Wednesday, July 24, 2024

Flooding in University Park is used by the City of Cambridge to illustrate its Flood Viewer tool. (Photo: City of Cambridge)

As government officials and residents in Vermont, New York and Western Massachusetts recover from unexpected early July torrential rains and devastating floods, Cambridge planners say they have improved predictions by factoring climate change into flood preparations and projections. That has made a big difference in the city’s recognition of risks.

It also factors into zoning that takes effect in September and will affect larger construction projects.

Traditional flood projections by the Federal Emergency Management Agency look back at past events to make predictions. “That works if the climate is stable,” Department of Public Works commissioner Kathy Watkins said. “But we know that the climate is changing; temperatures are increasing, rainfall intensity is increasing, sea levels are rising and coastal storms are increasing in intensity.”

An example: FEMA flood maps predict that in any year, a flood with a 1 percent chance of occurring would affect 2 percent of Cambridge properties, Watkins said. Cambridge’s Climate Change Vulnerability Assessment foresees that a flood with a 1 percent chance of occurring in 2070 would impact 27 percent of buildings.

Department of Public Works commissioner Kathy Watkins at a July 2018 meeting. (Photo: Marc Levy)

Climate change means extreme rainfall will become more common in later years. A 10 percent probability storm would dump 4.9 inches of rain today, 5.6 inches in 2030 and 6.4 inches in 2070, according to projections in the report.

Another example of incorporating climate change into predictions: A storm with a 4 percent chance of occurring today will have a 10 percent chance of occurring in 2070, Watkins said. She said it “highlights the need to use scientific-based precipitation projections incorporating climate change and not rely on FEMA mapping to determine flooding risks or to plan adaptation strategies.”

“Looking at the past is not useful”

Consultants, scientists and other experts helped the city develop the two-part report, completed in 2015 and 2017. It includes excessive heat risks as well as flood impacts. The report describes how flooding and heat would affect virtually every aspect of city life, from public transit to businesses, and it lists specific at-risk buildings such as individual public housing developments and hospitals.

Cambridge’s decision to diverge from the federal agency was echoed recently by a major national flood risk report from the First Street Foundation, a nonprofit in Brooklyn, New York, that is focused on climate risk. The widely publicized report said FEMA’s look-back method underestimated flood dangers in many areas, including Boston. “I agree with [First Street Foundation’s] bottom-line conclusion that looking at the past is not useful,” Watkins said.

Still, the federal flood maps remain the basis for requiring borrowers with federally backed mortgages to get flood insurance – the standard despite the fact that “the majority of floods that occur happen outside the FEMA flood plain,” said Indrani Ghosh of environmental consultant Weston & Sampson, a senior technical leader who helped develop the city’s predictions.

The city’s Flood Viewer

The city has used its flooding projections to produce a tool called Flood Viewer that details risks to individual properties from sea-level rise and rainfall today; in 2030; and in 2070. People can look up the results by address. Flood Viewer also predicts how high the water will rise, along with information on the elevation of individual buildings.

The tool will provide the basis for new zoning rules effective Sept. 1 intended to deal with climate change. Developers planning new construction or substantial renovation of large buildings must protect occupiable space and living areas against flood risks from rainfall and from sea level rise and storm surge if the Flood Viewer shows that projected flooding would raise the water level above the elevation of the building.

Watkins said the city is working with Somerville and the Massachusetts Water Resources Authority to develop a plan to reduce overflows of combined sewage and stormwater from pipes emptying into the Charles River and Alewife Brook. The plan will account for climate change, she said.

“To our knowledge we are the first group in the country looking at climate change when updating a CSO plan,” she said, referring to combined sewage overflows.

This post was updated July 19, 2023, to correct how a location was identified in a photo caption.