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CHARLOTTE, N.C. --On Wednesday, Duke Energy revealed a telling detail about the pipe that burst at one of its coal ash ponds this week. The spillsent as much as 82,000 tons of ashinto the Dan River in Rockingham County, near the Virginia border. Here s a diagram Duke Energy provided that shows what happened:

But Wednesday, inspectors found that 48-inch reinforced concrete pipe wasn t made of reinforced concrete like first thought, but rather corrugated metal, asreported byThe Charlotte Observer:

Duke officials had said that it was definitely unexpected that a reinforced concrete pipe would break.

But crews that unearthed the pipe s break point learned that two-thirds of its length is corrugated metal and the third closest to the river is reinforced concrete, Duke spokeswoman Erin Culbert said Wednesday.

Knowing the material helps inform the engineers who are designing a permanent solution, she said.

The pipe was installed in the 1960s, Duke says, before the ash pond was expanded and divided into two basins.

AnI-Team analysis of North Carolina s 5,600+dams back in November found that while most dams are inspected on a regular basis, only about a third of the riskiest dams have Emergency Action Plans, which spell out in detail what to do, who to call, and who would be inundated if a dam overflows or bursts. But state regulators we talked to also pointed out another problem, specifically with the type of piping used at Duke s coal ash pond on the Dan River:

The average North Carolina dam is 50 years old, and a third of the state s dams are older than that.Many of those dams now need repairs in the 1960 s, many used corrugated metal piping which has a lifespan of roughly 50 years. [State dam inspector Steve] McEvoy said it was an inexpensive alternative, often used for spillways that s now no longer allowed.

The corrugated piping is, in many cases, decaying.

Here s a chart showing the number of dams built in North Carolina each decade. This is only a ballpark estimate, since the database of dams maintained by NCDENR only has construction dates for 2 out of every 5 dams registered:

Closer to Charlotte, corrugated metal piping is in use at a coal ash pond at Duke s now-decommissioned Riverbend Steam Station, which sits along Mountain Island Lake, the source of Charlotte s drinking water. According to areport sent to the Environmental Protection Agency, that pipe was installed in 1958, but found to be in good shape during an inspection in 2009.

In short, a lot of North Carolina s 50-year-old dams were built using a type of piping that s only supposed to last 50 years. Stay tuned.

UPDATE:Duke Energy said there are no pipes running underneath any of its other coal ash ponds in the Carolinas, including the one at Riverbend.

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