Monday, May 17, 2010

A 1000 Year Flood

NOAA has estimated that the Great Mayday Flood of 2010 in Tennessee exceeded the expectations for a once-in-1000-years flood in many places in the state. I'll duplicate the map of estimated recurrence intervals from the linked article here (click for full size version):

Our place is in the northwest corner of Lewis County, right on the 500-year contour. To our north large areas experienced a rarer-than-once-per-millenium rainfall.

So, what does this mean? Is it a sign of the apocalypse? Is this one of those beyond-the-range-of-historical-variation freak events that would be the vanguard of rapid global climate change? At this point, one would have to say "probably not." This may seem odd given that it shattered records across the area for rainfall totals and flood heights. But rare events must always be interpreted in the large scale.

First, these estimations of quincentennial and millenial extremes are based on extrapolations from roughly 100 years of real data. We have a general idea of the frequency of extreme rainfall events within regions based on data from many stations, but we don't have 500 years of real data from any single point to validate these estimates. So there is some room to question them. But more fundamentally, the occurence of extreme events, even hyperextreme events, is to be expected. If you have 1000 watersheds, on average you would expect one of them to experience a 1000 year flood every year. Someone wins the lottery, and someone gets hit by the meteorite.

In the case of this recent event, there's an important starting point. Tennessee is a rainy place. It is one of the rainiest of the 50 United States on average. At our homestead, my records for 2002-2009 average about 58" of precipitation per year, which is typical for highland rim locations. That's just shy of being a rain forest by some criteria. The Nashville Basin averages a bit drier, but it still runs well over 40" per year on average. Rainfall is also quite variable here. Droughts and heavy rains are common. Before this year, again just from 2002-2009, we have had months with over 15" of rain and months with no measurable rain at all. Daily totals over 3" occur every year; totals over 5" have happened a couple of times before in our brief experience. So, while a 48-hour storm total of 18" might be a millennial extreme occurrence just about anywhere, Tennessee is a good place to look for one.

As for the specifics, if you look at the map and look at the area enclosed within the 1000-year contour, you'll see that it is only a small fraction of the total area of Tennessee. It's actually probably only about 1/1000th of the total land area of the Lower 48. Which means (I'm sure you are ahead of me here already), a rain event this extreme might be expected to hit an area about this size every year, SOMEWHERE within the Lower 48. It's just not very likely to hit any single particular spot. This time it hit populated areas including a State capital. Next time it might be all rural areas and attract far less attention.

You can never infer a trend from a single incident. But even given that, it appears at this point that the 1000-year flood in Tennessee is still likely to be within the expected range of extreme climate variability for the region and continent as a whole. The red flag will be if these types of events become more common. There have been some studies suggesting that this is indeed happening, but you still can't come to a solid conclusion yet.

Personally, I remain suspicious, but will defer to the probabilities at this point pending further recurrences. I'm also still keeping an eye out for a second South Atlantic hurricane; this has not yet happened, leaving the 2004 storm categorized as an isolated event without long-term implications. I also derive considerable comfort in knowing that our own bottomland homestead evidently survived the 500-year deluge with only minor consequences.

Tuesday, May 04, 2010

More on flooding in Hickman and Lewis Counties

Yesterday was a sunny, mild day in Tennessee, which allowed people to get started on cleanup and repair after the unprecedented rains and flooding over the weekend. The small streams have mostly fallen back below flood levels, but the larger rivers are only now passing their crests and inundation around them remains extensive. Large areas in downtown and metro Nashville remain submerged.

Around our place in Lewis County, I found an unfortunate flood victim in the middle of our yard, doubtless a casualty of stranding when the waters receded (click any image for a larger version):

These next two shots show how the stream channels realigned themselves in just a few hours, sometimes by as much as several meters. You can see how one bank has been eroded away, leaving undercut steep bluffs, while a gravel bar has accreted on the other bank. There are so many of these changes around our place that it is taking me a while to find old landmarks:

Over the next few months the vegetation will begin stabilizing these new banks, the beavers will rebuild their dams in new locations, and the landscape will settle in to a new normal until the next gullywasher comes along.

A mile down our road is a typical scene:

Before the county road crews barricaded it, people were still driving over this bridge as it was their only way out. Many people remain stranded behind scenes like this. It was this possibility more than anything else that lead us to evacuate our place rather than risk being trapped for a week.

From home we headed north on Highway 48 into Hickman County. Hickman had been entirely isolated by flooded roads and cut off from communications until yesterday. Peggy's commute transects the county, so we felt the need to check out if the roads were open and passable before she attempted them in the pre-dawn darkness. As I had feared, the lowlands south of Centerville along Highway 100 had been hard hit. A typical scene:

Note the mangled fences bearing high water marks that are well up into the buildings. The jumbles of mangled debris are ubiquitous.

Another common sight is flooded and overturned vehicles:

One hopes that they were not occupied at the time!

The power substation at Centerville was flooded:

All power in the area went out, including ours, about 8:30 a.m. on Sunday, probably when this station failed. Our power came back in less than an hour; most of Hickman outside of downtown Centerville remained in the dark through yesterday.

When people don't have tap water for days on end, they get it where they can find it:

Just north of downtown Centerville our excursion came to an end at the Duck River bridge on highway 100:

Normally this bridge sits high in the air over soccer fields.

The water was clearly well over the bridge at the crest:

This would help explain why phone service has been out county wide:

As the power outage also knocked out the cell towers, there was no communication in or out for about 36 hours.

At the north end of the bridge the roadway drops down a bit into the Defeated Creek drainage. It was probably still under 6-8 feet of water; the fish camp and restaurant has disappeared entirely below the river. Clearly Peggy was not going to be commuting past this point anytime soon:

Monday, May 03, 2010

Duck River at Centerville

The all-time record flooding on the Duck River at Centerville Tennessee this afternoon. This is the USGS gauge on the Highway 100 bridge which earlier recorded a crest nearly 10 feet above the previous all-time record. As you can see from the debris on the supports, the water was about five feet above this level at the crest, and well over the road surface on the bridge. Ordinarily this bridge sits far above the river, with a city park and soccer fields well below it. The bridge was still closed by floodwaters at this time and had become the major attraction in town. Centerville is without drinking water and has only had phone and electric service restored very recently.

More photos from Hickman County tomorrow.

Flood photos

A few images from the flood yesterday and its aftermath here at home. So Google searches can find the right post, these pictures are from western Lewis County, Tennessee, near Hohenwald, in the drainage of Cane Creek. Click on any image to see a larger version.

Two shots snapped during our escape as the waters were rising rapidly; as you can see we might have waited a few minutes longer than would have been ideal. Looking out the windshield:

The water was less than a foot deep, but it was still unnerving. Especially considering the view to the right:

That is our across-the-street neighbor. The water is pouring over the levee of the pond to the left and across the yard. In the background on the upper right you see the roiling raging torrent of mud that is Cane Creek, normally a placid headwater stream a few feet across and a few inches deep.

Later that afternoon as the waters receded, they left us many gifts and flattened fences:

At the downstream end of our property the road was washed out:

This is not as impressive as it may seem; this road washes out about every other year on average.

Today we drove up into Hickman County, which seems to have been hit as hard as anywhere. More details and photos will follow.

14.21" / 361 mm

That is the two-day storm total rainfall at my official CoCoRAHS reporting station in the orchard. There are two things that are remarkable about this number, other than its just being FRIGGIN' HUGE. First, this is a typical number for this storm! Total of of 12-16" are widespread; covering many counties, hundreds of small stream drainages, and the homes of probably close to a million people. Second, this number is so far beyond unprecedented it is hard to believe. Nashville's official NOAA 2-day storm total was 13.53". This is a new record, breaking the old record of 6.68" set in 1979 (records go back to the 1870s). In other words, this is more than twice as much rain in a 48 hour period than has ever before been recorded there in 140 years. In our case, the bulk of that rain fell between noon Saturday and noon Sunday, so if I had measured hourly increments I would probably have a 24 hour period with about 10" of rain in it.

To our north, Hickman County seems to have fallen down a black hole. Radar estimates indicate that they were one of the hardest hit counties. They are very rural and hilly, much like Lewis County where we live. The last report was at noon yesterday, when the county government issued a civil emergency statement declaring a county wide flood emergency and that the entire area was without power and communications. Another report indicated that the drinking water treatment plant had shut down. It is typical in these situations for isolated areas to drop off the media radar, to resurface days or weeks later with harrowing tales. As an indication of how bad things might be there, the automated USGS gauge on the Duck River at Centerville has been transmitting data through the flood. It shows the Duck having crested nearly 10 feet above the previous all-time record crest, with data going back at least 119 years.

I know that you can't infer much from single events, especially in an anecdotal context. But you combine things like this with the snowstorms last winter in the mid-Atlantic, and it does make you begin to seriously wonder about global weirding. We were noting that the last three places we have lived (here, Fort Collins CO, and Georgetown SC) have all experienced "100 year floods" during our time there (the Great May Day Floods of '10, the Spring Creek Flood, and Hurricane Hugo). It isn't just that these events were in the general area; they directly affected our immediate location. Something does not add up about this...

It also appears that the first person who was killed by the flooding in Nashville was a young man whom we actually know.

What a weekend...

ADDENDUM: The Nashville NWS office has prepared a summary of storm total rainfall from CoCoRAHS data. They have us mapped as 14.39" in the western edge of Lewis County; I guess they included my 0.18" from Friday in the total.

Sunday, May 02, 2010

Hard to comprehend...

...what is happening in middle and west Tennessee over the last two days.

Widespread rainfall totals in excess of 12" from a train of storms that refuses to quit. Every body of water in this region is setting a new "all time" flood record. Water rescues are happening in places that have not seen a drop of floodwater in decades.

I tried to ignore the sound of the rain when it started back up at 6:00 this morning, but I just couldn't. Within an hour we had gotten 4 more inches on top of already swampy ground. The creeks on both sides of the house climbed higher than we had seen before; when the water touched the corner of the house we grabbed the dogs and changes of clothes and headed for high ground. We almost waited too long, as the neighbors pond was overflowing its levee in a solid sheet of water and washing across the road. The floodwaters weren't just covering the low spots, they were forming cascading blankets across the slopes.

Sometime later this afternoon we should be able to try to see how close to home we can get to find out what has been happening there. Meanwhile the videos coming out of Nashville look like archival footage from Katrina.

ADDENDUM: Made it home, the house is fine, water entered the crawlspace but not the structure Much of our drinking water supply infrastructure was obliterated and there is damage to fences and such, probably not much that a few hundred dollars in supplies and a week's labor can't fix. We definitely are among the lucky ones in Tennessee today!

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