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How Boston wants to change its zoning rules to prepare for sea level rise

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The sunrise at Long Wharf in Boston on a recent October morning. Craig F. Walker / The Boston Globe

Every so often, Boston’s waterfront communities get a glimpse of the not-so-distant future, whether from a seasonal king tide or a storm surge.

However, at the current rate, it won’t just be the waterfront inundated by flooding.

City officials expect 40 inches — or more than 3 feet — of sea level rise between 2070 and 2100 due to climate change if global greenhouse gas emissions are not significantly reduced. And for development in the city’s many low-lying neighborhoods, that is going to require changes.

Last week, the Boston Planning and Development Agency released a new draft zoning overlay district in which new buildings and retrofits would be required to take additional steps to limit the impacts of future flooding.

The overlay covers parts of Boston that would be flooded during a major storm — or what’s known as a 1 percent annual chance flood event — in a future with 40 inches of local sea level rise, including the South End, Seaport, Back Bay, downtown, Fort Point, North End, East Boston, South Boston, Charlestown, and the Dorchester waterfront.

COASTAL FLOOD RESILIENCE OVERLAY DISTRICT —BPDA

The proposed overlay and its standards are based on advisory resiliency design guidelines released in 2019 aimed at encouraging the owners of at-risk buildings to elevate living space and flood-proof lower floors.

The overlay district would effectively require new apartments and condos to be located above a certain minimum base elevation, which varies from neighborhood to neighborhood based on flood projections. Chris Busch, the BPDA’s assistant deputy director of climate change and environmental planning, said those heights could range from 1 foot to 5 feet above street level. Stores, restaurants, and offices could have floors below that level, but only if they’re flood-proofed.

Some, but not all, developers are already taking steps to make new buildings resilient against future flooding; however, the new zoning overlay district would require them to comply with the standards put forth in the 2019 guidelines.

“If it’s a residential use, your first occupiable floor needs to be at or above that to ensure we’re not having residents susceptible to being flooded,” Busch said in a recent interview, explaining the rationale for the proposed base elevation.

“And it will also serve as a flood-proofing elevation for commercial, retail, industrial uses, where you need to be ensuring you’re utilizing materials to keep water out of the structure up to that elevation,” Busch said.

According to the proposed rules, non-flood-proofed floors below the required height could still be utilized for other uses, such as a lobby, stairwell, parking, or storage — or even a temporary shop, such as a farmer’s market.

Busch also noted that the BPDA will work with urban design staff during the review process to try to meld the required elevations into existing sidewalk elevations and landscaping measures “so you just don’t have this three-foot wall and five-foot wall right up against the sidewalk.”

If approved, the standards would apply to projects that result in the addition of at least 20,000 square feet in gross floor area. The BPDA is planning two virtual Zoom public meetings on the proposed standards this Wednesday and Friday.

According to Busch, the BPDA decided to use the projected sea level rise between 2070 and 2100 because “that’s really within the usable lifespan of a lot of buildings and projects we’re evaluating.” Still, the city expects to see harbor levels rise by an additional 9 inches and 21 inches by 2030 and 2050, respectively.

“In order for Boston to grow and thrive for generations to come, we must make sure that what we are building today is resilient and protected from impacts of climate change,” Brian Golden, BPDA director, said in a statement.

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