A lot of dams come with a person attached. When a big storm rolls in, somebody at the controls watches the lake climb and decides how far to open the gates, and an awful lot can ride on that call.<\/p>\n
The newest piece of concrete at Michigan’s Edenville Dam doesn’t need that person at all.<\/p>\n
If the name rings a bell, it’s probably because of May 19, 2020. After days of heavy rain, a section of Edenville’s earthen embankment gave way and dumped Wixom Lake into the Tittabawassee River. That flood then poured over the Sanford Dam downstream, which failed a few hours later, and Midland officials put the number of people evacuated at around 10,000.<\/p>\n
The Four Lakes Task Force, the nonprofit that Midland and Gladwin counties put in charge of the lakes, has spent the years since rebuilding all four dams on that stretch of river. In June, it announced that Edenville’s new auxiliary spillway was finished<\/a>. Its October updates show the rest of the system getting close, too, and Secord, the dam farthest upstream, is due for substantial completion this month.<\/p>\n An auxiliary spillway is basically an emergency overflow. It sits off to the side of a dam’s main spillways and only gets used when there’s more water than they can handle. The task force says every one of its four dams needed one to pass large floods.<\/p>\n Edenville’s is 282 feet wide and 211 feet long, according to the task force. Edenville’s trick is in the top edge. Instead of running in a straight line, the concrete wall the water spills over is folded back and forth into zigzags, and the task force says that gives it an effective width of 590 feet.<\/p>\n Why go to all that trouble?<\/p>\n Water flowing over a wall (engineers call that kind of wall a weir) pours off along its top edge, so the longer the edge, the more water you can get rid of at once. Fold that edge into zigzags and you can fit 590 feet of it into a channel that’s 282 feet across.<\/p>\n It’s pretty much the same trick as the pleated air filter under your car’s hood. Folding the paper packs a lot more filter into the same little box, and nobody had to make the box any bigger.<\/p>\n Building it took a while. Crews started digging in July of last year, and the final concrete pour went in on June 3. The task force says the job took 7,000 cubic yards of concrete and 692 truck deliveries. That’s about 10 cubic yards a truck, which is roughly a full load for a typical mixer. And since ordinary concrete weighs somewhere around 4,000 pounds per cubic yard, my rough math puts the whole thing at about 14,000 tons.<\/p>\n Under the concrete is a compacted earthen embankment with layers built in to catch any seepage and pipe it back to the river. I’d argue that’s the most important detail in the whole announcement, given what took the old dam down.<\/p>\n The Federal Energy Regulatory Commission hired a five-member independent forensic team to work that out, and its final report<\/a> blamed something called static liquefaction. Basically, loose sand that’s soaked with water can suddenly lose its strength and start behaving more like a liquid than a solid. When that happened in a section of Edenville’s embankment, the downstream slope slid out and the lake came through behind it.<\/p>\n According to the report, the dam went up in the 1920s in a way that strayed a long way from its own plans and specs, which left parts of the embankment built from very loose sand. Some of its slopes were also steeper than modern stability standards call for. On May 19, 2020, Wixom Lake climbed about 3 feet above its previous record high, set in 1929, and that’s when the embankment gave way.<\/p>\n Sanford’s failure was more straightforward. The flood from Edenville ran over the top of it, and the team found that the overtopping breach started almost three hours after Edenville let go.<\/p>\n The report called both failures foreseeable and preventable, though it didn’t pin them on any single person or company. It looked at the gates, too. Opening them the full 10 feet would’ve left the lake about a foot lower at its peak, the team found, and it couldn’t say whether that alone would’ve prevented the failure, “but it might have.”<\/p>\n I’m not going to pretend I know better than five forensic engineers. But that’s the backdrop for a spillway with no gates at all, and frankly, it’s hard to think of a better place to build one.<\/p>\n The task force describes the new structure as passive. It doesn’t need anyone to operate it, and it starts passing water once the flood coming into the lake hits the size of a 200-year flood.<\/p>\n That name’s a bit misleading. A 200-year flood isn’t one that shows up every 200 years like clockwork. It’s a flood big enough that there’s a 0.5% chance of seeing one in any given year, so you could get one next spring and another a few years later if your luck’s bad enough.<\/p>\n Below that line, the dam’s main spillways handle the water. Above it, the zigzag wall starts spilling whether anyone’s in the control building or not.<\/p>\n What I couldn’t find is a flow rating. The task force hasn’t published how many cubic feet per second the auxiliary spillway can pass, at least not anywhere I could track down, so I can’t tell you how it stacks up against the flood that took out the old dam. That’s a number I’d genuinely like to see.<\/p>\n Spillway capacity is arguably the thing dam owners lose the most sleep over right now. We’ve covered a 245-foot earthen dam the Army Corps moved to the top of its risk list<\/a> after a study found a rare flood would push 30 percent more water through it than it was built to pass. And a 131-foot concrete dam in California is getting its spillway cut 35 feet lower<\/a> because its concrete is too far gone to repair.<\/p>\n Edenville is the last of the four, and the furthest behind. Downstream, Sanford Lake started refilling on April 15 and reached its maximum legal level on May 8, according to the task force. Upstream, Smallwood Dam hit substantial completion on August 4, and its lake is officially back at its normal legal level.<\/p>\n Secord is the one to watch this month. The task force’s October construction newsletter<\/a> says crews have finished its primary spillway and low-level outlet, and that the crest gate in bay 1 is grouted in place and ready for its hydraulic cylinders. The task force’s own October 7 webinar slides still listed the spillway and outlet as in progress, though, so I’d take “finished” with a small grain of salt.<\/p>\n The bay 2 gate has had a rougher year. In July, crews found two sections that wouldn’t line up during installation, so both halves were shipped back to the factory. The repaired gate was back on site in September.<\/p>\n Secord’s estimated substantial completion date is November, but its lake can’t start refilling until April 15, 2027. Michigan’s environment agency, EGLE, doesn’t allow these lakes to be filled between October 1 and April 15, and the task force says Secord could take anywhere from one to four weeks to fill once it starts.<\/p>\n Edenville is a more complicated job, since the dam spans both the Tittabawassee and Tobacco rivers, with a main spillway on each. Both are being rebuilt as stepped concrete chutes. According to the task force’s October 7 webinar<\/a>, the auxiliary spillway’s concrete is done, both principal spillways are in progress, and the seepage cutoff wall and the flattened dam slopes are nearly complete. Flatter slopes, for what it’s worth, go right at one of the problems the forensic team flagged.<\/p>\n The first stage of the Tobacco spillway is expected to wrap up by the end of this year. A cofferdam, which is basically a temporary wall that keeps the work area dry, will stay on the Tobacco side from this fall into the spring of 2028. Next fall, the water on the Tittabawassee side should rise about 5 feet, and you’ll be able to see the change about 3 miles upstream of the dam.<\/p>\n The task force expects Edenville to be substantially complete in September 2027, but that doesn’t mean Wixom Lake comes back right away. Because of the October-to-April rule, refill can’t start until spring 2028, and the task force says it’ll take at least 47 days. If the fill starts after the spring flows have passed, that stretches to two or three months.<\/p>\n None of this is cheap, either. The task force’s estimate for Edenville alone is $142.8 million, its latest estimate for all four dams is $370 million, and property owners in the lakes’ special assessment district are covering a big chunk of it through charges on their winter tax bills.<\/p>\n By the time Wixom is full again, the people along its shore will have spent the better part of eight years looking at a lakebed. That’s a long time to wait for a lake, and I’d guess they’re pretty tired of hearing the word “cofferdam.”<\/p>\n The Four Lakes Task Force’s next community webinar is set for December 2, and the earliest Wixom Lake can start filling back up is April 15, 2028.<\/p>\n","protected":false},"excerpt":{"rendered":" A lot of dams come with a person attached. When a big storm rolls in, somebody at the controls watches … <\/p>\nSo what’s a labyrinth spillway?<\/h2>\n
So why did the old dam fail?<\/h2>\n
Nobody has to open anything<\/h2>\n
The other three lakes are mostly back<\/h2>\n
Wixom Lake is last in line<\/h2>\n