COLUMBIA — Two temporary dams being built in the Pee Dee could prevent Myrtle Beach from essentially becoming an island when flooded rivers wash out major roadways. 

Several main arteries to the Grand Strand are expected to be flooded Monday as water dumped by Hurricane Florence gushes downstream.

Rivers will reach historic levels after up to 2 1/2 feet of rain fell in North Carolina, which will flow into northeast South Carolina, which received up to a foot, said John Quagliariello, a National Weather Service meteorologist.

Along the Lumber River, closures are expected on S.C. 9 near Nichols and S.C. 917 near Mullins. U.S. 501 will close near Gallivants Ferry, after the Lumber becomes the Little Pee Dee River. The Waccamaw River will also wash out S.C. 9 near Longs, said Department of Transportation Secretary Christy Hall.

That's "based on actual rainfall amounts that's happened and what we expect to happen for the remainder of the storm," she said.

Several other Grand Strand routes — including S.C 22, known as the Conway Bypass, U.S. 701 near Georgetown and U.S. 17 at Georgetown — are being closely watched, as they may also wash out, depending on rainfall, she said. 

Starting Monday, U.S. 378 from Columbia to Conway will become the main way in and out of Horry County for first responders, utility crews and Grand Strand residents, Hall said. 

Traffic will then flow to U.S. 501 to the beach. 

But that will require the temporary dams, as rivers would otherwise wash out portions of those routes too, Hall said.

"To make this work, we have to build dams to hold the floodwaters back off the road and keep the road operational," she said.

Crews are quickly working to build a nearly mile-long dam along U.S. 378 where Lynches River flows under it in Florence County, and a 1.5-mile dam along U.S. 501 by Lake Busbee and past the Waccamaw River, Hall said. 

The joint effort by the DOT, state National Guard and county officials will be completed by Monday night, she said.

At Lynches River, where the water is expected to rise a foot above the bridge, the dam will consist of 3-feet-tall concrete barriers and plastic waterproofing, with sandbags and rebar to hold it down. By Lake Busbee, thousands of one-ton sandbags will form the dam. The Waccamaw would otherwise put three feet of water on the roadway there.

The one-ton sandbags are being filled by inmates and staff at medium-security Wateree River Correctional in Sumter County, which has a sand pit used specifically to assist with flood control and storm preparation, said Corrections Director Bryan Stirling.  

Their work will continue Sunday, he said. 

Flooding closed a 17-mile section of Interstate 95 in Dillon County Sunday morning. There was no river crossing at that wash out. The heavy rainfall caused flooding throughout the county bordering North Carolina.

State officials are also watching rivers that could top I-95 in several places in the Pee Dee.

In 2016, floodwaters from Hurricane Matthew closed nearly 75 miles of I-95, a main link from Florida to the Northeast.    

Follow Seanna Adcox on Twitter at @seannaadcox_pc.

Assistant Columbia bureau chief

Adcox returned to The Post and Courier in October 2017 after 12 years covering the Statehouse for The Associated Press. She previously covered education for The P&C. She has also worked for The AP in Albany, N.Y., and for The Herald in Rock Hill.