The National Underground is a bad idea that should be buried

The area along Constitution Avenue including Federal Triangle is prone to flooding, as it was once a major low point and the location of Tiber Creek and the Washington Canal. Climate change is increasing the flooding concerns, and a host of federal agencies have taken an interest in solutions.

A small private group called the National Mall Coalition has offered up a plan that adds more cars to the mix. The latest version of their proposal, which was presented to the Commission of Fine Arts in October 2021, consists of a large concrete structure between the Smithsonian Castle and the Museum of Natural History. The lowest level would serve as a cistern, storing stormwater that’s either detained temporarily or retained to irrigate landscaping in the area. Above that would sit multiple levels of bus and car parking and a visitors center.

The twist is that it envisions that the middle bus parking level could be intentionally flooded to detain up to 30 million gallons of floodwater and host a field of geothermal energy wells, although the details of this last feature are vague. The buses would have to be evacuated before the flooding occurs, and the designer has suggested using robotic parking systems on this level to minimize risk.

Proponents suggest the project takes after flood mitigation in the Netherlands. But the experience of the flood-prone country is that no single piece of infrastructure can do everything. Resiliency requires simplicity and redundancy, not vaporware parking robots.

While lobbying for the project has increased recently, decision-makers should instead turn to a range of surface-level changes that make the Mall greener and more accessible without motor vehicles.

The Federal Government has been seriously studying the concept of a large flood water cistern under the National Mall and considers it a viable option. And the concept of a parking garage and a cistern in the same project is not new. Most notably, a cistern was built adjacent to the garage at Rotterdam’s Museumpark in the Netherlands. However, adding more vehicles only adds complications.

Many ideas that don’t add up

Designs have largely been executed by Arthur Cotton Moore, an influential architect in the 1980s who is best known for the Washington Harbour project on Georgetown’s waterfront. Notably, a portion of the harbor’s garage is designed to flood under certain water conditions to prevent the building from becoming buoyant, rising from the ground, and suffering structural damage.

The concept begins to fall apart if you understand that Washington Harbor’s approach addresses flooding from a high river. In that scenario, flooding can be anticipated a week in advance and vehicles can be removed within that time frame. On the Mall, this kind of flooding is primarily addressed with a levee, which is visible on 17th Street.

The kind of flooding that is most likely to harm the buildings along Constitution Avenue is instead storm-based, for which there may be less than an hour’s warning — not enough time to safely evacuate the garage. As a result, a 2018 charrette suggested that a combined garage-cistern cannot reliably detain runoff from floods like the 2006 event that damaged multiple buildings, including the National Archives.

Beyond that, the garage and visitor center vastly increase the complexity of the project, relative to a simple cistern. Because combustion engine emissions are toxic, underground garages generally need more airflow than any other part of a building. Further, they add major fire protection and emergency egress requirements, to say nothing of the security that every other large, enclosed structure around the Mall requires.

Despite positive press around the project, its cost estimates are sketchy. They propose the project will cost less than $300 million and be paid for with tax credits and revenues from parking. But the 2019 interagency study calculated the cost of a cistern alone, with no human or vehicle occupancy, at $400 million. Further, the 2018 analysis by the US Army Corps of Engineers noted that the geothermal system, mechanical parking, and separation from the sanitary sewers on Constitution Avenue were not included in previous estimates. As a result, the analysis concludes that it was not possible to evaluate the cost-effectiveness of the design, despite what press releases say.

The study also showed that although green infrastructure like rain gardens cannot absorb major flooding, researchers say that they could be used to reduce the size of a storage cistern or pump facility. They would also treat the rainwater before discharging it into the Potomac or aquifers, something a cistern would not do as well.

All of these unknown costs, for what advantage?

As any tour guide will tell you, the Mall is not a single site. Tourists cannot take in the monuments and museums in a single day (pleasantly, at least). School groups generally do not want to march around from a single location. A parking garage in front of the Smithsonian may work for the Sackler and the Hirshhorn, but not for the Capitol or the Lincoln Memorial.

As for the visitors center, not only is it unnecessary when visitors can look things up both in advance and on the go, the Smithsonian is renovating the Castle to perform similar functions. More significantly, a visitor center would require an Act of Congress to build, as orientation buildings are forbidden under the Commemorative Works Act, which governs the development of the Mall.

The proposal adds these unnecessary programs to a floodwater cistern, presenting it as the one neat trick to solving major issues on the Mall. However, its only innovations are the least feasible elements, and it is far from a holistic solution to transportation and flooding in the area. Perversely, it places motor vehicles at the center of a piece of infrastructure meant to address the effects of climate change.

Transportation issues on the Mall could be improved with better investment in features already on the Mall: the Circulator, bike lanes, and the Metro. Several of the streets around the Mall are oversized for the amount of traffic they see. Long term, redesigning streets to remove permeable areas, making the Circulator more convenient, and adding bicycle facilities may exceed the water detention expected by the 2019 study.

It wouldn’t hurt to add more public bathrooms, as well.

Proponents of the National Mall Underground are not the only people who have failed to present a forward-looking vision for the area. The most recent updates to the Federal Comprehensive Plan (a cousin to the DC one) emphasize the importance of micro-mobility, mass transit, and green infrastructure, but have not put forward a granular vision of what that would look like through its streetscape manual.

A more ambitious version of green infrastructure has been developed by an architecture professor named Karolina Kawiaka who proposed building a swale—a big rain garden— alongside Constitution Avenue and regrading the fields north of the Washington Monument to detain a storm-based flood like the one that happened in 2006.

As green projects such as Kawaika’s proposal have already been built globally, any thought of building parking under the Mall should be buried.