Modern cars are built to wreck themselves in a very specific way. The crumple zone up front folds like an accordion in a crash, soaking up energy that would otherwise end up in you. Nobody calls that a design flaw. You get the car fixed, and you’re glad the front end took the hit instead of your ribs.<\/p>\n
A dam in southeast Tennessee runs on pretty much the same idea, just with lumber. And after the November 5 flood, it needs a new set.<\/p>\n
The dam is Ocoee No. 1, also known as Parksville Dam. It sits on the Ocoee River in Polk County, about 16 miles east of Cleveland, and the Tennessee Valley Authority runs it. Along the top of its concrete spillway there’s a row of wooden boards, and TVA says those boards are meant to get swamped and snap loose when the river runs high, which opens up more room for floodwater to get past the dam. That’s according to Rocket City Now<\/a>, which reported TVA’s comments on Friday, November 6.<\/p>\n TVA also said the boards will have to be replaced. So it sounds like they did their job.<\/p>\n The agency told the outlet its Dam Safety team started looking the dam over on Friday, the day after the flood, and described that kind of inspection as standard practice after a big one. To get new boards in, TVA said, it’s dropping the water level in Parksville Lake, which sits behind the dam, roughly 5 feet.<\/p>\n They’re usually called flashboards. In general, they’re a temporary extension that sits along the top edge of a spillway, so the lake can stay a bit higher than the bare concrete would hold it. When a big flood shows up, they’re supposed to get out of the way.<\/p>\n Why go to the trouble?<\/p>\n Well, a dam that doubles as a power plant generally wants as much water stacked up behind it as it can safely keep. More height usually means more energy from the same river. But during a flood, that extra lip of wood would only shrink the opening the water needs, so it’s designed to give up first. I’d argue that’s a pretty clever trade. You lose some cheap wood, and the expensive concrete gets an easier day.<\/p>\n Ocoee No. 1 has worked this way for a long time. Its National Register of Historic Places nomination<\/a>, written by a Tennessee Historical Commission staffer more than three decades ago, describes wooden flashboards on the spillway that wash out once enough water is running over them.<\/p>\n I can’t tell you whether today’s boards are the same height or built the same way as the ones in that form. It’s a historic-preservation document, not a current engineering spec, and the recent reports don’t say how big the boards are.<\/p>\n The same nomination has the rest of the specs. Ocoee No. 1 is a concrete gravity dam, which basically means it stays put by being heavier than the water leaning on it. Its spillway section is curved, and the form says that arch shape is there to pass the lake’s push out to the abutments, the ends of the dam where it ties into the hillsides.<\/p>\n Construction started in the early 1910s, and the form says the plant was already delivering power before World War I broke out in Europe. It also credits the project with giving Tennessee its first artificial lake. That’s a fun bit of trivia for a reservoir most people around there just call Parksville Lake.<\/p>\n Here’s how the dam measures up, according to that document.<\/p>\n If you caught our piece on the Australian dam that lets floods pour straight over its concrete arches<\/a>, this is a slightly different philosophy. That one is built to take the water on the concrete itself. Ocoee No. 1 puts a layer of wood up there that’s expected to lose the fight.<\/p>\n The reports I’ve seen don’t explain why 5 feet is the number. My guess is it’s about getting the water below the top of the spillway so people can actually work up there without a river running over their boots. That’s an educated guess on my part, though, and I haven’t found TVA saying it publicly.<\/p>\n And the timing matters here. When TVA talked about the drawdown on Friday, it was describing a plan that was underway, not a finished job. I can’t tell you how far the lake has actually dropped since then, when the new boards will go in, or who’s doing the work. If you’ve got a dock or a boat on Parksville Lake, you’ll probably find out before I do.<\/p>\n Frankly, I find it a little reassuring that what broke here was designed to break. It’s also a lot simpler than some dam repairs we’ve covered. A hydro dam in Canada has engineers sawing slots through its own swelling concrete with diamond wire<\/a>, over and over, just to keep the turbines lined up. Swapping out lumber sounds like a pretty good deal next to that.<\/p>\n Bad enough that the USGS had to send a team out to fix one of its own river gauges. The U.S. Geological Survey<\/a> says an isolated storm on November 5 dropped more than 10 inches of rain near McCaysville, Georgia, and Copperhill, Tennessee, pushing the Ocoee River to historic levels. The agency says the flooding forced a number of evacuations and rescues in the area.<\/p>\n The USGS team came from Knoxville and got the Copperhill streamgage working again, but it missed the peak. A streamgage is basically a monitoring station that logs how high a river is and how much water’s moving past. If it’s down when the river tops out, somebody has to work the high point out later from the marks the water left behind. The team measured how much water was moving and marked where the floodwater reached, and the agency says it’ll take more surveying and analysis before it can confirm a record peak.<\/p>\n You’ve probably seen headlines calling it a record crest. Several outlets reported one. USGS just hasn’t signed off on a number yet, so I’m not going to give you one.<\/p>\nSo what are these boards, anyway?<\/h2>\n
This dam’s been around since before World War I<\/h2>\n
So why lower the lake?<\/h2>\n
How bad was the flood?<\/h2>\n