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Common Dreams reports
A Maryland city was devastated [May 27] after 6-inches of heavy rain caused a downtown flash flood. Major damage is reported and many cars have been swept away.
Ellicott City was still recovering from a flash flood two years ago that killed two and forced the historic city to rebuild much of its Main Street. Residents said Sunday's flood seemed even worse than the storm in July 2016--which was called an extremely rare "one-in-1,000 year event", and cost the city tens of millions of dollars in damages.
Additional information at:
The Baltimore Sun
The Washington Post
USAToday
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Second "1,000-Year" Flood in Two Years Slams Into Historic Maryland City
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In Tulsa, Oklahoma, it is against the law to open a soda bottle without
the supervision of a licensed engineer.
(Score: 2) by Runaway1956 on Tuesday May 29 2018, @02:01PM (7 children)
Your request for citations is not unreasonable - but I'll pass on it just the same.
Meanwhile, it is a known fact that urban sprawl continues. A ten acre Walmart parking lot is not known for it's permeability - nor are the hundreds of miles of streets, sidewalks, and concrete foundations and driveways in a typical medium sized neighborhood. There is little place for water to soak in, and many ephemerals are just paved over, preventing water from freely running off.
Your citation of 8 inches of water in two hours? Interesting - but, how widespread was that? Did it extend a hundred miles in every direction, or was that a local event that only involved a hundred square miles? From the paragraph above your direct link,
So, one has to ask, just how far outside the ordinary is this particular flood? Hmmmmmm . . . Only 7.4 inches of rain fell, compared to 8 inches in the article you cite. But, was this truly a "worse flood" than all those others recorded in the town's history? Maybe this kind of thing happens every fifty years on average?
it makes ME think, "How in hell do these guys know that? There are no records going back 1000 years, they must be pulling numbers out of their asses! In most of Europe, China, and parts of the mideast, where people actually kept written records, they probably have a pretty good idea how often bad floods happen. In the US? Written language was introduced with the arrival of the Europeans, and we haven't deciphered the various records maintained by previous civilizations. Tying knots in yarn is probably perfectly understandable, to the people who were raised tying knots - but we don't understand it at all. Mayan and Aztec both had something roughly equivalent to cueniform writing, but we haven't truly deciphered that either. So - where are the records?
I know for a fact that all of the rivers in America flooded routinely before the Euros started reforming the land to their liking. There were no dams, except beaver dams. Torrential downpours rolled straight down from Bimidji, to New Orleans, with nothing to slow it down.
(Score: 4, Informative) by c0lo on Tuesday May 29 2018, @02:21PM (6 children)
Their specific meaning [usgs.gov]
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Learn first, be dismissive later (if you still feel the need) 100-year flood/Probability [wikipedia.org]
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 0, Troll) by khallow on Tuesday May 29 2018, @02:50PM (4 children)
As to the 1 in 1000 year event, what of stalled hurricanes (that is, very slow moving hurricanes)? They can dump a lot more than 6 inches in a very short period of time. And I bet any place in Maryland (even far inland) would get more than one of those things in a millennium.
I think there's a simpler explanation here. The flash flood wasn't a 1 in 1000 event.
(Score: 3, Insightful) by c0lo on Tuesday May 29 2018, @03:00PM (3 children)
It was you that took pride you had maths at least as a hobby?
If so, would you mind to explain to Runaway how do you determine that N when you assume a binomial distribution and have a representative data set, even if you don't have the full 1000 years "population"?
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 1) by khallow on Tuesday May 29 2018, @03:18PM (2 children)
I wouldn't do that since that wasn't what he asked. But I'll also note that weather is not a binomial distribution. It commonly has a long tail.
(Score: 1) by khallow on Tuesday May 29 2018, @03:23PM (1 child)
And by that, I mean that the size of extremes are not determined by normal weather conditions. The binomial condition you described doesn't give you a means to extrapolate extreme weather conditions. It merely describes how often those extremes are expected to occur assuming certain conditions (such as independence of weather from year to year), and the span of time that one looks at.
(Score: 2) by Osamabobama on Tuesday May 29 2018, @06:22PM
The important thing here is that there is some distribution being used, with the data being fit to it, leading to probability estimates about possible flood levels. Let's assume the model uses the appropriate distribution.
Does the model get updated, though? For instance, if a farm upstream were to build a levee around a field in the flood plain, the crops in that field may survive the flood (yay!), but some measure of water would no longer be able to spend a few days flooding that field, instead being diverted downstream to flood someplace else more severely.
Or, maybe the models are accurate, and we hear about a 1000-year flood in one location, but nothing about the 999 other locations that get a normal level of flooding. This one seems less likely, especially when the same location gets more than one such flood in a lifetime.
It's like a hypothetical statistician and a gambler, both observing a series of coin flips come up heads over and over. The statistician is confident in his knowledge that, eventually, it will even out to about 50-50 heads/tails. But the gambler will quickly start to suspect that the coin isn't fair.
Appended to the end of comments you post. Max: 120 chars.
(Score: 0) by Anonymous Coward on Tuesday May 29 2018, @03:11PM
WTF, that's the geometric distribution, not binomial. https://en.m.wikipedia.org/wiki/Geometric_distribution [wikipedia.org]
Besides that, it also assumes that there is no correlation for the reasons behind any two floods and that the probability of a flood is exactly the same every year. If this is the type of back of the napkin, first-pass analysis being used you should be scared for your town. There is no reason to expect those assumptions to hold, so no reason to be surprised that the model predicts the wrong thing, and so no reason to try attributing it to any specific cause.
And anyway, even if those assumptions were good enough, we need to know many towns are seeing too few or the expected rate of "hundred year floods", keeping track of a bunch and only reporting the places with "excess" floods is cherry picking.
There are so many things wrong here, its total cargo culting. I don't see any hope of coming to a correct conclusion using this process.