowing back toward the sea; for each fan, you can see the erosional 'drainage' area and little tributary gullies merging into a single large channel seaward, that turns from erosional to depositional as it widens. This is more clear in the zoomed-in pictures below.

Before:



After:



Note how some of the streets and roads that were perpendicular to the coast have become sites of preferential water flow and therefore locations for these channels.

Of course, all this cool sedimentology (I cannot wait to be able to get out there and have a closer look) also means that many-many homes have just become part of the stratigraphic record. These beaches and islands are the product of the interaction between storms like Ike, when large amounts of sand is eroded from the beach and transported offshore, and fairweather conditions, when sand has some chance to be deposited on the beach (if there is a large enough source of sand somewhere - usually not the case along most of the present-day Gulf coast).

The message should be already boring, but apparently it is not: these beaches and the barrier islands they create are geologically extremely active creatures, and in general it is not a good idea to build homes on them, certainly not right next to the beach. Hurricanes will be around for a while (and some experts say they are getting larger and stronger); and they are very good at creating large storm surges that are highly destructive on shallow shelves with low gradients, such as the continental shelf in the northern Gulf of Mexico.

4 comments:

BrianR said...

whoa ... those images are amazing ... glad to hear you got through the storm w/out much damage.

Silver Fox said...

Barrier islands are great places to visit and walk on - but building, except maybe for touristy businesses who don't care about their future? - seems a bit nutty.

Glad you could do some first hand reporting without getting too caught up in it.

Anonymous said...

Z - just found your blog by accident - it is wonderful!

- Cari in Utah

zs said...

Thanks Cari -- blogging is one way to spend the weekends in Houston... (although I haven't posted much lately). Greetings from South Africa, Z.

 
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