Models used to manage water in Lake Wylie show the lake could be losing more than 100 million gallons per year due to sedimentation.
Yet, experts aren’t sounding the alarm. If anything, they see reason for optimism.
“Sedimentation is an issue,” said Jeff Lineberger, hydro strategy and licensing director with Duke Energy. “I think right now we have a pretty good handle on it.”
Lineberger occupies Duke’s seat on the Catawba-Wateree River Basin Advisory Commission. When he talks about handling siltation, he isn’t just looking at it from the residential perspective. Though many do.
David Smith called it a “really messed up situation” after a late August sediment spill on Wylie from a construction site onto his property, one neighbor, Jim Allison, called the “biggest mess I’ve ever seen on this lake” in more than 60 years. Tea Hoffmann worried over the summer her cove could “fill up with dirt” after seeing runoff from a work site.
Lake Wylie resident Don Clarke has been pleading for months to any group that will listen, with concerns about a potential Duke sale of land surrounding Catawba Nuclear Station for fear of the “major disaster” that could occur if dirt is turned so close to the water.
Those residents are just a sampling. In recent years others have sent letters to the media and environmental regulators. They grilled elected officials. Some even ran for office on platforms of controlling growth and sediment runoff.
Homeowners along Lake Wylie don’t need reams of data to convince them it’s an issue. They have their eyes. Yet, the science on sedimentation tells a larger story.
“It was not filling in quite as fast as we had thought,” said Barry Gullet, director with Charlotte Water.
Gullet is chairman of the Catawba-Wateree Water Management Group, a collection of 18 public water utilities within the Catawba River basin and Duke. That group put together a five-year sediment study ending in 2015. The 111-page document found sediment infill isn’t standard across the 11 Catawba River reservoirs.
“It was more localized than it is widespread,” Gullet said. “Some of the coves are impacted greatly. Some aren’t.”
The model Duke uses to manage the river system estimates Wylie loses 355 acre-feet — an acre wide, a foot deep — of water each year. The four-year change during the study period would estimate a loss of more than 461 million gallons of water storage. That figure is .6 percent of the total available storage.
Yet the study showed a cyclical impact of sedimentation, with Wylie actually gaining 2 percent of available water storage in five years based on the lake bed depths recorded at monitoring sites.
Still, there are reasons to believe the lake didn’t get any deeper. The study only set up two test sites on Wylie. Though they combine to account for about 67 percent of the water flowing into the lake, the South Fork and Crowders Creek sites weren’t in the shallowest or deepest parts of the water. The study assumes less sediment at those sites means it moved into deeper and wider parts of the lake.
Mark Oakley, licensing project manager for Duke, said reservoirs are tapered and models typically account for lost volume due to sedimentation. Some sediment comes in naturally. Some comes with construction site or other runoff. Experts like Oakley understand why residents living on coves have concerns. Sedimentation starts there.
“They’re sort of filling in around the edges,” Oakley said.
As sediment fills in coves, the angle of the lake bottom lessens, creating more exposed land when the water dips, compared to the same water drop against a vertical surface. Which is why residents may see coves that look drier than in past droughts, even though measurements show the water a foot or more higher.
Lineberger said even though Duke looks at a larger picture, he gets concern from homeowners.
“There are some coves that have some real issues,” he said.
What isn’t as big a concern, at least not yet, is the ability of Lake Wylie to provide drinking water. From its full pond, the top 7.5 feet of water can be used — almost 73,000 acre-feet — before water intakes become exposed. It’s another couple of feet before steam plant intakes would be exposed, and about 16 more feet before hydro plants are threatened. Almost 35 more feet remain before the “dead storage” point, as one study illustration terms it, where the bottom of the lake can’t be used.
The lake typically runs a few feet below full pond to avoid spilling, which still leaves about five feet of usable storage. Duke is looking at raising target elevations half a foot in the summer to create more storage, which brought up the whole issue of fill-in among river basin advisory group members.
“How much of that six inches is being affected by siltation?” asked former chairman and Rock Hill resident Rick Lee.
Experts say they continue working on long-term solutions to keep the Catawba viable for existing and coming growth. Wylie supports the drinking water intake in Rock Hill, which serves much of York County. At full pond, the lake sits 569.4 feet above sea level. The third largest reservoir by surface area in the Catawba chain, Lake Wylie spans 12,177 acres and 348.5 miles of shoreline.
Though the five-year study is complete, water use modeling is ongoing and the water management group will continue to learn more about the impact sediment has on reservoirs. Starting with what they now know following the study.
“We’ll have a much better understanding of those sedimentation rates and how those impact storage,” Gullet said.