Friday, September 10, 2010

NOLA Needs Scenic Canals, Not Grim Levees?

Seriously? THIS is the recommendation of "experts"?

That NOLA needs scenic canals, and not "grim" levees?

Do I even have to do the calculations to refute the idiocy of this suggestion?

Does anyone really believe it's possible to dig enough canals and lakes in New Orleans so that when the next deadly storm surge arrives that water could be safely and efficiently stored to avoid flood damage? This sounds like a plan concocted by third graders--and I apologize to any third graders reading this if that offends you.

Even if we had huge canals and lakes that could contain all the water from a hurricane storm surge, wouldn't they be filled with water all the time? I mean, you can't store more water in a container that is already filled with water, right? So the whole concept would rely upon being able to pump down those canals and lakes to almost empty just before a hurricane storm surge hits us. And then we'd have to devise some way to funnel that storm surge neatly into the canals and lakes before anyone was hurt.

I know we'd all like to think that we don't need those "grim" levees, but friends, there are no other options. I know we'd like to be able to make this hurricane problem go away if we just had more canals and lakes for storage, or just more swamp to knock down the storm surge, or more river diversions to build "natural" defences...

But it just doesn't work.

Okay, so the engineer in me wins out and I will run the numbers just for fun. I know it's stupid but I can't help myself. Let's see what happens if you try to catch hurricane storm surge with canals.

Assume an ordinary hurricane about 9 miles in diameter. Assume it delivers a 3-ft storm surge to the city limits. That's not a very big storm, but let's just see what happens. Assuming a perfectly round eye and a perfectly cylindrical storm surge from the proposed storm, we get about 5.3 billion cubic feet of water.

Let's check my math: volume is pi * r^2 * h = 3.14 * (9 miles * 5280 ft/mi * 1/2)^2 * 3 ft = 5.3 billion cubic feet of water. Check.

Now for storage: assume a typical canal, trapezoidal shape, 20 feet wide at the bottom, 1 on 3 side slopes, 10 feet deep, 80 feet wide at the top. How much can that canal hold? Area of a trapezoid is h * 1/2(b1 + b2) = 10 * 1/2(20 + 80) = 500 square feet.

Okay, so how many miles of canals of that typical shape will we need to be empty and ready to receive that hypothetical storm surge to save our city? Why, it's just simple division:

5.3 billion cubic feet / 500 square feet = 10.6 million linear feet = 2,015 miles of canals.

Yep, that's the answer. Over two thousand miles of canals to store the water from a hurricane delivering 3 feet of water to the city limits. That's a storm many, many times smaller than Katrina. It's a storm smaller even than Gustav.

What if we make the canals bigger? Double the depth--make them 20 feet deep. Okay, so now I'm calculating that we would need 630 miles of canals that are 140 feet wide at the top. Still a completely ridiculous number. It's less than 10 miles from Lake Pontchartrain to the river, so you're talking at least 63 canals crammed into that space.

So how stupid do you have to be to publish an article with the title, "New Orleans needs scenic canals, not grim levees"? It's idiotic to the nth degree.

Now, please, can we focus on building better levees and quit with the artistic and whimsical ideas about protecting our city?


Leigh C. said...

Yeah, that was also one guy's assessment in "The Big Uneasy" based on what the Dutch have done.

Superdeformed said...

*posting positive feedback to encourage more posts and whatnot*

I have nothing to add really. Beyond fucking stupid.

LatinTeacher said...


I am not posting to dispute your numbers or engineering expertise or anything other than to ask a question that has been at me since I read your post a couple of days ago. I don't know the answer to this and that is why I am asking (there is no ulterior motive!) - There are no levees around Bayou St. John. What happened to the Bayou and surrounding areas when the surge came in? Is there a structure at the lake that prevents the Bayou from flooding? If not where did the water go? If so, I guess that answers that question. If not, why weren't things flooded quite as severely in that area?

PS - The word "grim" is stupid. I can't dispute the need for levees, but I do wonder if there are ways to use the water to our advantage, too.

Tim said...

Bayou St. John is not directly connected to Lake Pontchartrain. There is a gate at the lake that keeps lake water from entering the bayou. There is also a small pump station. At the opposite end the bayou is connected to the canal along the Lafitte corridor. So storm surges do not enter Bayou St. John.



LatinTeacher said...

If I may ask a follow up, why was there a gate on BSJ and not on the other entry points (i.e., 17th St canal, Orleans Ave Canal, etc.)? Is it because they are used for drainage? Would the exact same kind of flooding have occurred had there not been the gate and small pump? Also, it looks like they are building (or have built) a larger structure. Why? (Feel free to respond by email if you would prefer)