The 500 year flood

noway2

noway2

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Came across this article: http://www.msn.com/en-us/news/us/th...nderprepared-for-harvey/ar-AAqQp7V?li=BBnb7Kz

I decided to put it in the preparedness section because it raises a point that is most definitely a preparedness issue, that the statistical models used to predict severe weather events don't appear to be accurate. Consequently, the risk of a severe weather event, of a magnitude that is devastating may be higher than the official statistics indicate.

The US appears to be getting hit with major storms with unusual frequency. From August 2015 to August 2016, there were eight 500-year flood events recorded by the National Weather Service. There were six “1,000-year” floods in the US over the five years from 2010 to 2014; in 2015 and 2016, though, there were at least three each year.

Tomball, Texas, Public Works director David Esquivel told a local paper there this year that the Houston area had “two 500-year storms back to back”: over Memorial Day weekend of 2015 and early April 2016. That means that Hurricane Harvey constitutes the third “500-year” flood in three years.

Theoretically, the odds of a 1-in-500 event occurring three straight times are one in 125 million. Because Houston is a big city and the same spots aren’t necessarily reaching 500-year levels each time, those odds don’t quite apply — but we’re still, as the Memorial City example shows, talking about events that FEMA estimates to be vanishingly unlikely.
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Maybe the weather and climate models for this stuff aren't that good at predicting macro trends like these events (generally, not each event).

Who knew?
 
Does the article take into account all the pervious lands, forests/grasses, etc, that are being replaced by impervious areas??
Housing developments, apartments, shopping centers, that just remove all the forests, ground cover is a big cause of the increase in flooding.
The water reaches the major streams much more quickly when going over impervious vs forested/pervious surfaces and flooding will be a problem.

IDK...lots around here, especially down east, blame the Raleigh area for the problems with flooding, but maybe don't take into account the above...
 
Undoubtedly the destruction of pervious land will exacerbate the situation. How is a city going to plan to handle > four feet of rain over a 24 hour period? I think that is one of the points that the article does try to address. It mentioned that after the last flood event, Houston, talked about implementing some anti flood measures, but it didn't happen. Also, those measures would have been designed for a much smaller type of event because one of this magnitude wasn't supposed to happen. Still, they probably would have helped reduce the damage.

While climate and storm data isn't available for the last 1000 years, it should be available for the last 100 or so. I am curious about the frequency and intensity of storms and whether or not it has been increasing? Even if it hasn't, places like Houston weren't anywhere near as big 100-150 years ago.
 
That's a great article. It does a really good job of explaining the probability vs assumption of what 500-year floods are (or 15, or 100, or 1,000).

There comes a time in risk mitigation that you cannot prepare for the eventuality because it just doesn't make sense (usually financially). That said, Houston is flat with a lot of water going through it to reach Trinity Bay, Galveston bay, and the Gulf...it happens enough now that they should make some plans.
 
noway2 said:
While climate and storm data isn't available for the last 1000 years, it should be available for the last 100 or so. I am curious about the frequency and intensity of storms and whether or not it has been increasing? Even if it hasn't, places like Houston weren't anywhere near as big 100-150 years ago.
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That's a great question. We know events like El Nino/La Nina will "cluster" certain types of bad storms to a time period and a region. I am curious if other events are correlating.
 
I spoke to a colleague at work who does storm water design vs my structural engineering. She mentioned a new approach that some areas are taking. There will always be a bigger event than you can design for and some times designing to control a large event can cause problems when you have an event that is even bigger than the one you anticipated. An example would be designing flood control devices to retain larger and larger amounts of flow instead of releasing them to let areas drain faster. Once some flood control measures are breached, they serve to lock water in behind them and slow draining.

This is similar to earthquake engineering for me. The Kobe earthquake in Japan caused widespread devastation because the Japanese had designed huge structures to take no damage under their design earthquake. Then a larger earthquake happened and the extra mass of the structures resulted in the structures being shook to pieces. Now we design structures to fail but have energy absorbing characteristics that allow people time to get out of them so life safety is improved.
 
noway2 said:
While climate and storm data isn't available for the last 1000 years, it should be available for the last 100 or so. I am curious about the frequency and intensity of storms and whether or not it has been increasing? Even if it hasn't, places like Houston weren't anywhere near as big 100-150 years ago.
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That depends. What do you want the data to show and what do you think of my grant proposal?
 
wolfpack65 said:
Does the article take into account all the pervious lands, forests/grasses, etc, that are being replaced by impervious areas??
Housing developments, apartments, shopping centers, that just remove all the forests, ground cover is a big cause of the increase in flooding.
The water reaches the major streams much more quickly when going over impervious vs forested/pervious surfaces and flooding will be a problem.

IDK...lots around here, especially down east, blame the Raleigh area for the problems with flooding, but maybe don't take into account the above...
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Yep, when you remove all the grass and trees, put down unending miles of asphalt and concrete, buildings with huge rooflines, and give the water no place to go, it's gunna flood!
 
I ain't real smart, but I do know a few smart people. When buying land, but otmin high spots. Seems to work very well. Big hills are very nice. Little hills are good too. That creekside and ocean front property? Well, it will get wet someday.
 
@wolfpack65 on my way home last night I heard a report saying that much of the land that has been "developed" was prairie grass that will absorb a tremendous amount of water. While it wouldn't have been enough to prevent this incident, it greatly contributed to the problem. In addition, Harris County apparently allowed substandard anti flood building practices compared to the surrounding areas (it didn't go into detail) that are making the situation worse.
 
noway2 said:
the statistical models used to predict severe weather events don't appear to be accurate.
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Well that's it then, you're a Climate Denier and you don't get to speak anymore. The models are perfect, hockey sticks, solar panels, wind turbines, electric cars.



[/sarcasm]
 
It looks like the flood effect area is expanding: http://www.msn.com/en-us/news/us/le...orces-evacuation-in-houston-suburb/ar-AAqWqEN
If I am reading this right, the rivers around the area are overflowing and this is impacting a community about 50 miles from Houston proper with residents being told to evacuate to higher ground.

A levee along the Brazos River south of Houston breached on Tuesday due to heavy rains from Tropical Storm Harvey, forcing officials to call for residents to evacuate low-lying areas.

Matt Sebesta, the chief administrator of Brazoria County, urged residents of the Columbia Lakes neighborhood to leave. "They need to get out. Get to higher ground in Angleton," the county seat, he said in a televised interview.

Columbia Lakes is about 50 miles from Houston, close to the Brazos River. The Houston area's rivers have started to flood. Harris County officials earlier on Tuesday warned residents of six northern Houston neighborhoods to evacuate around two water reservoirs that had started overflowing.
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It is pretty much a feature of our models of any complex process that we will underestimate the tail risks. For those interested in this, check out Nassim Taleb's books, in particular, The Black Swan.

On a slightly different note, a huge number of cars in Houston (4th largest city in America) are flood damaged. Just like after Katrina, many of these cars will be dryed out and repaired to functioning-for-now condition. Definitely do not want to buy a used car that was owned in that area around this time. Also saw that many new car dealership lots were flooded. With all the warnings, you'd have thought more of them would have driven all the cars farther inland. Even 15 min drive farther inland could be the difference between high and dry vs flooded.
 
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