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Archive for October, 2012

Now that I’ve showed you a practical example of a levee system, let’s discuss a technical challenge that a levee system has to address and a short case study using the Metrocenter levee and the biggest  flood to occur in Nashville since it was constructed.

Take a look at the map above.  The red shaded portions represent the heavier urban areas on higher ground south of Metrocenter while the white and green portions are the lowland floodplain areas.  You can see a major stream immediately west of Bush Lake, and another running along the bottom of the hilly area and going into the river next to the Bordeaux Bridge.  You can also see a swampy area near the center of the floodplain area.  In order to provide flood protection you need high ground or the human built equivalent. 

By 1968 the embankment for I-65 (then referred to as I-265) has closed off the southeastern gap between the southern hillside and the river, and the southwestern gap is closed off by the Clarksville Pike embankment.  Roadway embankments aren’t built to be impermeable to water the way a good levee is, but that isn’t necessarily obvious to a non-engineer and they do provide a pretty decent physical impediment to water flow (but more on that later). 

When the levee was built in the early 1970’s the arc was completely closed off.  At this point we have the entire area enclosed by higher ground and we can assume it’s protected from the river.  But, and this an important, we have essentially built a hole that water can’t get out of.  The river is kept out, but anything that does get inside our protection is going to be stuck in there.  That water comes from a combination of rainfall inside the levees, and runoff from higher ground to the south.  It’s not that much water compared to the Cumberland River, but if we don’t get it out it will build over time.  (And interior rainfall can become a problem quickly since so much of the area is paved or covered by building roofs that don’t allow water to percolate into the ground.)

This is what the lake in the center was most likely built to deal with.  The interior drainage is diverted to this lake and a pumping system is provided to pick up the water and dump it over the top of the levee into the Cumberland.  If the pumps do their job properly then all is well.  If the pumps can’t get water out as fast as it comes in, the lake gets bigger and water starts to back up along the canals and ditches inside Metrocenter.  Eventually you reach a break point where things start getting flooded.  The design difficulty is figuring out how much pump capacity to install.  Pumps are expensive, and most of what you’re paying for won’t get used unless there’s a flood.  So you end up having lots of expensive pumping gear sitting idle unless there is a major flood.  That’s not the kind of thing real estate developers want to spend money on.  Not only do you have to buy it, but you have to do keep it in working order, which requires periodic maintenance even if they haven’t been used.

During the May of 2010 flood in Nashville there were reports of some flooding inside Metrocenter.  A significant amount of flow also came in through the road embankments.  When I-65 was built the designers knew there was a flooding problem in the area, so the road was built on a base of rock that allowed water to flow in and out without damaging the road or interrupting traffic.  There was no levee back then and no expensive development to flood so a little water flowing through the embankment wasn’t a big concern.  The rain falling inside Metrocenter, runoff coming down the hill from the south, and water seeping in from the road embankments combined so that a lot more water was coming in than the pumps could handle and caused some flooding inside the levee system.

I’m not slinging any blame here, it’s a problem you have to expect when dealing with large areas behind a levee.  The original pump station was built in 1970 and it just didn’t have the capacity to handle the water coming in.  Metro is currently in the process of expanding the pump station and doubling capacity to handle a 500 year rainfall event.

This was also a significant problem for New Orleans for months post Hurricane Katrina.  Once the water gets in the hole, it’s hard to get out and New Orleans had waaaay more water since their levees didn’t hold up.

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Metrocenter through the years

As a follow up to my last post on some of the history of development and the levees at Metrocenter I wanted to post this sequence of maps from the US Geological Survey.  They’re all on the same scale and cropped to roughly the same area.  You can find them for yourself at the USGS map store.

This is the 1952 map, and it’s the base map all the others get their contours from.  I want to point out Cumberland Airfield in the left-center area, along with the big swamp and the railroad spur.  You’ll also want to note Bush Lake.  A helpful commenter from the Nashville Scene tells me Bush Lake was originally a quarry created by  W.G. Bush & Company.  In a few paragraphs the location of Bush Lake will be important.

Next up we have the 1968 map.  Bush Lake is still there, but Cumberland Airfield is gone and the swampy area in the center seems to have been drained with the exception of a few small ponds.  You can see the proposed location of what is now Rosa Parks Boulevard on the hill south of Buena Visa Park.  This would have been after Cheatham Dam was built, but before the levee.

Now we have the 1983 map with the purple items showing what has changed since the previous map.  This would have been 5-10 years after the levees were built.  Bush Lake is gone and several buildings are where it used to be on the eastern edge of the area.  Most of the streets and drainage canals are in place but only the eastern side seems to have many buildings.  The western portion is now the site of a golf course.  Rosa Parks Boulevard has been built, but it’s north of Buena Vista Park and at the bottom of the hill rather than the proposed location from the previous map.  (I assume this was changed in order to avoid the heavy residential area on top of the hill.)  The large purple blob in the center is much as it is today.

The last map is from 1997 and is largely as it appears today.  Bush Lake is mostly gone with all the buildings along Great Circle Road built over its old location.   If you look closely you’ll see a sliver of blue still on the old Bush Lake site.  Google maps still calls it Bush Lake but these days it’s not much more than a stormwater detention pond.

That’s the geographic history of the Metrocenter as best I’ve been able to trace it.  I’m particularly bummed that the earliest map was 1952, but this type of mapping requires aerial capabilities that weren’t really available for civilian use until post World War II.  If you’re so inclined, you can find the 2010 map at the USGS map store link above.  It’s not significantly different from the 1997 map above other than including

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Note: I pieced together this narrative based on archived articles from the Tennessean and the Nashville Banner, as well as accounts from friends who lived in Nashville in the 1970’s.  All conjecture is based on my own personal experience.

Metrocenter was already a going concern when I moved to Nashville in the mid 1990’s, but I’ve heard a lot of stories about the building boom out there over the years.  A quick look at the map makes it obvious the entire area is a floodplain, and yet it’s also the site of a huge office park that includes the Titans practice field, one of Nashville’s four TV networks, a Comcast facility that provides internet for a good portion of middle Tennessee, an art school, and a Metro park and golf course.  During its heyday in the mid 1990’s there was even an outdoor mall with a multiplex until it went under and was repurposed as the campus for Watkins Art Institute (Watkins moved from downtown to make room for the main Nashville library building in the late 1990’s).  After an acquaintance at Metro Water Services told me the infrastructure and improvements in the area were worth $4 billion I decided to spend an afternoon sifting the library’s newspaper archive just to satisfy my curiosity about how it went from the city dump on a floodplain to the tail that wags the dog in Nashville’s flood mitigation plan.

Road and terrain map of the area. Note how the land rises to the south toward Capital Hill

Historically most of the land contained wetland areas or was used for farming.  The earliest reference I found to the area was a story about the girls from St. Cecilia Academy sitting out watching gunboats duke it out on the river during the Civil War.  (The school is on West End now, but back then it was on the grounds of the convent on the hill above Metrocenter.)  There was a small airfieldon the floodplain in the late 1940’s and early50’s and the Nashville landfill was there for a while back in the days when nobody cared what floated down the river.  I’ve also been told that a glass factory was located on the hill and used the floodplain to dispose of a lot of their excess product.  Rumor has it if you dig deep enough in the right areas you’ll find glass under some of those parking lots.  (An old timer told me Capital Nissan’s new lot is on top of the glass disposal area, but I have no idea if it’s true.)

Metrocenter as it looks today

In the early 1970’s the property owner realized he had a huge swath of unoccupied land within shouting distance of the state capital and downtown Nashville.  Add the new Interstate being built next door and it had the potential to be the best real estate in town, if only something could be done about the regular flooding.  This all happened before my time so this is a bit of speculation based on some old editorials from the Nashville Banner, but as best I can tell the property owner basically hired some bulldozers to pile up dirt along the river bank, called it a levee, and started subdividing tracts.  (I’m sure it was a little more complicated than that, but the details are hazy so I’m going with poetic license.)  In the end he had a home-made levee bounded on one end by Clarksville Highway on a high fill as it goes up to the bridge at Bordeaux, and on the other end by what’s now I-65 (called I-265 until TDOT redesignated it in 2000) on top of roughly fifty feet of fill as it goes up to cross the river and merge with I-24.

I don’t have any good idea of how the levee construction was done in this case, but a levee is usually constructed with an  impermeable clay core surrounded by well compacted soil and built well above the expected flood elevation.  It’s a very expensive undertaking, and from the reports I’ve seen the Metrocenter levee wasn’t constructed to any that exacting standard.  It stood for nearly 30 years on it’s own so it obviously worked well enough to keep out the intermittent small floods on the Cumberland, but by the 1990’s the Metrocenter levee was starting to look shaky, and there was a lot of concern about it holding up through a major flood.

The 2000 flood map to the right shows the flood situation in the 1990’s.  The flood studies showed the levee would keep out a 100 year flood (darker gray), but anything greater than that would overtop the levee and completely flood the interior (lighter gray is the 500 year floodplain).  This analysis also assumed the levee doesn’t fail at some point, which may not have been a valid assumption.  (This has been the legal floodmap since 2000.  An update is underway to revise it to take into account the Corps recent modernization project.)

By this point the area was covered with office parks, and a breached or overtopped levee would cause major damage ($4 billion per Metro Water Services), so the landowners and tenants called in Metro, and the Corps of Engineers.  This resulted in a multi-phase levee modernization project.  Phase 1 was completed in the mid-2000’s and consisted of strengthening the levee and raising its top to protect from higher floods.  Phase 2 to remove the vegetation on the levee and plug a couple of holes should be finishing up any time now.

And with that, somebody made a ton of money and managed to get Metro to do all the new levee work and assume all the risk of future flood damage.  It’s really the only logical policy choice for Metro considering the huge potential damages and the modernization project proved to be timely by keeping Metrocenter mostly dry during the May 2010 flood.

I’ve got several more posts in the works discussing some of the details of the Metrocenter levee including how it held up during the 300 year flood in May of 2010, and how I became involved in the project.

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Starting October off right…

In honor of the beginning of October, I’m highly recommending you go read this story written by my friend Betsy Phillips and published in the September, 2011 issue of Apex Magazine.  I don’t want to give anything away because that would ruin the fun of it, but let’s just say it’s a zombie story.  It’s not very reminiscent of the way pop culture usually presents zombies, you won’t find anything reminiscent of George Romero or Robert Kirkman

The story opens with a driving lesson.  This mundane opening scene may make you wonder if you’re reading the right story.  Never fear though, things go creepy pretty quick. 

The pacing and economy of the story telling are what really make me a fan.  Things unfold a little at a time and just when you think you’re clear about what’s going on a new, supremely creepy, revelation comes out.  Phillips specializes in setting a creepy scene without veering into outright horror and this subtlety embues the story with the feeling that everything going on could happen at any time.  The horrible things going on are presented so matter of fact that you don’t always realize what was just said until you’re two sentences past it.

On a more academic note… I usually try not to over think my reading, but I really have to wonder if Phillips intended the story to be a metaphor for how day-to-day life makes us all into zombies and the toll that takes on our relationships.

If you like the story, you might want to look at some of Phillips’ other work.  She wrote a book of Nashville ghost stories, and she links to some of her other work on her blog.

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