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About the Coastal Stormwater Discharge Analysis

Boston's storm drain system collects rainfall runoff and discharges it to the Charles River, Boston Harbor, or the Neponset River using a network of pipes that move water by gravity. Rainfall runoff is collected by catch basins in the street. These catch basins are connected to large pipes that convey water towards coastal structures where the water can be released. These coastal structures are called outfalls. 

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Higher sea levels and storm surge, due to climate change, could hinder this system from working as designed by preventing water from flowing out of the system by gravity. During some rain events, this could lead to backups and street flooding. Sea level rise could even lead to inland flooding during dry weather, as high tides could cause sea water to flow up through outfalls and into the city, as shown in the figure below, even with coastal flood barriers and berms. 

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One of the first major steps in protecting outfalls from storm surge and sea level rise is the installation of tide gates. Tide gates allow for water to flow out of outfalls during low tide but close during high tide to prevent backflow into the outfalls. 

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For some outfalls, tide gates alone are not a sufficient solution to prevent flooding. If rain weather events occur during times when the tide is high, storm water cannot discharge by gravity and the buildup can cause inland flooding, as shown in the figure below.

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To address this vulnerability, the Commission undertook the Coastal Stormwater Discharge Analysis to begin the process of adapting coastal flood vulnerable outfalls to higher sea levels. 

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The end goal of the project is to protect the City of Boston from both interior flooding and from coastal flooding with solutions that facilitate stormwater discharge despite higher sea levels in the future.


The Commission's Coastal Stormwater Discharge Solutions shown on this website, when paired with Climate Ready Boston's solutions, are effective at preventing major flooding due to climate change.

Project Objectives

The Commission undertook the Coastal Stormwater Discharge Analysis to identify its coastal flood vulnerable outfalls, design conceptual solutions at several locations, and develop a roadmap to adapt all its coastal flood vulnerable outfalls. The project involved:

  1. Identification of coastal flood vulnerable outfalls. Outfalls were considered vulnerable if they were below a projected 100-year flood elevation in 2070 (approximately 13.8 feet, NAVD88). 

  2. Ranking of coastal flood vulnerable outfalls. The Commission considered a multitude of factors including flood risk, size, location, social vulnerability, and others to identify outfalls that most urgently require adaptation. 

  3. Development of conceptual solutions to adapt outfalls with high coastal flood vulnerability. These solutions offer regional coastal flood protection benefits. 

  4. Creation of an Implementation Timeline to adapt all Commission owned coastal flood vulnerable outfalls with possible solutions. 

  5. Two-dimensional flood modeling to predict flood reduction benefits and characterize economic damage avoided. 

Throughout the project, the Commission coordinated closely with the City of Boston and the Climate Ready Boston team currently developing solutions to protect Boston's shoreline from sea level rise and storm surge.

This graphic depicts the Commission's methodology for determining if an outfall is vulnerable to coastal flooding. For the purpose of this project, outfalls located below a projected 100-year flood elevation in 2070, 13.8 feet NAVD88, were considered vulnerable and included. 

Project Progress

The graphic below illustrates how the Commission defined which portions of the City are coastal flood vulnerable. Drainage areas that lie below a projected 100-year flood elevation in 2070 were considered flood vulnerable. Substantial portions of the City that drain to the Charles River were not considered flood vulnerable because the Charles River Dam protects the outfalls that serve these areas from higher sea levels. 

The map below illustrates the Commission's progress with the Coastal Stormwater discharge analysis, and the outfalls which are considered vulnerable to higher sea levels. The project resulted in design concepts and planning level solutions to adapt all of these vulnerable outfalls. 

Coastal Flood Vulnerable Outfalls - Conceptual Design Progress

Coastal Flood Vulnerable Outfalls - Implementation Timeline

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