As sediment builds, one dam faces its comeuppance

Officials at a Colorado reservoir are reckoning with decades of accumulation.

 

This story is a part of the ongoing Back 40 series, where HCN reporters look at national trends and their impacts close to home.

In October 2014, Paonia Reservoir, a skinny, 3-mile-long basin tucked between a winding mountain highway and a forested ridge in western Colorado, had a problem. Waterlogged branches and wood, plastered with silt and clay, had accumulated six feet high on the metal grates outside the reservoir’s outlet, partially plugging it. “It’s like building a beaver dam around your outlet,” says Tim Randle, the manager of a sedimentation and river hydraulics group for the Bureau of Reclamation. Once a blockage covers a dam’s outlet grates, “you can’t get the water out.”

The root cause of the problem at the reservoir had been building since the dam was finished in 1962. Year by year, sediment quietly collected on the reservoir bottom, gradually raising its floor. Once the sediment was level with the dam outlet, where water is released downstream, any debris that washed into the reservoir threatened to clog the opening and make the dam inoperable. In the fall of 2014, personnel worked 10-hour days for two weeks to clear logs, branches and dirt from the outlet, by hand and with an excavator. Some of the workers stood directly on waterlogged sand, digging out the grates with pitchforks. When the dam was newly built, they would’ve needed a crane 70 feet tall to reach the same spot.

When Paonia Dam was first constructed, the top of the outlet sat 70 feet in the air on a concrete tunnel. Today the structure is almost entirely buried in sediment.
E.J. Peterson/Bureau of Reclamation

Silt, sand, gravel and clay are accumulating in nearly all reservoirs in the West, leaving less room to capture water for storage or for flood prevention. Behind many dams, space is starting to run out. Ignoring the problem could jeopardize the West’s water supply in the coming years. To avoid that, water managers are dredging behind dams, retrofitting outlet works and looking for ways to safely pass sediment downstream. “It’s not an immediate crisis,” Randle says. “But it is sort of a ticking time bomb.”

When dam-builders were constructing the network of reservoirs that waters the West, they knew that sediment would accumulate. Dams fundamentally alter waterways by severing the flow of not just water, but also things like fish and aquatic insects, seeds and logs — and sediment. Sand and other particles flow in with the creek or river that feeds a reservoir, then settle out when the water slows and stills behind the dam. The outlet at most reservoirs was built high enough to allow sediment to gather without blocking it; the space below is called “dead pool,” because any water stored there can’t be sent downstream. Engineers typically designed reservoirs to last for 50 to 100 years before gathering enough sand and gravel to threaten the outlet.

But for many Western dams, that deadline is quickly approaching or even passed, forcing some reservoir managers to reckon with the problem now. More than half of Reclamation’s reservoirs in the West are 60 years old or older, nearing the end of what engineers call their “sediment design life.” Some, like Matilija Dam in California, which is slated for removal, are already inoperable thanks to sediment buildup. At Paonia Reservoir, more than 8 million cubic yards of sand, silt and other particles have piled up behind the dam since it was built, according to a 2015 Bureau of Reclamation survey. That’s enough to fill nearly 100,000 shipping containers 40 feet long. By October 2014, the space below the reservoir’s outlet was full, obliterated by a muddy mass of sediment.

Now that the Paonia Dam’s outlet is nearly blocked, officials are preparing to operate the dam in a way that will prevent even more sediment from piling up. Eventually, dam managers plan to start letting sediment pass through during the spring, when much of the yearly sediment load washes in with melting snow. The best, most cost-effective way to do that isn’t yet clear; the options, which include digging a bypass tunnel and waiting until later in the spring to close the dam, come with a wide range of price tags. “(You) put the reservoir in ‘river mode,’ as I call it,” says Sean Kimbrel, a hydraulic engineer who also works at the Bureau of Reclamation, “to pass sediment downstream.”

This fall, after water managers drained Paonia Reservoir in mid-September, the surface of its sediment floor glistened in the slanting sunlight of a crisp October morning. Muddy Creek, the source of the water that fills the reservoir when the dam’s gates are shut, cut extravagant curves through the brown expanse. A handful of people were clustered around the metal grates of the outlet, concrete dust billowing up from where they stood working on the frame of the structure.

  • In October, Paonia Dam sits empty, with just a trickle of water weaving its way through the sediment. Fire Mountain Canal and Reservoir Company and Bureau of Reclamation workers do maintenance on the aging outlet after the reservoir was drained.

    Brooke Warren/High Country News
  • Workers drill anchors for a new trash grate at Paonia Dam so debris doesn’t clog the outlet.

    Brooke Warren/High Country News
  • Muddy Creek cuts through the sediment that has built up at the bottom of Paonia Reservoir.

    Brooke Warren/High Country News

At Paonia Reservoir, managers need to address a few other problems before they can tackle sediment buildup. A few days before I visited the dam, workers had begun fixing one long-standing issue: A 10-ton concrete bulkhead that can be fitted into the dam outlet like a bathtub plug had broken. Officials worried that a chunk of concrete might fall in and get stuck in one of the dam gates. Workers jackhammered the bulkhead into nine sections, then carefully removed them. “Worst-case scenario, the dam would’ve become inoperable,” Steve Fletcher, the dam tender at Paonia Reservoir, told me. “We needed to get rid of this bulkhead, and that scenario.”

Fletcher and other officials will install a new bulkhead in the coming years so they can temporarily close the outlet, allowing them access to the dam gates and outlet tunnel for monitoring, maintenance or repairs. That will be crucial once the dam begins regularly passing sediment, since small particles in the water can sandblast metal and concrete.

Once that’s done, reservoir managers can move forward with figuring out the best way to pass sediment downstream at Paonia Dam. Thanks to decisions made decades ago, today’s Western water managers and engineers are left a difficult task: Faced with an onslaught of sediment, how can they maintain reservoir capacity and safeguard the West’s water supply? “Each individual grain of sediment or sand, silt, clay that’s depositing — any one of those is not a big deal,” Randle says, “but cumulatively, they are.”

Emily Benson is an editorial fellow at High Country News. 

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