Mighty big job ahead on dams

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PIERRE, S.D. — Priority repairs at the Missouri River dams bruised and battered by last summer's flood are surging toward completion before spring runoff season.

At the Oahe and Big Bend Dams in central South Dakota, repairs include pouring boulders, stones and gravel into deep, earthen holes scoured behind the muscular concrete walls that channeled record-setting floodwaters out of the dams and down the river below.

The work at Oahe and Big Bend is only a trickle of the $234 million in dam repair and maintenance chores the Army Corps of Engineers plans to begin this year across the sprawling basin. Another $280 million was allocated for levee rehabilitation, work that includes the recent repairs of breaches near Hamburg, Iowa.

The dam and levee funding are part of $530 million in congressional spending for basin-wide relief from the disaster spurred by record runoff in the northern Plains and Rocky Mountains.

Corps commanders have bolstered the Omaha District, which regulates the reservoirs, with engineers and others to accelerate the pace of restoration.

Despite the damage, the civilian engineers who manage Oahe and Big Bend said the dams operated as designed throughout the unprecedented three-month flood.

Spillway gates and outlet tunnels released record volumes of water in regulated flows. Hydroelectric power plants ramped up and down.

"We had no issues," said Keith Fink, operations manager at Big Bend Dam near Fort Thompson, S.D.

Eric Stasch, operations manager at Oahe Dam near Pierre, said the structures weren't stressed by the high water.

Eric Stasch

Oahe and Big Bend are two of six big dams constructed by the corps in the mid-20th century along the main stem of the river from Nebraska upstream to Montana. They create the nation's largest reservoir system.

Big Bend forms Lake Sharpe, which stretches 80 miles upstream toward Pierre and Oahe Dam. Big Bend passes on virtually all water that flows into Sharpe because it was designed to accommodate only 1.1 percent of the system's 16.3 million acre-feet of flood storage.

Oahe Dam creates Lake Oahe, a 231-mile-long reservoir responsible for more than a quarter of the system's flood control storage. Only Garrison Dam in North Dakota is designed to hold more.

The water volume and energy the six dams unleashed in the flood of 2011 were tremendous.

Oahe Dam's two-mile-long earthen spillway had never been used, and it wasn't during the flood for fear that it would send tons of mud, trees and other debris into Pierre, South Dakota's capital city, as well as into the adjoining community of Fort Pierre. Both cities are about six miles downstream of the dam.

"The reservoir control office in Omaha did a stellar job of manipulating the system and keeping us out of that spillway," Stasch said.

Instead of using the emergency spillway, most of Oahe's floodwater was funneled through six outlet tunnels beneath the dam.

Day and night, water exploded out of each 19-foot-diameter tunnel at about 42 mph.

Each tunnel discharged about 18,000 cubic feet per second — or 134,600 gallons, Stasch said. That's enough water from each tunnel to cover nearly two football fields with a foot of water.

The six tunnels combined discharged enough water each second to cover 11 football fields a foot deep.

Peak releases from each dam hit 160,000 cubic feet of water per second, when outlet water combined with flows channeled through the hydroelectric plants.

For comparison's sake, Big Bend's releases are expected to average 22,600 cfs this month. Oahe releases averaged 26,700 cfs in January.

Last summer's high flows and increased water elevations in the stilling basins immediately below Oahe and Big Bend — designed to dissipate the energy of fast-moving floodwater — swamped the concrete walls along the outlet chutes and created havoc.

The churning outlet water washed away prairie topsoil behind the concrete barriers, called wing or chute walls. It also exposed the slate bedrock that dam builders had chiseled away to create the outlet channel.

Once over the walls, floodwater floated and flipped concrete paving slabs the size of the floor of a small two-car garage. At Big Bend, the slabs pummeled the submerged steel railings atop the wing walls.

A scour hole behind one wing wall at Oahe was about 20 feet deep. Ice anglers at Big Bend's stilling basin earlier this month said they were fishing over a new 30-foot hole in the river bottom.

Once over the wing walls, high water went on to damage low-lying roads and campgrounds, picnic and fishing areas immediately below Big Bend.

Corps employees monitored the dams day and night during the flood.

Public viewing areas often drew crowds until 3 a.m., Fink said.

"People were interested. The force of the water was astonishing," he said.

At Oahe, the discharge channel below the dam rose about 15 feet. The surge of floodwater carved away 70 to 80 feet of the towering north bank of the channel near the stilling basin bridge.

Stabilizing the eroding cliff with rock riprap is a priority for some of the $17.1 million in disaster funds for Oahe, Stasch said.

Fink estimated that about 60 percent of the $10.1 million for Big Bend would be spent on new gate seals and cables, concrete repair and gate painting to prevent corrosion, among other repairs. The remainder would fix the road, campground and picnic-area damage in time for summer use, as well as repair and protect American Indian burial sites the floodwaters exposed.

The wing-wall repairs at Oahe are nearly complete after three months of work. Similar repairs at Big Bend are under way.

Last summer was only the third time since Oahe's construction that the outlet tunnels had been used to release water.

The first was during the flood of 1997, when 16,000 cfs was released for 30 days. The tunnels were used for 30 days again in the fall of 2010, when up to about 18,000 cfs was released to flush away floodwater.

Last summer's peak flows out of the Oahe outlet works were six times that much.

"We used these projects pretty hard last year,'' Stasch said. "The dams performed magnificently."

Contact the writer: 402-444-1127, david.hendee@owh.com

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