Should "500 year flood" be renamed? Redefined? Redrawn?

On Sunday, Trump tweeted, "Wow - Now experts are calling #Harvey a once in 500 year flood!"

Not the best wording, but it would pass a fact check.

I live in Houston. I'm a mile south of Brays Bayou, where it passes through Meyerland (two miles east of the Med Center). The Brays Bayou (among many other waterways in Houston) flooded past the 500 year flood zone.

Recent Brays Bayou / Meyerland floods:
Tropical Storm Allison, June 2001, a 500 year flood.
Memorial Day flood of 2015, a 100 year flood.
Tax Day flood of 2016, a 100 year flood.
Hurricane Harvey, a 500 year flood

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined? Should the maps be redrawn?

Definition of a 100 year or 500 year flood:
There are maps of Houston showing the "100 year flood" zones and "500 year flood" zones. The maps are created by the Army Corp of Engineers and used by insurers. If an area has a 1% chance of flooding in any given year, it is called a "100 year flood zone". If it has a 0.2% chance of flooding in any given year, it is called a "500 year flood zone".

A map showing the 100 & 500 year flood plains for Houston

Karl R said:

On Sunday, Trump tweeted, "Wow - Now experts are calling #Harvey a once in 500 year flood!"

Not the best wording, but it would pass a fact check.

I live in Houston. I'm a mile south of Brays Bayou, where it passes through Meyerland (two miles east of the Med Center). The Brays Bayou (among many other waterways in Houston) flooded past the 500 year flood zone.

Recent Brays Bayou / Meyerland floods:
Tropical Storm Allison, June 2001, a 500 year flood.
Memorial Day flood of 2015, a 100 year flood.
Tax Day flood of 2016, a 100 year flood.
Hurricane Harvey, a 500 year flood

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined? Should the maps be redrawn?

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Multiple years ago, while trying to explain global warming, I used this very analogy. Such things as 100 year flood's; or once in a century heat waves; or "storm of the century", will all become "storm of the decade", or "five year flood", or similar.... as the slow but relentless effects of climate change simply become the new norm. Until places like Miami, and Houston, and Charleston, etc....simply become the 21st century versions of Venice, Italy.

I think they just need to scale it back a bit with the hyperbole.

Karl R said:

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined?

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This seems like a question for an expert, not for me but for fun I think it would depend on a preliminary estimate by the Army Core of Engineers. If the preliminary result is they think that the probability is changing rapidly, then they can propose a study. That study will cost money, probably. Another option is to simply have less accurate data and higher insurance premiums. Does this sound like a good guess?

Karl R said:

Should the maps be redrawn?

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That is guesswork. It depends on if they already have physical data about the land. They probably already do. Then it depends upon if they can use that to estimate the new chance of flooding per year.

Wow, the insurers are sharks. Making sure its a lower percentage so they don't have to pay up when the stuff hits the fan.
Report: ‘Most’ Hurricane Harvey Victims May Not Have the Right Insurance

Karl R said:

iven climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined? Should the maps be redrawn?

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It isn't just climate change that is the problem. It's also what humans do to the topography.
I've lived in my town since 1981. There wasn't a major flood for at least 10 years after. But during the last couple of decades we've had a couple of "100 year floods" and a "300 year flood".
The difference is not the climate. The difference is that upstream developers have convinced authorities to let them channel the creeks and fill in flood plains. Economic development "needed" cooperation and they could afford to buy cooperative authorities.

I don't know if that's relevant to the disaster in Texas or not. But around here, flooding is mostly a manmade disaster.
Tom

Given climate change with the increasing instability of the weather, those older definitions are indeed obsolete. But @columbus is correct that what we've done to the ecosystem is part of the problem.

It's probably caught in a cycle, 500 years ago it was in the same cycle and flooded like that all the time. So call it the non-climate change 500 year flood cycle. Like the business cycle but wetter, and not as good for macro economics.

Those names are just that....names.
It ain't like anyone calculated the frequency.
But yes, some names could stand to change.

idav said:

Report: ‘Most’ Hurricane Harvey Victims May Not Have the Right Insurance

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That's a little hard to say (and the article is somewhat misleading). It is correct that homeowners' insurance won't cover flood damage. (Mine doesn't.) I separately purchase flood insurance through the National Flood Insurance Program, as do many others.

About 373,000 homes in the affected counties have flood insurance through NFIP. About 250,000 of those policies are in Harris County, about 120,000 in Houston itself. It's too soon to tell how many homes were flooded, or how well the insured match up with the flooded. (For example, I have flood insurance, but I didn't flood.)

columbus said:

It isn't just climate change that is the problem. It's also what humans do to the topography.
I don't know if that's relevant to the disaster in Texas or not. But around here, flooding is mostly a manmade disaster.

Click to expand...

There's a bit of both going on with Harvey. It took less than 60 hours for Harvey to go from being a tropical wave (winds less than 35 mph) to a Cat 4 hurricane (winds over 130 mph). That was due to the exceptionally warm waters of the Gulf of Mexico.

Since Allison, Houston has been working on reversing the effects of topography. Bayous have been widened and deepened. Flood retention areas have been added. Most public parks and recreation areas double as floodwater retention areas.

In 2001, every hospital in the Medical Center flooded ... except Ben Taub, the hospital furthest from Brays Bayou.
This week, Ben Taub was the only hospital in the Med Center to flood. All of the others had installed infrastructure to prevent a repeat of Allison. Ben Taub was the only one that felt it was unnecessary.

idav said:

Wow, the insurers are sharks.

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I think that the correct term is not sharks, but capitalists.
Tom

Karl R said:

On Sunday, Trump tweeted, "Wow - Now experts are calling #Harvey a once in 500 year flood!"

Not the best wording, but it would pass a fact check.

I live in Houston. I'm a mile south of Brays Bayou, where it passes through Meyerland (two miles east of the Med Center). The Brays Bayou (among many other waterways in Houston) flooded past the 500 year flood zone.

Recent Brays Bayou / Meyerland floods:
Tropical Storm Allison, June 2001, a 500 year flood.
Memorial Day flood of 2015, a 100 year flood.
Tax Day flood of 2016, a 100 year flood.
Hurricane Harvey, a 500 year flood

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined? Should the maps be redrawn?

Click to expand...

It's a good point, climates change, expectations should also.

According to NOAA - historically about 2 major hurricanes make landfall every 3 years in the US

While Harvey is the first hurricane we've seen in over 12 years, a record span of time, breaking the old record for quiet hurricane activity set in 1860- 1869.


But even if Hurricanes are trending less frequent, flooding is really more a factor of land use and water management than anything else

idav said:

Wow, the insurers are sharks. Making sure its a lower percentage so they don't have to pay up when the stuff hits the fan.
Report: ‘Most’ Hurricane Harvey Victims May Not Have the Right Insurance

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Just yesterday, I heard on NPR that insurers have been selling flood insurance at below actual cost.
This was at government's behest, to encourage more home affordable ownership in flood prone areas.
To keep things going, government subsidizes insurers. I blame government for using crony capitalism
to encourage irresponsible consumer behavior. And note: NPR is no champion of private industry.
It's a perfect storm of stupidity & corruption.

Karl R said:

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined?

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It's nothing to do with climate change, they were never '100 year floods' or '500 year floods' in the first place.

Climate change might make the terms 'more wrong', but they weren't accurate to begin with.

Karl R said:

Given climate change, should the terms "100 year flood" and "500 year flood" be renamed or redefined? Should the maps be redrawn?

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Yes. And this is happening.

The terms more accurately apply to the 100-year/500-year/etc. storms.

How a particular storm translates into a flood depends on things like the topography of the land, size, shape, and roughness of waterways, whether the ground is saturated from prior rain, etc. These assumptions get thrown into computer models to figure out the flood plain mapping for a given storm in a specific area. Those models and mapping change with development regardless of whether anything changes with the storms themselves.

But yes: agencies that are responsible for maintaining the intensity/duration/frequency graphs that describe storm behaviour are working to figure out what the new shape of those graphs should be, because in many areas, severe storms are happening much more frequently than the old curves would predict.