Notes from the Field, June 28, 2011, A Problem at Big Bend Dam

Fort Thompson, South Dakota.  The Army announced today (June 28, 2011) that it will halt flows through the emergency spillway at Big Bend Dam on July 1, 2011.  The engineers need to stop the violent movement of water through the spillway so they can approach it with a boat.  They want to examine the soundness of the concrete spillway and the structures around it.  The Army engineers will deploy a sonar system to inspect the portions of the spillway that will remain underwater.  The Army stated, “The purpose of the inspection is to assess the spillway’s performance….”  In other words, the Army is concerned about erosion backward toward the dam (known as back-cutting).  The press release underplays the seriousness of the situation, which is understandable.  The Army does not want to panic the public.  But the Army has been downplaying the gravity of this flood, and its potential dangers, since May.  If the back-cutting requires the closure of the spillway for any length of time, the Army will have to increase the water discharge rates through the dam’s tunnels to compensate.  Big Bend Dam cannot just shut down on the Missouri.  Since the reservoir behind the dam is small and was not built as a flood control reservoir, it cannot long store the high flows coming down from Oahe Dam.  As a result, Oahe Dam would have to cut back on its discharge rates.  But Oahe is almost full, so it cannot pull back on its outflows for long.  Oahe will have to let water go eventually.  If Big Bend Dam cannot release enough water through its tunnels, where will it go and how will it be evacuated through the dam without an operational spillway?  If the Army loses the spillway at Big Bend Dam for a period of time, it could face a crisis situation.  At Big Bend Dam, the Army is operating on a very thin margin, but this is nothing new for the military.  It has been functioning for weeks on the very edge of an even greater disaster.  Along the Missouri, things could still go horribly awry for the Army, but so far the Army has been able to advert a worse disaster through a combination of hydrological expertise and sheer luck.

Mud Lake (Update).  On Monday June 27, 2011, the Missouri River took back Mud Lake.  After a soaking 1.5 inch rainfall on Sunday, the Missouri easily overwhelmed the Army’s flimsy levee across the lake’s riverside mouth.  Almost three weeks ago, when we first reported on the levee at Mud Lake, a South Dakota official doubted whether the levee would hold against the river.  His fears have now come to fruition.  Once the river breached the levee, it filled the old oxbow with fresh, muddy water.  After Mud Lake could take no more of the Missouri, water poured from it into another oxbow named Lake Goodenough.  From there it moved on toward the housing development known as “Wynstone.”  Residents in Wynstone are now fearful that another hard rain, or an increase in outflows from Gavin’s Point Dam, will send the Missouri into their community from the north, rather than from the south or riverward side.  But the new flood threat to Wynstone aids Dakota Dunes.  As the old oxbows fill with floodwater, they act as natural reservoirs, lessening the river’s crest as the river spreads out and fills in the lakes.  The failure of the levee at Mud Lake will take some of the pressure off the levees at Dakota Dunes.

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