Assessing Compound Flood Risk in a Low-Lying Coastal-Urban Catchment: A Case Study of East Boston

Abstract: The recurrence of flood events under climate change and sea level rise has increased the vulnerability of major coastal cities to compound flooding resulting from the combination of pluvial, fluvial, and coastal flood processes. Traditional flood assessments often evaluate these hazards independently, overlooking their dynamic interactions and limiting their ability to capture the severity of future flood impacts. This study presents an integrated flood risk assessment framework for East Boston, a highly urbanized, low-lying coastal community that is particularly susceptible to compound flooding. InfoWorks ICM was employed with a 2D rain-on-grid approach to simulate surface flooding at 1m spatial resolution, coupled with a 1D stormwater drainage network and a dynamic coastal flooding component. Pluvial flood modeling evaluated 10, 25, 50, and 100-year design rainfall scenarios for 2070 climate projections for Massachusetts obtained from the Executive Office of Energy and Environmental Affairs. The compound flood modeling approach combined extreme rainfall with storm surge (20 and 100-yr return periods) and relative sea level rise (1ft and 3.3 ft). Model outputs were coupled with a risk quantification framework (probable annual risk) to assess infrastructure risk by integrating flood hazard with business risk exposure, providing a practical basis for prioritizing vulnerable assets and supporting adaptation planning. Results indicated that areas downslope of the Orient Heights and Eagle Hill neighborhoods exhibit the highest susceptibility to pluvial flooding. Compound flood analysis revealed that combined effects of extreme rainfall, storm surge, and sea level rise substantially increase flood extent and depth compared with rainfall-only scenarios, highlighting the importance of considering multiple interacting flood drivers. Model validation against Boston Water and Sewer Commission flood reports demonstrated strong predictive accuracy. The PAR framework was further used to evaluate the effectiveness of alternative investments in flood protection actions. These results underscore the necessity of multi-hazard modeling and asset-focused risk prioritization for effective urban flood adaptation planning in coastal environments under climate change.