Where is my "Oh shit" dashboard?

April 30, 2009 10:43 AM

How do I understand floodplain maps? Just how high does the river near my house need to get before my house floods?

This is just out of curiosity, I am in absolute no immediate threat of this happening. (As far as I know)

I live on Lawrence, Kansas just a couple hundred feed from the Kansas River. My house is protected by a levee and is listed by a FEMA flood plain map as being in a "Zone X" which I interpret to mean a 500 year flood plain.

Here is the flood map of my neighborhood. I live on Ash Street - In case the map link does not work, here is the same street on Google Maps) Here is the closest USGS river measurement station.

Every spring I watch the river get high and wonder "Just how high does it need to get before my house floods?" Can anyone point me to the data indicator I could watch and at what level I should start worrying?

posted by jlowen to science & nature (14 comments total)

I've got a screen capture of my floodmap here if it helps.
posted by jlowen at 10:53 AM on April 30

The screen capture link doesn't work and neither does the first link in your post.
posted by If only I had a penguin... at 11:13 AM on April 30

…and a bungled link.
posted by Dick Paris at 11:13 AM on April 30

You can probably use Google Earth for this, with some accuracy. When you have topography checked on, it'll tell you the elevation where the pointer is. So, hover the pointer just outside of the riverbank, and that elevation is the flood stage. If you know the elevation of the top of your levee, subtract that from the flood stage, and that's the river elevation that you need to worry about.

So far as the 500-year thing, that means that every year there's a .2% chance that it's going to happen. I wouldn't worry about it.
posted by hwyengr at 11:21 AM on April 30

Thanks Dick.. Don't know how I messed that one up. Thanks for fixing it.
posted by jlowen at 11:21 AM on April 30

Here is The FEMA guide to reading flood maps, which may be of some assistance to you.
posted by Lame_username at 11:26 AM on April 30

That flood map doesn't have any good information other than which zone you're in. I just had a quick look on Google Earth. The elevations are usually fairly reasonably accurate, and it looks like you're at 250m on Ash St. and the river's elevation is 243m. So, 7m or about 21ft. From a quick scan you're sitting in the bottom of a bit of a basin, but even so that river's gotta get pretty high for you to be worried about it.
posted by jimmythefish at 11:28 AM on April 30

Thanks to all.. I think I have it figured out..

This page says that at a level of 34.0 the river reaches the top of the levee protecting my house. Its currently at 13.05 feet

Historical Crests
(1) 29.90 ft on 07/13/1951
(1) 29.90 ft on 07/01/1844
(3) 28.00 ft on 05/31/1903
(4) 26.80 ft on 07/27/1993
(5) 25.10 ft on 06/23/1951

Based on my reading of the flood map am I to assume that my house is safe until the water crests over the top of the levee? Or do levee's typically "break" before then?

Thanks all!
posted by jlowen at 11:43 AM on April 30

The USGS link you provide gives you the current gauge-height (how high the river is, AKA stage) and the current discharge (how much water is flowing down the river). The missing piece of data you want is the calculated 500-year flood discharge. I've done a quick search, but I can't find anything more than 100-year flood discharge (PDF, site 1873). Here are a couple interesting fact sheets about previous floods (which had discharges greater than the 100-year level).

You could ask the USGS what the 500-year flood discharge is, but be aware that flood predictions at the 500-year level are fairly rough. Note that discharge is much more useful for your purposes here than gauge-height because the cross-section of the river at the USGS station may be quite different from the cross-section near your house. In other words, a doubling of discharge at the USGS site may only increase the gauge-height by a couple feet, whereas it may increase the height near your house by much more or less if your cross-section is deeper or shallower (e.g. because of your levees).

That said, I'm sure you would know when the river was rising anywhere near your flood level, because there would no doubt be other areas flooded out before you (in other words
posted by ssg at 12:13 PM on April 30

(oops) ... in other words, a 500-year flood area is considered fairly safe).

But now that I've looked at the FEMA map a little closer, it looks like Zone X indicates, in your case, that your levee is designed to protect you from at least a 100-year flood, which is an entirely different story.
posted by ssg at 12:15 PM on April 30

Thanks Sag.
So I am to understand that I live in a 100 year flood that is artificially made to be a 500 year flood plain via the levee? That makes sense because I think my area was underwater in both 1903 and 1951, before the levee but not in 1993 after the levee was built.

Does one expect a levee to survive until it is over ran at 34 ft? What about the Led Zeppelin song?
posted by jlowen at 12:53 PM on April 30

I'd feel pretty comfortable with a levee built in '93. They're designed to survive an overtopping, but the odds of you seeing even seeing a flood that's an additional 4' higher than the all-time historical high is incredibly slim. (.2% per year, in fact :-) )

Could it break during a lower flood? Sure. But, there's really no way to say that you should worry about it at a certain elevation.
posted by hwyengr at 1:09 PM on April 30

So I am to understand that I live in a 100 year flood that is artificially made to be a 500 year flood plain via the levee? That makes sense because I think my area was underwater in both 1903 and 1951, before the levee but not in 1993 after the levee was built.

No, (assuming you live in a levee protected area) all you can determine from the FEMA map is that your levee will protect you from at least a 100-year flood.

Also, the 1903 and 1951 floods were greater than the 100-year flood, but the 1993 flood was less than the 100-year flood. That said, you can see from the NWS site that your levee is 34ft high and that the 1951 flood only reached 29.9ft, so you know that the levee is designed for much higher water than just a 100-year flood.
posted by ssg at 1:23 PM on April 30

Reiterating that the map clearly states that they believe you are protected from a "100 year" flood", and this doesn't mean you get 100 years between floods. The better way to think of it is that there is a 1% chance every year of a flood that *exceeds the levy design specifications*.

Of course, it is possible that the levy would hold past a 100 year flood, but they aren't promising.

As to the question about the levy holding, there are multiple designs for earthen dams. Generally they are subjected to only temporary loads during flood events and thus will hold until over-topping occurs, but again, this isn't guaranteed. It is possible that (like a bridge with a weight designation) there can be levies that will hold at a certain water level only (or only for a certain time period while saturated).
posted by meinvt at 4:33 PM on April 30