Devastating flooding from a July 2019 storm has propelled the completion of a stormwater vault under Cardinal Elementary School in Arlington, Virginia.

The vault, located in the Westover neighborhood’s Torreyson Run Watershed, is part of Flood Resilient Arlington, Arlington County’s strategy to mitigate the major impacts of flooding.

Building the Vault

Cardinal Elementary School’s stormwater vault project was a USD $18 million joint effort by Arlington County and Arlington Public Schools which took 2 years and was separated into two phases. Image courtesy of Arlington County

The 4,366-m2 (47,000-ft2) vault system can hold more than 15 million L (4 million gal) of water – a volume comparable to six Olympic-sized swimming pools. Located beneath the school’s athletic fields, the system is one of the largest precast concrete stormwater vaults in the Mid-Atlantic.

Initially, plans were in place to install a smaller stormwater vault, but following the 2019 flooding and subsequent community appeal, Arlington County collaborated with Arlington Public Schools to build two larger detention vaults to better address the flooding risk.

The construction, a USD $18 million effort, took 2 years and was separated into two phases.

First, crews installed underground pipes and junction boxes to divert water from an existing storm sewer beneath the school property to where the newly built vault would sit. Then, in December 2021, construction of the vault began. Construction concluded in June 2023. View the full construction timeline here.

Putting It to the Test

During intense rainfall, stormwater diverts into the vaults through a 2.1-m (7-ft) pipe where it is detained for a few hours. Then, gravity carries the water slowly out of the vaults into a 90-ha (222-ac) drainage area.

The Arlington community celebrated the vault system’s completion with a ribbon-cutting ceremony on Sept. 20, 2023, held as part of the Westover Day neighborhood festival at Cardinal Elementary School. The event featured live music and a variety of food trucks. Image courtesy of Arlington County

“The vaults have been functioning well,” said Aileen Winquist, Stormwater Communications Manager for Arlington County Environmental Services. “Although we have not had any extreme storm events since the vaults were completed, the vault system has been activated 24 times since June.”

A month after its commissioning, the vault system underwent its first test on August 8, 2023, when the Arlington area rapidly received 3 cm (1.2 in) of rainfall within an hour. The system functioned according to its design – the water level within the vaults climbed significantly and held until the storm passed and the captured runoff became safe for discharge.

Arlington County marked the completion of the vault with a September 20 ribbon-cutting during a neighborhood festival at the school, complete with live music and food trucks.

“The community is very pleased with the county’s and Arlington Public Schools’ collaboration and quick action to improve flood resilience in the watershed,” said Winquist.

Watch the Cardinal Elementary School stormwater vault project come together:

Michelle Kuester is a staff member of the Water Environment Federation, where she serves as Associate Editor of Stormwater Report and Water Environment & Technology magazine. She can be reached at