The Inundation Model and Coastal Stormwater Discharge Analysis are important preliminary steps towards characterizing coastal flooding risks in Boston and developing adaptations to manage flood risk. As the Commission and the City continue to evolve their climate adaptation programs, the assumptions that led to these projects will need to be updated.
Sea Level Rise and Storm Surge
Data for SLR and storm surge were obtained from the Massachusetts Coastal Flood Risk Model (MC-FRM) developed by Woods Hole Group. According to Woods Hole Group, "the Representative Concentration Pathways (RCP) greenhouse gas concentration trajectories developed as part of the Intergovernmental Panel on Climate Change (IPCC). These pathways describe a wide range of possible scenarios that may occur due to future anthropogenic greenhouse gas emissions. The RCP pathway utilized in this assessment (RCP 8.5) assumes that no changes are made to human based emissions. The sea level rise produced under this scenario (RCP 8.5) was developed specifically for the Commonwealth of Massachusetts, is being used in the MC-FRM, and is consistent with the projections being used in the Massachusetts State Hazard Mitigation Climate Adaptation Plan. The Commission obtained data from the MC-FRM in 2019 during the Inundation Model Project for scenarios in 2030 and 2070. In 2030, SLR projections amount to an increase of 1.3 feet in Boston from a baseline condition (2008 centered tidal epoch), and in 2070 projected SLR amounts to an increase of 4.3 feet. According to Woods Hole Group, “this scenario is based on the RSLR projections under RCP 8.5 and represents elevations that have a 99.5% probability of not being exceeded within the respective timeframes”.
Coastal Related Flooding is the Focus of this Website
While some inland stormwater flooding was accounted for in the Commission's Inundation Model, the primary focus of the modeling efforts was coastal related flooding. Coastal flooding can occur when water flows directly from the ocean onto land or when water levels in the ocean are too high and prevent outfalls from discharging stormwater by gravity. When outfalls cannot discharge, backups in the storm drain system can occur, leading to flooding. These types of flooding were accounted for in the flood modeling. Stormwater flooding can also occur due to intense rainfall and/or localized conditions that prevent drainage (such as a clogged catch basin). The Inundation Model was not developed to characterize these sources of flooding. As a result, if flooding is not shown on the Inundation Model viewer in an interior portion of the City, it is possible that flooding could still occur at that location.
Additionally, the Inundation Model only takes into account stormwater outfalls owned by the Water and Sewer Commission. It was assumed that the owners of all other outfalls within the City effectively manage their outfalls to allow for coastal stormwater discharge under all wet weather conditions. This assumption is not accurate; coordination between several stakeholders will be required to protect all vulnerable outfalls within the City. As a result, if flooding is not shown on the Inundation Model viewer in a coastal portion of the City, it is possible that flooding could still occur at that location because of non-Commission-owned outfalls. The Commission intends to expand and update the Inundation Model during a future study.
The graphic below depicts the type of coastal related inland flooding that the Inundation Model was developed to characterize.