FLOOD RISK
Resilient Cambridge
Climate Change is defining a new normal that is already affecting our way of living and how we are inhabiting our cities. But Cambridge is designed and built for the climate of the past.
The Stormwater Journey
During normal rain and snow events, water flows off the land to rivers and streams through both built and natural systems and reaches the ocean without flooding the City’s streets and properties. However, when heavy rain occurs or snow melts quickly, or both occur at the same time, the capacity of the systems to move water toward the ocean can be overwhelmed and areas of Cambridge can experience flooding.
Water flow and flooding in Cambridge is influenced by two rivers and a stream: the Charles River to the south, and the Mystic River and its tributary, Alewife Brook, to the northwest. The Mystic River and Charles River watersheds define the City’s hydrology – or the movement of water – and determine where water flows.
About one-third of Cambridge’s rain and snow melt flows to the Mystic River and two-thirds to the Charles River. Both rivers flow to Boston Harbor and the Atlantic Ocean.
The ability of the water to flow to the ocean is influenced by the topography – elevation and slope – of the land, roughness of the land’s surface, size of the storm drains, and the holding capacity of the rivers and streams. Flooding can occur rapidly during extreme precipitation, with topography and impervious, or impenetrable, surfaces affecting the City’s hydrologic system.
Topography plays a role in defining stormwater pathways. Swipe left to see the watershed boundary along with the flow directions.
What is the Risk?
Precipitation Flooding
Cambridge already experiences flooding. But with climate change, precipitation-driven flooding is likely to become more frequent, expansive, and deeper in the region.
Preliminary data indicate that the area of Cambridge at risk from this type of flooding is projected to nearly double between now and 2070 for a 1% Annual Storm – a storm that has a 1% chance of occurring in any given year, also known as the 100-year storm.
More frequent and extreme rain events will test the capacity of the stormwater system to move water toward the ocean.
Significant areas of Cambridge saw street flooding on July 10, 2010, when about 3.5 inches of rain fell in one hour (for context, a moderate rain event should have a peak intensity of less than 0.5 inches per hour). The storm drainage system could not convey all of the water and it and backed up onto streets, flowing over curbs onto property. There was also overbank flooding from Alewife Brook that flooded streets and properties.
Doing nothing, or a no-action scenario, to address precipitation flooding would disrupt business, mobility, housing, and infrastructure in large swaths of the City. Implementing strategies that are outlined in Resilient Cambridge would mitigate the extreme effects of more frequent rainstorms and minimize the impact of flooding when it occurs.
Caption: With climate change, precipitation-driven flooding is likely to become more frequent, expansive, and deeper in the region.
An Encroaching Ocean
This map of 1777 Cambridge, Massachusetts, and the vicinity illustrates the extent of marshlands and flood plain abutting the Mystic and Charles rivers. Source: S.F. Batchelder/Cambridge Historical Commission.
Prior to the construction of dams on the Charles and Mystic rivers in 1910 and 1966, Cambridge experienced storm surge flooding, which is when ocean waters are pushed onshore by storms centered over the ocean. For example, on May 10, 1851, extreme storms coinciding with high tide submerged areas of Cambridge by as much as 3 feet. In November 1871, an easterly storm caused storm surge that overflowed into low-lying areas and residents of those areas were forced to seek refuge on higher ground. With the construction of the dams, the hydrology of the Charles and Mystic Rivers was significantly changed, and the waterways ceased to be tidal.
With sea level rise driven by climate change, storm surge flooding could return to Cambridge.
What is the Risk?
Storm Surge Flooding
Currently, Cambridge does not experience storm surge flooding from the ocean during coastal storms, like hurricanes and nor’easters, because Cambridge is located behind the Charles River Dam and the Amelia Earhart Dam on the Mystic River. The dams effectively block storm surges.
But current science predicts that the Boston Harbor area will experience 1 foot of sea level rise by 2030 and more than 3 feet of sea level rise by 2070. This means storm surges will become increasingly likely to be able to go around and over the tops of the dams.
The City’s projections indicate a 1% chance that storm surge flooding will reach Cambridge as early as 2045. Without action, the ocean waters would push up the Charles River and the Mystic River and Alewife Brook, reaching Cambridge and flooding streets and properties.
Caption: Storm surge elevations from recent storms are shown next to flanking elevation under existing conditions. Climate change will force sea levels to continue to rise, with the potential of storm surge flooding returning to Cambridge.
What is the Impact of Flooding?
Flooding caused by the effects of climate change is threatening the wellbeing of the City’s people.
Extreme precipitation, sea level rise, and storm surge caused by climate change are threatening the wellbeing of the City’s people, buildings, and infrastructure.
The City’s aging stormwater pipes are designed to channel rainwater to rivers and the ocean. But rising sea levels and consequently higher river elevations stress piped infrastructure. These increased water levels will then flow upstream through existing outfalls and connected pipes and disperse into areas within Cambridge via catch basins and manholes, causing localized flooding.
A photo of a neighborhood before an extreme precipitation event.
A rendering of a flooded neighborhood during a present day flood.
A rendering of a flooded neighborhood during a 2070 flood.
Flooding has obvious implications in causing physical damage to buildings and infrastructure, as well as making areas inaccessible and creating an immediate public safety concern.
Federal Emergency Management Agency (FEMA) has identified 287 buildings in Cambridge that are at a higher risk of being flooded than others. This number, however, does not consider the threat of climate change. Conversely, the City projects that approximately 800 buildings are at risk of flooding during a 2070 10% storm surge/sea level rise or precipitation event – a climate-related event that a 10% chance of happening over a 24-hour period in any given year – and approximately 3,500 to 3,700 buildings are at risk for flooding for the 2070 1% precipitation or storm surge/sea level rise event – a climate-related event with a 1% chance of happening in any given year.
There are also public health consequences associated with flooding events. Flooding can carry contaminants into buildings and create conditions for indoor mold growth. This poses significant negative impacts on indoor air quality. This risk is exacerbated in buildings that have poorly sealed exterior windows and roofs and those that use forced hot air, which can become a conveyor of air from damp basement areas. Indoor dampness is a well-known cause of adverse respiratory effects.
If not prepared, any residential or commercial structure that experiences flooding, and its occupants, can potentially experience long-term challenges related to contamination and mold growth and their remediation.
Alewife is an area within Cambridge that will likely be the first impacted by sea level rise and storm surge. The effect would be felt citywide as Alewife is also a hub for the City’s infrastructure and ecosystem and a neighborhood of regional importance, connecting Cambridge to the larger Boston metropolitan area. Alewife is critical to the City’s economy and houses some of its most vulnerable populations.
Cambridgeport, The Port and Wellington-Harrington are the City neighborhoods at risk for precipitation flooding, while Cambridge Highlands and North Cambridge are most at risk for sea level rise/storm surge flooding.
Protecting You and Your Property
The City has developed the Flood Viewer, an informational tool for the Cambridge community to assess climate change threats from flooding and to prepare for flooding by implementing specific strategies.
What is Your Risk?
To see if your residence or building is at risk of flooding, click the map to interact. Zoom in with the +/- buttons in the top left corner. Once zoomed in, left click once to identify the property with the risk information. For the full web mapping experience, click the launch button listed in the top right corner.
Cambridge FloodViewer Tool
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Planning for this "new normal" requires imaginative and responsive changes by the City and the community. Resilient Cambridge is substantive and optimistic and provides a comprehensive set of actions and strategies.
Click the button below to find out about the City’s Flood Strategies.
