What is a 100 year rain anyway?

14 Aug 2014

Anonymous_two_red_diceAfter the recent flooding, news reporters came out and pronounced Monday a “100 year rainstorm.”  What does that mean anyway?  Is this mere hyperbole, like “trial of the century”?

The term has a specific impact for both insurance purposes and for planning.  Your flood risk determines whether you should buy (or whether you are eligible for) flood insurance.  And I assume that, as to sewer backup insurance (there is such a thing), it also affects your rates.  So, given that we’ve had rain storms in Detroit of over 4 inches 4 times over the last 100 years, what is a 100 year storm?

A 100 year rain storm (like a 100 year floodplain) does not mean that it happens only once every hundred years but rather that, statistically, planners believe that there is only a 1% chance of it happening in any one year.  Think about rolling a die.  If you roll a 3 four times in a row (assuming the die is fair), when you roll it a 5th time, its chances of coming up a 3 (or any other digit) is still 1 in 6 or 16.7%.  Weather is a bit more subjective and variable than rolling a die or flipping a coin but the same concept applies.

The problem my neighbors faced this week with flooding comes, in part, from the fact that when the City of Detroit and suburbs were designed and built, no one designed for a 100 year rain. Typically, they designed for a 10 year storm event – a storm with a 10% chance of happening each year.  This was the case, for example, in Huntington Woods  – consequently, the system, as many of us learned  the hard way, was too small to handle all that water in such a short period of time – resulting in basement and sewer backups. This is still the design standard today.

These concepts are difficult for even experienced meterologists and planners as witnessed by this recent NPR story.  And if it is difficult to understand with respect to the “chance” of rain – it must be much harder to predict whether too much rain for the storm system will fall (vs whether it will rain or not) given the dynamism of our climate and weather systems. Perhaps the statisticians are off and these storms are more frequent (or are becoming so). Certainly, a 100 year storm is not the same in Michigan as it is in Oklahoma or in California (which I suspect would’ve been grateful for Monday’s storm, flooding and all). The formulae used to predict a 100 year storm must be based on some past experience and should evolve – but it appears that they haven’t been re-evaluated any time recently.

In any case, one thing is certain, when dealing with our infrastructure, understanding the math, better design, better maintenance and bigger capacity are all things that should be seriously considered to prevent or minimize the kinds of problems that happened on Monday.

Leave a Comment to “What is a 100 year rain anyway?”

  1. Stuart Neiman 18. Aug, 2014 at 6:40 pm #

    My first job out of Grad School was as the Water Resource/Geology Manager for a Regional Planning Agency. As part of my duties I had to work with the Flood Insurance Agency which was just being born and the USACE. The standard norm for determining flood heights was determined by rainfall collected at specific locations that were geographically located no further than 20 miles apart. Most of these stations were maintained by either the Illinois Water Resources Division or the USGS. Using their data and statistical probability curves, a 100 year rainfall for a single day was predicted based on those “collected and statistcally verifiable” data, not the newspapers. The 100 year floodplain was then calculated and mapped using the rainfall quantities and the latest in computer formulae, more commonly known as HEC1 and HEC2 programs. The 100 year flood or rainfall was never allowed to be used to sell papers or stories. I learned a long time ago never to talk to reporters because no matter what you say, they’ll twist it to their advantage not yours and you’ll look like a fool. And guess what? They don’t care.

    So now you know what a 100 year flood and rainfall really is. Only 4 inches of rainfall? That’s not even a 20 year rainfall in the midwest, especially Michigan. I would suspect that a 100 yr flood or rainfall would be at least 10 to 12 inches of rain over a single day. Northern Illinois uses the 1987 rainfall of 18 inches for the 100 yr flood.

    Hopes this helps you.

    Stu Neiman, LPG

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