Sonar scan will aim to get to bottom of Big Lake's woes

Boiling Spring Lakes | With their main lake nearly dried up, officials here are looking beyond leaky floodgates for a cause.

City Manager David Lewis said discovering any sinkholes in Big Lake has become the first priority on rehabilitating the lake.

A Wilmington engineering and surveying firm, Gahagan & Bryant Associates, has been hired to do a sonar scan to produce a composite image of the lake basin. Lewis described the process as similar to medical imaging, such as CAT scans, that will bounce sound waves in a way that produces a fairly accurate representation of the density of the lake bottom.

Experts can use it to determine if any sinkholes exist.

The city has signed a $9,604 contract for the survey, which should start next week and take two to three days to complete, Lewis said.

"They'll run a boat up and down the lake to get a complete picture of the bottom," he said. "Because of the history of the past few years, we thought it would be a good idea, especially with the lake level low already."

Big Lake, which normally has a surface area of about three square miles, has shrunk to about half that size during the spring and summer. Officials point to a combination of causes including leakage, dam repair operations and overall lack of rain.

City utilities director Larry Modlin got a head start on the lake survey this week by discovering a sinkhole while eyeing the lake bed during a walk-around in areas where the lake bottom was exposed. That and another foot-deep depression in the lake bed were filled by city crews with about 150 yards of red clay dirt.

Five years ago during a similar dry spell, the lake was drained and sinkholes were discovered and repaired after the lake's water level dropped precipitously, Modlin said.

If more sinkholes are discovered during next week's survey, the city would have to lower the lake level to fill them, Lewis said.

Big Lake is currently 8 feet below normal, and "we're still losing water," Modlin said, because of normal evaporation, the floodgate repair work at Sanford Dam and lack of rain.

Three of the four dam gates have already been replaced.

In addition to replacing the floodgates, city officials have given thought to a novel plan to pump water into depleted Big Lake directly from flood-prone areas.

"Certain areas of the city flood during prolonged rains or tropical storms," Lewis said. "So we thought of a plan that would basically put pumping stations in those areas that would pump the water out before it became a problem."

Also briefly considered was a costly, time-consuming and unlikely plan for the city to install a deeply submerged pump tapping directly into the Castle Hayne aquifer to maintain the lake level.

"That would need a lot of regulatory approvals and would only add about an inch a week to the lake level. That's about what evaporates in a week's time," Lewis said.

Paul Jefferson 538-2955

paul.jefferson@starnewsonline.com