The Nonlinear Increase in Flood Risk with Mean Sea Level Rise and the Zone of Shared Risk Concept

Branford, Conneticut
Submitted by:
James O'Donnell
Professor of Marine Sciences, and Executive Director, Connecticut Institure for Resilience and Climate Adaptationn
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Project Description

As a consequence of its geologic history, the rocky shoreline of Connecticut is incised by numerous inlets where salt marshes have formed. The coastal settlements that have developed around these marshes are now experiencing flooding when storm surges coincide with high tide. Residents and their political leaders have become concerned that increases in the mean sea level will inevitably lead to more frequent flooding and are searching for strategies to mitigate the increased risk. In a study of two locations in Branford CT, we used high-resolution LIDAR data, sea level observations, and mathematical models to diagnose how flooding occurs and how it will change in the next 50 to 100 years. At both sites, we find that the coastal geometry, the presence of culverts and tides gate, the elevation of coastal roads, and saltmarshes combine to provide flood protection. At one site, we show that an increase of mean sea level of 25 cm will increase the frequence of flooding of a crucial state highway by a factor of 5. A nearby neighborhood of 60 homes is currently protected by marshes and tide-gates and was not flooded during super-storm Sandy even though the water level in Long Island Sound exceeded the elevation of the houses. Simulations show that as sea level rises, the effectiveness of the flood protection decrease. The maximum water level in the neighborhood during a strom like Sandy is predicted to rise faster than mean sea level until it reaches 40 cm when all the flood protection value is lost. Understanding the role of the small scale structures and the evolution of the patterns of flooding identifies possible adaptation strategies, the zones where property owners share the same risk, and where benefits from interventions occur.

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