The Extreme Engineering Of Washington, D.C.

Imagine causing a sinkhole that collapses the Washington Monument?
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DC Flood Risks https://www.washingtonpost.com/climat...
Potomac Basin https://potomacdwspp.org/resources/ab...
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CBS: Rising Waters in Tidal Basin    • Rising waters in Washington, D.C., th...  
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DC Clean Rivers video    • DC Clean Rivers Tunnel Alignment  
Lady Bird Tunnel Boring Machine    • Lady Bird Advances  
Tour of Blue Plains Wastewater Treatment Plant    • #205: DC's Blue Plains: Tour Of A Wor...  

That nightmare scenario haunted the team excavating the 75 foot-deep site for the National Museum of African American History and Culture after they accidentally hit the aquifer under the National Mall. If too much water gushed into their giant pit, they could destabilize their neighbor, the great structure just a few hundred feet away, causing it to shift or even fall over.

So they acted quickly to pump water back in to stabilize it. When their instruments showed the ground pressure holding firm, they plugged up the leak and focused on modifying the museum’s design to mitigate this new risk.

To maximize its exhibit space, while limiting its above ground footprint, the museum’s first four levels are underground. To keep them dry, visitors safe, and the 91,000-ton monument next door standing tall, they encased the subterranean levels in two walls, with eight feet of porous stone fill in between, to catch and hold any water that might penetrate the foundation.

It was an ingenious solution, but for builders in the federal District of Columbia, dealing with a shallow water table is nothing new.

When President George Washington chose this idyllic spot to be the national capital, it was a land of farms and woods, rich with surface water. Small brooks trickled into streams, like Rock Creek. Marshes and tidal flats swelled and receded as the 14,700 square-mile Potomac basin drained through the District and mixed with the tidal flow of the Chesapeake Bay–the largest estuary in the United States. But as the city's boulevards and grand buildings were constructed over the past two-and-a-half centuries, 70% of these natural streams have disappeared: diverted into sewers, culverts, or filled in and paved over.

Tiber Creek flowed through what has become the National Mall. First, it was converted into a canal, and then piped into sewers buried beneath Federal Triangle. Today, this group of large neoclassical buildings - between Constitution and Pennsylvania Avenues - is home to numerous large government agencies. It’s also the lowest point in downtown.

In 2006 it rained heavily for an entire week, leaving the Triangle submerged, causing tens of millions of dollars in damage and disrupting government operations.

It wasn’t the first time this has happened here.

Experts fear the most extreme storm would put the American History Museum under 16 feet of water.

Engineers have taken steps to protect the buildings, like installing temporary barriers, retractable gates, and raising core utilities out of their basements.

There’s even now a temporary levee that’s trucked over and dropped in along the Mall for added protection ahead of big storms.

But throughout the rest of DC, an estimated 16,000 homes and businesses have had little protection from flooding when it rains.

Some residents have water gush in from the street, others confront it bubbling up from their toilets when old sewers overflow and long-buried “zombie” streams gurgle back to life.

On the southernmost edge of the district, along the Potomac, another major flooding threat looms. Over the past century, the waterline has effectively risen four feet, a combination of sea level rise and the land sinking under the weight of development.

I’ve definitely noticed the water getting a little higher every year. The area where the water is is actually a portion of the seawall that has settled over the years.

During high tide, when the ocean pushes the Chesapeake Bay further up the Potomac, 250 million gallons of river water enter the Tidal Basin. But it was constructed in 1897, it can no longer contain the river at high tide, causing the walking paths around the monuments and iconic cherry trees to flood twice a day like clockwork.

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