Report: Charles River flooding to increase dramatically with climate change

A new report finds that communities along the Charles River will experience a dramatic increase in flooding within the next 50 years.

Due to climate change, extreme storms that are now less common are expected to become more frequent and discharge more water. Using these existing rainfall forecasts, a report by the Charles River Watershed Association modeled the impact of future storms on 20 cities along the river and found many places where flooding could damage key infrastructure, including Newton, Dedham, Watertown , Waltham and Wellesley.

“One of the big takeaways has been that it’s going to take a lot of adaptation efforts, a lot of investment to be able to address the flooding that this tool tells us is coming,” said Julie Wood, director of the Charles River Watershed Association’s Climate Compact initiative. . She said the report will help cities make planning decisions.

The research made clear that cities need “bold, regional action,” Wood said. “You can’t just do one thing and fix the problem.”

While cities like Boston, Cambridge, and Somerville had already produced future flood reports, smaller cities didn’t have climate change precipitation forecasts applied to their areas. So 20 cities along the watershed, from Hopkinton to Watertown, banded together and received a grant from the Massachusetts Municipal Vulnerability Preparedness to commission the study of flood impacts and possible solutions.

Here are five key things you need to know from the report:

Flood risk is significant in short-term and long-term scenarios

One source of concern outlined by the report is what is called a 100-year storm, so called because historically it has occurred about once a century. By 2070, extreme storms like these will be four times more likely to occur. And by 2070, a 100-year storm could dump more than 11 inches of rain, a 61 percent increase in runoff volume over today, the report said. In the event of such a storm, the report notes that more than 2,600 acres along the Charles River that are not currently flooding would experience flooding.

The risk is also high for more frequent but smaller volume storms that occur once every two years, called biennial storms. In 2030, those storms would inundate 886 acres, 25 percent more than today’s floods. And by 2070 those storms could inundate 2,733 acres, an increase in area of ​​78%.

To calculate these risks, engineers commissioned by the Charles River Watershed Association used a scenario of severe warming of 4.5 degrees Celsius by 2070. Under these high-carbon scenarios, increased precipitation in the Charles River watershed would be considerable.

Paul Kirshen, a climate change adaptation professor at UMass Boston who was not involved in the research, reviewed the report: “If they used a more moderate scenario, the impact would still happen, but decades later,” he said.

Precipitation is increasing

The Northeast has seen the highest increase in extreme precipitation events of any region in the United States, according to the 2018 US National Climate Assessment report. In 2014, the national report found that when it comes to heavy precipitation, the region recorded a 70% increase from 1958 to 2010.

The trend is expected to continue: As the land warms, more rain is likely to fall in the Northeast, including the Charles River watershed. And storms that weren’t that frequent are expected to happen more often.

The Charles River Watershed Association commissioned engineers to use Cornell University rainfall projections to map potential flooding along the Charles River and model the impacts on local infrastructure such as drainage capacity and existing dams.

“We’ve actually seen quite a few points in that extreme scenario across the watershed where a lot of critical infrastructure, structures and roads as well as homes could be impacted,” said Indrani Ghosh, an engineer with Weston & Sampson who worked on the flood model. .

Ghosh said that in the extreme case of 100-year storms occurring within 24 hours, floodwater could reach three feet above the ground in low-lying areas. He also said the drainage pipes aren’t big enough to drain that volume of water.

Watertown Dam from above. (Courtesy of Sean McNamara/Charles River Watershed Association)

Downstream urbanized cities are likely to be most affected

Cities that are more urbanized, meaning a more concrete impervious surface that doesn’t absorb water, and that are located downstream are likely to be worst affected. Using the flood model, the team identified cities that would have the most critical infrastructure such as schools, hospitals, public water supplies, fire and police stations, and roads, at risk under different scenarios. The model’s projections show flood risk at scale and are not granular enough to predict specific damage to a single property.

In the case of a 100-year storm, they found that by 2070 the hardest-hit cities with the most urban infrastructure would be Needham, Newton, Wellesley, Waltham and Watertown. And in the event of a more likely 10-year storm — where runoff could increase 75% by 2070 — Newton, Dedham, Watertown, Waltham and Westwood would still experience flooding.

The model also highlights forests and wetlands that face flooding and recommends cities avoid development in those areas, said Wood of the Charles River Watershed Association.

“This [model] it gives us this information to understand which areas are vulnerable to flooding both now and in the future, and how we can perhaps preserve some of those natural areas that can and are providing natural flood storage.”

Downstream areas will see more water flowing during heavy rains and are more prone to flooding. For this reason, a regional approach is more effective than if each city tried to do it alone, Wood said. Different strategies work in different places: Upstream areas have more opportunities to store water to prevent it from flooding downstream. Downstream, the goal is to get out of the floodplain.

“Perhaps some of the upstream communities may need to hold water to prevent flooding downstream,” said Kirshen, a professor at UMass Boston. “The problem is who pays for the improvements and management strategies? Because the downstream communities are definitely benefiting from the actions taken by the upstream communities.”

Solutions will need multiple approaches

Urban executives, engineers and over 150 community members joined the conversation to generate solutions. The report team identified 50 potential projects that could be used to mitigate the impacts of flooding, from increasing the water-holding capacity of wetlands to building green roofs to adding permeable pavement and rainwater tanks to slow the rainwater runoff. The overall conclusion is that communities will need a mix of strategies to mitigate the impacts of flooding and that each city will have a different solution.

Using different rainfall scenarios, the team tested the effectiveness of different flood mitigation approaches in the model. For example, when engineers increased the size of existing wetlands by 20% using a 10-year storm projection of 2070, the team was surprised to learn that this would only protect 29 acres – less than 1% – of ground from flooding versus a no action scenario.

Another hypothesis they tested was to see what would happen if they implemented various strategies to hold back the rain that hits half of the impermeable surfaces existing today across the watershed. This had little impact on reducing the ballot. Compared to a no action scenario, it protected 500 acres – about 5% – from flooding.

The roots of trees, shrubs and grasses hold the bank of the Charles River together in Watertown.  (Robin Lubbock/WBUR)
The roots of trees, shrubs and grasses hold the bank of the Charles River together in Watertown. (Robin Lubbock/WBUR)

Cities are considering flood reduction projects

In Waltham, the community unearthed an engineering project proposed in the 1980s to use the 45-acre Hardy Pond as a way to retain rainwater. The idea is to lower the water on the pond days before a storm is expected so it can hold more precipitation. The city is also considering restoring wetlands and using green infrastructure such as permeable paving.

In Medway, one of the ideas for Oakland Park is to build a bioswale that would slow and filter runoff and before it discharges it to an underground water storage.

One of the concepts proposed by the City of Newton is to add green infrastructure while upgrading the Albemarle Field sports complex. Ideas include building an underground rainwater storage area and improving surface water absorption with permeable pavement and rain gardens.

The Charles River Watershed Association has shared the report with all 20 cities and will begin accepting public comments online and at its next meeting in January. The association also plans to continue testing various solutions to see how cities can move forward.

“Given the level of investment and action that will be needed to adapt to the changes that are coming, to reduce these impacts, to keep us all safe, we really need to get started,” Wood said.

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