COMMENT OF THE DAY: A BETTER WAY TO TELL IF YOUR HOME IS GOING TO FLOOD 
“My neighborhood flooded in Allison in 2001, and then again on Monday night. I can’t tell you how many ‘so much for the 100-year flood plain‘ comments I heard walking up and down the street.
What it really means is that it is a flood (or more properly a storm, or my favorite, ‘rain event’) that has a 1% chance of happening every year. So what that really means is that if you live in the ‘100 year flood plain’ you have a 26% chance of flooding during your 30 year note. And for many of these areas the 100-year storm on which these maps are based have 100 years or less of accurate rainfall data.
A better rule of thumb is to remember: (1) if you live near a bayou and it rains A LOT, you will probably flood at some point. (2) if it’s raining A LOT and the road you are on dips below the grade of the adjacent roads, it’s probably going to flood and (3) if it’s raining A LOT where you are in Houston, you can count on it flooding.” [Txcon, commenting on That Place on I-45 North of Downtown Where the Cars Always Seem To Hang Out After It Floods] Illustration: Lulu
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Probability math is confusing to a lot of people. This is a cumulative binomial, which is a common probability equation, but people still get it wrong all the time. It really is an incredibly misleading name: There’s actually only a 64% chance that you will have a 100 year flood in 100 years. Weird right? That said though, the chance of having 2 (or more) 100 year floods in 15 years is less than 1% (0.963% to be exact), so your neighbors aren’t exactly wrong in thinking that its odd to get hit twice that fast. Its statistically very unlikely.
I hope this post is sarcastic, because about two-thirds of it doesn’t hold true.
Aw, you can come up with statistics to prove anything, MrEction. Forty percent of all people know that.
MrEction:
My guess is that the “100 year flood” is incorrectly calculated (i.e., the probability of a particular flood is not really 1%, but more like 2-3%, maybe even 5%). Then the “2 in fifteen” number would change substantially.
@MrEction
That would be the case if the maps were perfectly drawn. Its more a map problem than a probability problem. The post above highlights that the lack of highly accurate rainfall and storm gauge information in many areas for a significantly long time frame. There is also rapidly changing land use patterns (more concrete=more runoff) and significant efforts at rainwater detention/retention, flood prevention etc. Add in differences in rainfall timing, speed, location and you have a recipe for maps that don’t establish the risk properly.
A house that flooded in Allison will be much more likely to flood again than a house that didn’t regardless of how the maps are drawn.
This is why you should always sneak a bomb onto a plane. The odds of one person having a bomb on a plane are small, but the odds of two people having a bomb on the same place? That’s damn near impossible.
Not sure “sneaking a bomb onto a plane” is the best way to make a point regarding probability.
Let’s make it even more confusing/interesting. Hitting 2 100 year flood events has approximately a 1% chance of happening. Seems really rare right? Let’s consider it a 15 year rolling period and see how often you could hit 2 events in that window over the course of 30 years. Well, we already did it! In a 30 year period, there’s actually a 26% chance of seeing 2 100 year flood events in a 15 hear window! Neat huh? Probably not as neat if your house is currently flooded….but still.
When looking for a home to buy, be sure and note if the floors in a house are all tile or brand-new (on an older home). That’s a pretty good sign the home has flooded in the past (and would probably flood again).
I was once at a meeting where long time county engineer Art Storey said that all of Harris County was in the flood plain, we just haven’t drawn the maps that way yet.
The way I can rack my brain around 1% probability and the numerous variables is that there is a 1% chance it will flood each time it rains.
What is often not taken into consideration when redrawing the maps, mostly because there is no way to consider it, is the change that occurs here in Houston on an hourly basis. The Houston of 2001(TS Allison) and of today are totally different places. The changes are often so small they could not be captured in computer models but when combined they can add up to significantly different responses to as much rain as we just got.
So while it is easy to compare rainfall rates year over year, what that rain falls on is so changing it is near impossible to predict anything. What will happen now will be responsive to this weeks rains. ACoE will be out developing new flood plans, maybe doing some work on major drainage ways and COH may improve some local street issues, but all of it will be responsive. New areas will flood next time this happens.
Since math is difficult to many, a shorter thumbnail is to just presume that your house will flood at some point. Odds are always greater for the longer one lives in Houston.
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I presume it is a “ticking time bomb” as long as I’m living on the ground level. Of course, even if you live in a 3-story townhome, your garage is still on the ground floor – and will eventually flood.
My house is near a bayou and flooded in 1998 but it has not since, even during Allison. The flood plain maps are not very accurate. the city and the bayous are changing constantly but the maps are only updated maybe every 10 years. The current FEMA flood maps are based on data from the TS Allison flood which was 14 years ago! Think about how much the city has changed and how much development has happened and how many improvements have been made to the bayous since then.
How to tell if your house is in a flood plain:
1. It is in Houston.
The flood maps are are all subjective . Aka : a big fricking stinkin crap shoot . Harris County / Houston is a sinking swamp and sinking approximately 1 ” per year or about a foot a decade. And with development using open spaces that used to absorb/drain off rain water, flooding is inevitable. That is just plain common sense. Which our public “officials” have little to none of !
Here’s a simple rule of thumb: when the water has risne over the curbs, the right-of-way,and the sidewalk and is advancing across your yard/lawn/etc. towards your home,more than likely it’s flood time!!!
@MH005 has a great point. Your house may not have flooded during Allison, but how much new construction has gone on around you? In my neighborhood, old bungalows raised on pier-and-beam to about 3 feet above the dirt are being replaced by houses and townhouses on slabs, and the developers prep the foundation by trucking in tons more dirt to raise the slab much higher than the lot was previously. So now all of the water flows off of the new construction slabs onto the yards of the neighboring older houses, simply because the older yards are lower than the new ones.
There are parts of the city that are not in any statistical flood plain, and that have never flooded (not even during Allison). My house is in one of these areas. It has not flooded since its construction in 1940, despite all the changes around the city. I observed the flooding this week and noted that the water level in Brays Bayou at its peak would have had to rise another 20 feet to even threaten my home. The blanket statements about the entire county being at risk are not true. I feel it necessary to object to them, because at some point someone is going to use statements like these from public officials to try to force me to purchase flood insurance against my will.
SuperDave: We may not have bought the right house, but we bought on the right block.
I’m at the top of a hill, Braes would have to rise another 25 feet to hit me. Hasn’t happened since 1935 when my house was built.
Does anybody else feel Houston looks really great in a flood? Other cities have mountains or snow or awesome historic architecture, colorful boisterous festivals… But from what I see, flooded Houston is green and peaceful – the perfect spot to live!
I’m think the actuaries have already done all of the math, and the likelihood of your house flooding is already built right into the cost of flood insurance. Having said that, I should say that hopefully the markets for that kind of insurance are the right kind of competitive…
Why does no one state the obvious, that the “1% of flooding in any given year” rule applies to the EDGE of the floodplain. Chances increase as you get closer to the bayou people.
@ movocelot.
I have wondered why they don’t just put a dam under I-10 north of downtown and just turn White Oak Bayou into a series of long, meandering lakes. That would be much nicer to look at than a big grassy flood channel.
@anon22
There is no market at all for flood insurance. It’s a massive federal subsidy that is merely administered by private companies.
You can’t effectively insure against floods.
This is one of those things many Texans like to ignore – that our coastal development is highly subsidized in the form of the government backed NFIP.