Flood damage at the National Archives in 2006. (Susan Biddle/The Washington Post)

Albert H. Small is the originator of the National Mall Underground idea and a member of the National Mall Coalition board. Arthur Cotton Moore is the architect of the National Mall Underground concept and vice chair of the National Mall Coalition board. Judy Scott Feldman is chair of the National Mall Coalition board.

On July 8, the District experienced torrential rains and flash flooding that in a matter of minutes overflowed storm sewers, trapped people in cars and created havoc throughout the region. News photos flashed around the country, showing cars sloshing through floodwaters at Constitution Avenue at 15th Street, in full view of the Washington Monument and the White House, and revealed the shocking vulnerability of the nation’s capital and the Mall.

It was shocking, but it was not a surprise.

Recent studies by the University of Maryland, the National Academy of Sciences and the National Park Service predict that urban flooding throughout the country will cause widespread inundation of heavily populated neighborhoods and our national parks. By 2050, the Mall museums and other vital parts of our American heritage could be underwater during a Category 3 hurricane from the combined impact of a rising Potomac River and runoff from heavy rains.

Although the deployment on July 8 of new floodgates at the National Archives successfully protected our nation’s treasures there, this is not a comprehensive solution. In fact, putting gates around all susceptible buildings along Constitution Avenue could force floodwaters north, toward Pennsylvania Avenue and downtown. That is not a desirable outcome, to say the least.

We’ve been here before. In June 2006, after three consecutive days of heavy rain, stormwater runoff from city streets at higher elevations flooded Smithsonian museums, the National Archives and other nearby buildings. An interagency, intergovernmental panel studied that disaster, predicted more frequent and more intense storms in coming years and considered several comprehensive solutions. Modest measures have been instituted, including the Archives floodgates. (The Park Service’s 17th Street levee is intended to prevent Potomac River flooding, not stormwater flooding.)

We are a long way from safeguarding one of the nation’s most iconic places: the Mall and the thriving tourism industry that bolsters our city’s and the region’s economic vitality.

There is a solution: The nonprofit National Mall Coalition has proposed an underground structure beneath the vast open space of the Mall, between the Ninth and 12th street tunnels. The National Mall Underground would function as a storm-water reservoir, absorbing 30 million gallons of floodwater — basically a 200-year-flood event such as occurred in 2006.

This innovative proposal would be more than an empty box below ground, awaiting the next storm. The lower level of this multipurpose facility, when not in use as a reservoir, would provide parking for 150 tour buses, which have few places to go after discharging their passengers. The upper level, designed not to flood, would include car parking for tourists and a visitors’ center with amenities including restrooms and food service.

Our coalition has briefed the District government, federal agencies, civic groups and other interested parties. One unexpected fruit of that coordination was a design modification to include a clean-energy component: a field of geothermal wells beneath the slab floor that would help cool nearby public buildings.

We engaged the Army Corps of Engineers, supported by a unanimous resolution of the D.C. Council, to review the proposal. Though additional studies are warranted to turn the concept into a shovel-ready project, the Corps of Engineers’ report concluded that flood risk “could be reduced significantly during a flood event by implementation of the Underground.” It stated that “revenue potential from parking fees and water credits may offer self-financing opportunities that attracts a public-private partnership.”

And here — the matter of funding — is where the National Mall Underground would provide additional benefits. Unlike other solutions that require public funding and political will that too often never materialize, the Underground could be built with private financing, the cost recovered over time from parking fees.

Not everyone is on board. Some oppose more parking under any circumstances, despite the fact that tourist buses are ever-present. Others, including the Park Service, have different priorities or claim such a structure doesn’t fit with their plans for the Mall. Ultimately, Congress — key members have been briefed on this project — is crucial to final approval.

The challenge isn’t going away. The District’s emphasis on resiliency, especially green infrastructure that can absorb rain and ameliorate its impact, is a welcome priority. But we need more. Which is why the National Mall Underground deserves serious consideration by everyone who has the Mall’s and the city’s future well-being in mind.

If someone has a better solution, then speak up, the sooner, the better. Let’s get on with it, lest we lose this iconic stage for American democracy, the Mall.