Monday, August 27, 2007

A reference

During the Q&A following my presentation at Rising Tide 2 on Saturday, someone asked for more details about a study I mentioned.

I had talked about the minimal effect of wetlands when it comes to storm surge reduction. The study I quoted estimated that wetlands reduced storm surge at rates from 2.8 to 3.1 inches per mile. As expected, my marginalization of wetland restoration as hurricane protection was not popular. We all so very much want to believe that the answer is simple and easy. Sorry. It's not.

Anyway, I did not get the name of the person asking for the source data, but just in case she looks here, or someone else wants to investigate further, here is the reference from USGS:

Lovelace, J.K. 1994. “Storm-tide elevations produced by Hurricane Andrew along the Louisiana coast, August 25-27, 1992,” U.S. Geological Survey Open File Report 94-371, Baton Rouge, LA.

I suggested to Maitri that they should post all the powerpoints presented at the conference on the web page. That might prove helpful, too.


mominem said...


I also found a web reference to a similar study for the Chesapeake area, with similar conclusions.

I never could locate the study.

ashley said...


I'm the keeper of the blog...send me the powerpoints and I'll put them up.


swampwoman said...

I inquired about the science behind the portion of your presentation concerning wetlands. Thank you for posting the reference.

Be it is 1 foot, or 3-4 inches of storm surge absorbed per mile, if New Orleans has 40 miles of land mass between it and the Gulf of Mexico, then that is a 10 foot reduction in storm surge. Please correct me if my math is wrong.

Aside from the storm surge, there are other factors important to wetlands, like habitat for wildlife and seafood, and the ability to weaken the power of hurricanes. The reduction in intensity from Hurricane Dean recently from a Catrgory 5 to a Category 1 hurricane as it passed over the ~ 250 miles of the Yucatan Peninsula.

I believe we probably will have to agree to disagree on this issue. Thanks for an informative presentation.

Grandmère Mimi said...

Tim, thanks for your tour de force presentation at Rising Tide. It was excellent.

I am no expert, but I tend to quibble with your diminishment of the the importance of wetlands in protecting from storm surges.

I live in Thibodaux, and we are told that we must leave, not only because of the destructive winds of a hurricane, but also because of the possibility of an 8 ft. surge of water coming from the Gulf, due to the disappearance of wetlands below Houma.

Tim said...


Thanks for checking the blog! I was worried this information would not be available to you.

I appreciate your comments, but as you probably guess, I disagree--or perhaps just have a different perspective--on some points.

1. Nothing "absorbs" storm surge. This is an imprecise and confusing way to talk about the benefits of land features. Marsh and forests help dissipate the energy of tropical cyclones, primarily by providing friction to slow the advance of storm surges and waves. And wetlands sustain a substantial amount of damage in the process, as we know the 2005 hurricanes did. Just keep that in mind.

2. You are correct that Hurricane Dean was greatly diminished by crossing the Yucatan. All hurricanes lose strength when they leave the warm water of the gulf. The Yucatan rises over 1,000 feet above sea level near the middle, so I don't know how that relates to coastal Louisiana. How much reduction was there in terms of storm surge along the coast?

3. It is simply not possible to build 40 miles of land mass around New Orleans as you suggest. That would require filling Lake Borgne, among other things. And even with the ideal benefit of reducing storm surge by 10 feet, it's not enough to protect New Orleans and the surrounding communities from the next big hurricane.

4. How long will it take to build a 40 mile buffer of wetlands around New Orleans? How much sediment does it require and where does that sediment come from? These are huge questions that further reduce the practicality of relying upon wetlands for storm protection. Levees can be built faster using less material that comes from local sources.

I absolutely agree that wetlands are important for many reasons, most of which have nothing to do with hurricane protection. But my talk was exclusively about hurricane protection.

I stand by my comments that wetlands have limited benefits for storm surge reduction. I'm not trying to convince or convert anyone here; I simply want us to be honest and use good science as we move forward.

It is wrongheaded to think that rebuilding the wetlands will save us from future catastrophes, which has become the mantra of many people. Josh Clark, for instance, emphatically stated several times that we need wetland restoration to save New Orleans and Louisiana--he did not even mention better levees, zoning or building practices. I, on the other hand, talked about the contribution of many factors to building a safer city here. That's the point I'm trying to make: don't put all your eggs in one basket!



Anonymous said...


I am a scientist actively studying the causes of the demise of south Louisiana. I have followed your blog with great interest since Katrina and have concluded that you are one of the very few bloggers and PEs (and fellow day-job folks) who understand the real causes of the NO vulnerability to storm surge. You are absolutely right regarding the lack of supporting science for the claim that " ... for every 2.7 mi of wetlands, surge is reduced...". The often cited ref is an internal CORPS report that you can get at work. I recently confronted your favorite geologist on this fallacy at a recent New Orleans Geological Society symposium. He, of course, couldn't come up with a body of scientific literature to prove his case.

Check out an interesting article about a new paper about the geology of New Orleans at the Discovery Channel News. Contact the "local" author if interested.

Keep up the good work!

Sophmom said...

I was sorely disappointed we didn't get to see a panel that involved both you and Mr. Clark. Still, I felt like the whole thing was an amazing success and that you hit the ball way over the fence. Thanks so much. I'm so pleased we have the ppt for reference. Great job, Tim!

Anonymous said...

Tim, Joshua Clark here. I'm not sure we disagree as much as you think. I don't know you yet, and haven't had the fortune of discussing the matter with you, so it seems strange that I'm having
people tell me about your references to my wetlands beliefs. As you say (and this is all I or anyone I know has said), "wetlands reduced storm surge at rates from 2.8 to 3.1 inches per mile." If we built wetlands (and we do know how), then there would be enough of them to kill almost any storm surge. No one is proposing to fill in 40 miles of land around New Orleans. There should be well over a hundred miles of it in all directions, yes, with lakes dotting that landscape.

Of course, I don't think anyone doubts (I certainly don't) that we need stronger levees, dikes, stronger, higher homes, a whole confluence of factors. The reason I harp on wetlands is

1) it is the most important, first line of defense against storms, and

2) most ppl in this country have no idea whatsoever about their existence. Everyone now knows about levees, they hear about them and see them on the news worldwide, but they don't realize simple things like the fact that LA doesn't have beaches (unless you're on a barrier island). They don't realize that New Orleans should not be at the risk it is now because of wetlands erosion. They don't realize how relatively small the price tag to fix them is (as opposed to things like Iraq; as opposed to building bigger and bigger levees every few years since the land will continue to sink without them), and how important they are to the nation.

I think you know all this. I hope you do. Please don't use me as an example on your site. I'd love to talk more about it some time, with you, with an audience, whatever. As you know, there's much more behind all this than I can get into in this blog comment. Keep burning, man.

Tim said...


Thanks for dropping by and leaving the note. I appreciate your willingness to discuss the issue, but then you go and repeat your same errors and make new ones.

For instance:

1. You claim wetlands are "the most important, first line of defense." You also say, "there would be enough of them to kill almost any storm surge." I strongly disagree with both of these statements, and I can provide the factual basis for it. If 40 miles of wetlands could be built around New Orleans, we'd get protection from about 10 feet of storm surge. If "the big one" comes at us with 30 feet of storm surge, wetlands protect us from only 1/3 of it. I maintain "the most important" part of our hurricane defenses is levees.

2. You say, "New Orleans should not be at the risk it is now because of wetlands erosion." What do you mean? That the hurricane of 1915 didn't damage New Orleans because we had more wetlands? Perhaps the 9 to 13-foot storm surge of that hurricane didn't know about the wetlands that were there to protect us prior to the Corps building the Mississippi River levees. History shows that this area has flooded repeatedly during the almost 300 years of western inhabitation and hundreds died almost every time. What makes you think wetlands used to protect the city and its citizens?

3. You claim how "relatively small the price tag to fix" the wetlands is compared to "building bigger and bigger levees." Again, what facts do you have on this? Because I can tell you that building levees requires (a) less sediment than building wetlands to provide comparable storm surge protection, and (b) building wetlands is neither as cheap nor as easy as people seem to think. A lot of people seem to think you can just breach the Mississippi River levee in a few spots and stand back and watch the garden grow. Not at all. It will require massive pumping and flow controls, it will require ongoing dredging to spead the sediment out. It will require more on-going maintenance than any levee.

As far as using you as "an example" on this site, you spoke at a public forum and I disagree with you. I have just as much a right to respond to your comments as you do to mine. Other bloggers have cited your comments at Rising Tide as proof of the need to focus more on wetlands. I wonder if you have similarly asked them not to use you as "an example."

Anyway, thanks again for stopping by. I would like to discuss more with you, but please, bring your science and facts with you, and let's both make an effort to encourage a full portfolio of responses to the threat of flooding and not just carp on one idea.



Sophmom said...

I think we're all very fortunate to have access to this discussion. I, for one, was sitting in the back during Tim's presentation wishing Joshua Clark was there to hear it. I had already read his moving and elegant Heart Like Water, hadn't been able to put it down, gripped by the story and it's riveting telling. Beyond being open to public discussion by his participation in RT2, it's my belief that Mr. Clark started the discussion when he published his book. I, for one, am grateful to both of you and, having asked for this discussion, will retire to my rightful position as an observer. Thank you, gentlemen. Peace.

Anonymous said...

I read your defense on NOLA's wiki on wetlands restoration. It featured this comment:

"It's the overflow that destroys levees. There were no levee failures where the water did not overtop. There were wall failures both with and without overtopping, but levees held up except where they were significantly overtopped."

This makes no sense. You state that there were NO LEVEE FAILURES (I assume this means "the levee held up," where the water did not overtop. You go on to say that there were wall failures (levee failures) both with and WITHOUT overtopping, a direct and obvious contradiction of your first statement. Would you like to respond to this?

Zeraph Moore

Tim said...

Yes, Zeraph, I would like to respond. The confusion is simply one of terminology. I know that for people who depend upon the hurricane protection system to keep storm surge out of their neighborhoods, there is little distinction between levees, walls, gates and transitions. But to engineers working on these structures, there is a world of difference.

It is a fact that no levees failed in Katrina except for the ones that were massively overtopped. This is an important point. Walls made of sheet piling failed, but earthen levees that were tall enough did not. It means that there is nothing wrong with the way earthen levees have been designed and built to resist storm surge up to the top of levee. It means we can focus instead on where the problems lie.

The problems exposed by Katrina include the problem of overtopping of walls and levees. This is something engineers did not design for prior to 2005. I have conducted searches of engineering literature from around the world and I can tell you there is a lot of research about how to resist wave forces on the sea side of levees, walls and other structures. But water washing down the backside of a levee or flowing over a wall has, until now, drawn limited attention. Until now.

Another problem exposed by Katrina is the weakness of walls that deflect under load. IPET called attention to the formation of gap between sheet pile walls and the earth foundation, and, they identified this gap as providing the path for water to exert enough force to move sheet pile walls to failure. This is what happened at the 17th Street Canal breach. It was a wall failure, not an earthen levee failure.

Again, I know this is rather esoteric for the general public, but it is crucial to the understanding of the kinds of forces our hurricane protection system must resist. It is crucial to the protection of all Americans that engineers attack the right problems and focus on solving the real issues.

Thanks for asking, and peace,


Anonymous said...

Thanks, Tim. As I understand it from your reply, 'levees' applies to earthen structures, and 'wall' to the structures we typically think of as levees.