Hurricane Harvey

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Gashouse
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Re: Hurricane Harvey

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Arthur Dent wrote:
Harvey is obviously an extreme event, but major hurricanes in the Gulf are not that uncommon. I'm super skeptical that something like this has a <0.2% chance of happening in any given year.
That's a common misconception. People keep saying that there is a 100+ year event every year - Katrina, Sandy, Harvey, CO floods... It's a 1% (100-year) or 0.2% (500-yr) chance of that event happening in each specific location. But if there are 500 storms in the US in a given year, then there's a reasonable chance that one of them is going to be a 500-yr event. The odds of two of them happening in the same location within a short period of time is very low. I can think of multiple 100+ year rainfalls in MN in the past 10 years, but most of them were isolated events where one town got hammered, but a handful of miles away, it was only a 10-yr event.

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mikechamp
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Re: Hurricane Harvey

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mikechamp wrote:Here are a couple of interesting reads on the topic of flooding & insurance:

http://www.politico.com/story/2017/09/0 ... sts-242275

https://www.usatoday.com/story/opinion/ ... 619926001/

The 2nd link has many interesting linked references within the article.
Here's another article I ran across, from a guy that went down there shortly after the storm hit:

https://www.bloomberg.com/news/articles ... heir-homes

Arthur Dent
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Re: Hurricane Harvey

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Gashouse wrote:
Arthur Dent wrote:
Harvey is obviously an extreme event, but major hurricanes in the Gulf are not that uncommon. I'm super skeptical that something like this has a <0.2% chance of happening in any given year.
That's a common misconception. People keep saying that there is a 100+ year event every year - Katrina, Sandy, Harvey, CO floods... It's a 1% (100-year) or 0.2% (500-yr) chance of that event happening in each specific location. But if there are 500 storms in the US in a given year, then there's a reasonable chance that one of them is going to be a 500-yr event. The odds of two of them happening in the same location within a short period of time is very low. I can think of multiple 100+ year rainfalls in MN in the past 10 years, but most of them were isolated events where one town got hammered, but a handful of miles away, it was only a 10-yr event.
I meant specifically in Houston. Hurricanes are a regular event in the Gulf, and some number are going to drop a lot of rain on Houston's drainage area. Doesn't need to be that close to a direct hit, as Harvey wasn't. I don't believe the chance Houston is hit by hurricane related major flooding in a given year is less than 0.2%.

Gashouse
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Re: Hurricane Harvey

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Agreed - houston has a much greater than 0.2% chance of getting hit by a hurricane in a given year, but to be clear, harvey was way bigger than a 500-yr event. As Beerstrike pointed out, the hydrology and the flows used to model the 100-yr and 500-yr events may have been off. Atlas 14 was published only in 2013 and that was a much more comprehensive look at rainfall data than the previous study (TP 40 - published in the 1970s?). Both papers reported estimated return period rainfalls, but in general the rainfall in Atlas 14 increased significantly (but did go down a little in some places), due to better data. The rest of the hydrology and floodplain mapping likely hasn't caught up yet.

Arthur Dent
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Re: Hurricane Harvey

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Gashouse wrote:harvey was way bigger than a 500-yr event
Is it though? How do we know that?

Looking at NOAA's historical hurricane tracks, I find 5 hurricanes that hit the Texas coast at category 4 or 5 from 1842 to 2016. Adding Harvey, that's 6 in 175 years, which scales to 17 over a 500 year period. Based on that, this seems more like a 30 year event than a 500 year one.

Gashouse
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Re: Hurricane Harvey

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The amount of rain was way bigger than estimated 500-yr events since it stalled the way it did.

Gashouse
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Re: Hurricane Harvey

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Gashouse wrote:The amount of rain was way bigger than estimated 500-yr events since it stalled the way it did.
ETA - I tried to find the Atlas 14 maps for Texas, and they aren't on NOAA's website (probably some Deep State conspiracy). But Louisiana's are. I can't find the way to get to the set of data I'm used to finding for MN, so I can't find the various 500-yr rainfall depths - 1-hr, 6-hr, 12-hr, 24-hr, up to 72-hr and all kinds in between. However, the 100-yr 48 hour rainfall depth for most of coastal LA is 16-19 inches. My basis of comparison is the upper midwest, but I assume the 500-yr rainfall over 2 days for LA is only a few inches more than that, and the rainfall for a 500-yr 72-hr may just be a few more inches. Let's be generously conservative and assume that it's reasonable to add 4-6-inches in both cases, so the 500-yr, 72-hr rainfall for coastal LA is roughly 26-30 inches. A town 30 min outside of Houston, which is not right on the coast got over 50 inches in 3 days.

here's the map I could find:
ftp://hdsc.nws.noaa.gov/pub/hdsc/data/se/ar100y48h.pdf

Gashouse
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Re: Hurricane Harvey

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Ran across the following analysis that estimates Harvey was between a 2,000 and 55,000 year event for Houston.

https://issuu.com/asce-ewri/docs/fall_2 ... s_-_issu/6

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lukethedrifter
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Re: Hurricane Harvey

Post by lukethedrifter »

Gashouse wrote:Ran across the following analysis that estimates Harvey was between a 2,000 and 55,000 year event for Houston.

https://issuu.com/asce-ewri/docs/fall_2 ... s_-_issu/6

Should be good for the next few generations then.

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