Government agencies rate things. Bridges get a condition score, levees get an inspection grade, and the 700-plus dams the Army Corps of Engineers owns each carry a risk classification that tops out at “very high.” On October 1, the Corps moved Oahe Dam in South Dakota up into that top category, and the label did exactly what you’d expect a label like that to do on the local news.<\/p>\n
So should anyone in Pierre, the state capital about six miles downstream, be packing a bag? The Corps says no, and having read what it put out, I’m inclined to agree. But the three reasons it gave for the bump are a lot more interesting than the label, and one of them is a number that changes what this 64-year-old dam is supposed to be able to handle.<\/p>\n
Oahe is big in the way only a mid-century federal project gets to be big. The dam is a wall of packed earth 245 feet high and about 9,300 feet long, built from 92 million cubic yards of fill, and the lake behind it runs 230 miles up the Missouri River to Bismarck, North Dakota. That’s enough to make Lake Oahe the fourth-largest man-made reservoir in the country, according to the Corps<\/a>. Seven turbines in the powerhouse can put out 786,030 kilowatts, and in an average year they make about 2.7 billion kilowatt-hours. John F. Kennedy showed up to dedicate it in August 1962, which tells you how much of a deal it was at the time.<\/p>\n The Corps’ classification is a ranking tool, and it’s a fairly blunt one. Engineers look at how likely a problem is and how bad it’d be if it happened, combine the two, and use the result to decide which of those 700-plus dams gets money and attention first. “Very high” is the top of that list. Col. Joe Sahl, who commands the Corps’ Omaha District, told reporters the change puts Oahe first in line when the agency decides where to spend, and that there’s no higher classification than the one it now has. The Corps’ own release is careful to add that the rating doesn’t indicate a current emergency and doesn’t trigger any emergency response.<\/p>\n The consequences half of that equation is why Oahe scores the way it does. Pierre and Fort Pierre sit right below the dam, and the Missouri keeps going past them through four more Corps dams and a long list of towns. For comparison, the other five mainstem dams on the river rate like this: Fort Peck in Montana and Garrison in North Dakota are “high,” and Big Bend, Fort Randall and Gavins Point, all in South Dakota, are “moderate.” Gavins Point is the small one near Yankton whose intake gates came out of the water 25,000 pounds overweight with zebra mussels<\/a> this summer, so “moderate” is doing some work there too.<\/p>\n Oahe was designed in the late 1940s, and the engineers who sized it had to guess how much water a truly bad year could throw at it, using the rainfall and snowmelt records they had. The Corps has now redone that modeling with modern methods and about 75 more years of weather, and the answer that came back is bigger. “The result of this updated analysis shows a 30% increase in the flows Oahe would need to pass downstream,” Andrew Nystrom of the Omaha District’s public affairs office wrote in an email to KELO<\/a>.<\/p>\n A quick translation, because “flows it would need to pass” is doing a lot of work in that sentence. A dam like Oahe has two jobs in a flood: hold water back, and let enough of it through, in a controlled way, that the lake never climbs over the top of the embankment. An earth dam that gets overtopped erodes, and that’s the scenario everyone in dam safety is actually worried about. Oahe was built to let through up to 415,000 cubic feet per second when it has to. A cubic foot is about 7.5 gallons, so that’s roughly 3.1 million gallons a second.<\/p>\n If the dam now needs to pass 30 percent more than its design, my math puts the new target somewhere around 540,000 cfs. Treat that as my arithmetic. The Corps hasn’t published the new figure or said exactly what the 30 percent is measured against, which is a slightly odd thing to leave out of an announcement that’s built around that number.<\/p>\n Now put that next to what the dam has actually done. The biggest release in its history came in June 2011, when spring rain and snowmelt filled the reservoir and the Corps pushed 160,000 cfs through the powerhouse and the six outlet tunnels, a flood that damaged homes and businesses along the river. That’s a bit under 40 percent of the 415,000 the dam was built for, and the spillway, the concrete chute with eight gates rated for 304,000 cfs on its own, stayed shut. The Corps’ project page says it hasn’t been needed yet. So you’ve got a dam that has never passed even half its design flow, and a new study saying the design flow itself is about 30 percent short.<\/p>\n The other two items on the Corps’ list are older news to the engineers who work there. One section of the earthen embankment has been slowly shifting and settling since construction, which is why it’s been instrumented and watched since the dam was built. Earth dams do compact a little under their own weight, so some settlement is normal, and the study’ll have to figure out whether this stretch has stopped moving, because the Corps hasn’t said that it has. The third item is the spillway again: the concern is that a release bigger than anything the dam has handled could erode it, and since the record is 160,000 cfs through the tunnels, no one’s ever watched that spillway run anywhere near its rating.<\/p>\n Which brings me to something I find slightly funny about the timing. The Corps bumped Oahe up its flood-risk list in a year when the Missouri basin can’t buy a flood. The Corps’ October 7 update<\/a> put 2026 runoff above Sioux City, Iowa, at 63 percent of average, which would be the 14th-lowest year on record, and the whole six-reservoir system held 46.1 million acre-feet on October 1, 10 million below the bottom of its flood-control zone. Lake Oahe ended September at 1,592.0 feet, down 1.7 feet in a month and about 25 feet below its maximum normal operating pool of 1,617.<\/p>\n Does that make the reclassification silly? No. The rating is about rare events, the kind that show up once in a few hundred years, and a dry stretch doesn’t cross any of those off the list. The 2011 flood came after a run of pretty ordinary years on this river, and the reservoir doesn’t care which year the rain decides to show up in. If anything, a low lake buys the Corps time to do the study while the embankment isn’t under full load.<\/p>\n What comes next is what the Corps calls a Dam Safety Modification Study, a multi-year process to work out what to physically change at the dam, pick a plan, engineer it and then build it, with state, local and tribal partners in the room and public input along the way. The Corps’ release<\/a> says the study is now among the highest priorities in its dam safety program. It hasn’t said how long the study will take or what the fix might cost, which I’d expect, since producing those numbers is what the study is for.<\/p>\n This isn’t starting from zero, either. After 2011 the Corps added drains under the dam to take water pressure off the foundation, put in more instruments to watch it, started surveying the spillway and embankment with newer gear, and dug out sediment that had been restricting flow through the spillway. On the money side, Nystrom told KELO the Corps has put about $73 million into running and maintaining the Oahe project over the past five years, roughly $15 million a year, with $15.4 million budgeted for this fiscal year, and that’s before counting upgrades to the hydropower side.<\/p>\n Operations don’t change.<\/p>\n Releases from all six Missouri dams are coordinated as one system under the Corps’ master water control manual, and the agency says no operational changes are planned at Oahe or anywhere else because of the new rating. Sahl’s line in the release was that the dam has “performed as designed for more than six decades,” and the Corps is betting, pretty openly, that it’ll keep doing that while the study runs.<\/p>\n If you live downstream of any Corps dam, the advice in the same release is basic and, frankly, sensible: know how your town would warn you, have a family plan, and talk to an insurance agent about flood coverage, because no dam eliminates flood risk, and that’s the Corps’ own wording. Out west, Hoover Dam is losing turbines to a lake that keeps dropping<\/a>. Oahe’s problem is the opposite kind, a flood that so far only exists in a model.<\/p>\n The Corps’ October 7 update forecast Lake Oahe at 1,592.5 feet at the end of October, with releases averaging 9,500 cubic feet a second, a little over 2 percent of the 415,000 the dam was built to pass.<\/p>\nSo what does “very high risk” actually mean?<\/h2>\n
Where does the 30 percent come from?<\/h2>\n
The lake is 25 feet down right now<\/h2>\n
So what does the Corps actually do about it?<\/h2>\n