How Boston's Biggest Development Is Defending Itself Against Catastrophic Flooding

When The HYM Investment Group bought the 161-acre Suffolk Downs site long occupied by a racetrack in 2017 for $155M, it had visions of a generational development. The next year, the site was literally under water during Boston’s worst storm in decades.

“As the water came over the MBTA tracks, it would start to fill into our site, it would spill down Washburn Avenue to the base of the T station and flood the MBTA parking lot,” HYM partner and Director of Development Doug Manz said.

The historic storm threw into sharp relief the challenge HYM is facing as it plans its 16.2M SF, $8B development: The site is flanked by the Atlantic Ocean to the east and west, has Sales Creek running through it, and is uniquely at the mercy of the changing climate.

The team behind the massive development is now gearing up to defend both its project and its neighbors from storms that have already pummeled Boston and only project to get stronger.

High-tide flooding may occur up to 95 days a year by 2050, sea levels are expected to rise more than 4 feet by 2070 and rain events will reach 10 inches by the late century, experts predict. To protect its ambitious mixed-use project, HYM is spending tens of millions of dollars in flood mitigation measures drawn from a complex, three-year-long engineering study.

Infrastructure upgrades and creative use of the public space will temper rising waters, the team behind the project told Bisnow in a series of interviews this month.

“Besides raising the site, there were a few points where some very simple measures helped hold back the Atlantic Ocean and eliminate flooding,” Manz said.

HYM's former racetrack was once mulled as an Amazon HQ2 contender, but now the developer is pledging to build 10,000 housing units over the next 20 years, more than any project in city history. The expanse will host more than 2.7M SF of life sciences space and 2.7M SF of commercial office, along with 450K SF of retail and 400K SF of hotel space, designed by master architect CBT.

Atlantic Ocean inlets flank the land with the Chelsea Creek to the west and the Belle Isle Marsh Reservation to the east. The Sales Creek freshwater stream cuts through the former racetrack loop in the northern portion of the site. Most of the area was ground fill and part of the Atlantic Ocean over a century ago, Manz said, resulting in a high water table and a topographic elevation of 17 feet above sea level on average. 

High-tide flooding, or “sunny day” flood events, pummel Boston more than any other East Coast metropolitan area, according to a National Oceanic and Atmospheric Administration study. The surges bring waters more than 2 feet above typical high tides, and such events have tripled since 2000, NOAA said. Suffolk Downs' location and topography put it squarely in harm’s way. 

“Certainly, we’ve had other large multi-acre projects that have come forth, but I think this is one of the more vulnerable,” BPDA Senior Waterfront Planner Chris Busch said. 

Civil engineering firm Vanasse Hangen Brustlin conducted its most in-depth study of a development ever, VHB Water Resources and Civil Engineer Mark Costa said, merging on-site data gathered from surveying, publicly available datasets, and light detection and ranging to map the area's surface.

The firm assembled presentations showing animations of floods at the site from the 2018 Winter Storm Grayson, which caused near-record-high tides in the Boston Harbor, as well as potential 100-year events, or storms with a 1% chance of occurring in a given year.

Worst-case scenarios in a pre-construction layout, including a projected simultaneous 100-year rainfall and 100-year tidal event, with 1.5 feet of sea-level rise, would overwhelm nearby areas with over 4 feet of water, according to a presentation Manz and Costa gave to the Environmental Business Council of New England. 

“We had to look at both coastal events as well as rainfall events, as well as a combined event, where it's both raining and it has a coastal storm surge, which has a very low probability of occurring,” Costa said. “But we looked at it anyway, so we could analyze that.”

A combination of a 4-foot-tall earthen berm along Suffolk Downs' eastern border near the Belle Isle Marsh, an upgraded water pumping station and open space would bring floodwaters down by over a foot in some areas, and in others prevent flooding altogether, according to the presentation.

HYM and general contractor John Moriarty & Associates will construct the berm in the project’s first phase, which will prevent Atlantic Ocean waters from flooding into Suffolk Downs as well as approximately 20 houses and the MBTA’s Beachmont Station to the north. The natural wall, which will be topped with vegetation, is expected to be complete by 2023. A relatively simple build, the reduction in flooding will be dramatic, Manz said. 

“That was a benefit beyond us,” he said. “It was impacting Washburn and Winthrop Avenue. It clearly showed a fairly simplistic approach and had a lot of benefits for a long period of time too as well. This is a fix that goes for decades.”

Construction crews are excavating the Central Common, Suffolk Downs’ open space, to a lower grade to withstand additional flooding capacity, Manz said. Sales Creek in severe weather events will overflow into the Central Common, which will act as temporary storage for up to 2.2 million gallons of water, up to 17 feet deep, modeling showed. A tidal gate, slated for Phase 4 of construction, will also mitigate some of the flooding.

HYM will pay $5.75M for the upgrade of the Bennington Street Pump Station to boost its capacity from 300 cubic feet per second to 600, according to the project’s lengthy master plan. The pump at the eastern end of Suffolk Downs will spew floodwaters out of Sales Creek and into the nearby marsh, back into the Atlantic Ocean. 

The master plan fits into the city’s coastal resilience design guidelines implemented in 2019. HYM will also pay a combined $500K for studies assessing climate change’s effects on local flood risks and the Belle Isle Marsh, according to public documents. 

Other large city developments, including the 100-acre build-out of the Fort Point Channel and the South Boston Power Plant redevelopment, face flood concerns but aren’t as vulnerable as Suffolk Downs because of the enormous project's freshwater and stormwater management concerns, Busch said.   

“I can think of a few sites I can’t mention today that are critically at-risk,” Costa said. “But I would say Suffolk Downs is probably the most unique, and the most complex and probably the most, really the most well-thought-out.”