It is an unremarkable wall a stone's throw from the White House, barely noticed by tourists and locals alike. Yet officials hope this updated but never-used levee system will protect downtown Washington from the wrath of Hurricane Florence.
The barrier sits on the National Mall, the famous grassy stretch between the Lincoln Memorial and the US Capitol, and in the coming days it could be put to the test as the capital city braces for potentially catastrophic flooding.
Three meters (ten feet) high in places, the stone wall seems to blend into the landscape just north of the Tidal Basin, the picturesque reservoir surrounded by monuments near the Potomac River.
But its role is suddenly a hot topic as city leaders declared a state of emergency and waters on the swollen Potomac surged past flood stage this week following recent heavy rains.
Washington, built on a swamp, is now bracing for what could be a devastating glancing blow from Florence, which is forecast to barrel ashore along the Carolina coast, some 300 miles (480 kilometers) south of the capital, delivering life-threatening winds and flooding.
The Washington wall bisects 17th street, meaning there is normally a gap in the defenses. But the dam can be tightly sealed in a matter of hours, should a flood threat emerge.
"The levee was completed in 2014. We've erected it four times since then" on annual training exercises, National Park Service spokesman Mike Litterst told AFP.
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The levee system has existed for several decades, but after Hurricane Katrina devastated New Orleans in 2005, Washington's system was deemed insufficient, and the Army Corps of Engineers was charged with building a more permanent structure.
While Litterst currently sees "no indication" of a need to use the levee system, which was designed to protect the White House and nearby neighborhoods and federal properties, authorities are at the ready.
"With Florence coming in and her path still uncertain, and depending on how much rain we get given that the system is already saturated, we'll continue to monitor" water levels, Litterst said.
The equation is simple. If the water is measured to be 12 feet (3.6 meters) above normal gage level, an alert is triggered, and the large concrete slabs to plug the gap in the wall will be trucked in and installed.
"It's a three- to four-hour process," Litterst said.
At last survey, the Potomac's water level was at five feet (1.5 meters) above normal, with a peak of nine feet reached on Tuesday. Flood stage is six feet above normal.
Washington has become water-logged in recent days, with particularly soggy soil after heavy rains and little sun.
While Florence does not have Washington in its direct sights, the spectacular forecast of up to 40 inches (one meter) of rain in some parts of the Carolinas could translate to dangerous rain levels for Washington -- and put the city's levee system to an unprecedented test.