Paonia Reservoir, CO - A Preview Of The Inevitable Silt-Filled End Of Western Dams
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.
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.
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http://www.hcn.org/articles/water-as-sediment-builds-a-colorado-dam-faces-its-comeuppance-paonia-reservoir
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