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.