Evaluation of Alternative Coastal Vulnerability Assessment Method at Point Allerton

Abstract: In response to the increased risk and intensity of sea level rise and storm surge due to climate change, state agencies across the United States have made strides to update hazard mitigation and climate adaptation plans to assess climate risk, program funding, and provide guidance. The State of Massachusetts has been at the forefront of this effort, providing data-driven guidance as early as 2015 through the completion of a Massachusetts Department of Transportation (MassDOT) Pilot Project funded through the Federal Highway Administration (FHWA) to assess coastal flood vulnerability and adaptation options for the Central Artery tunnel in Boston. The project was made possible through development of a detailed model that simulates the effects of current and future coastal storm conditions under extreme weather events in the City of Boston.

Since the Pilot Study’s completion, the flood model has been expanded to include the entire coast of Massachusetts through publicly available information including topographic and bathymetric data from LiDAR and NOAA Electronic Navigational Charts, respectively. The updated model has been integrated into the ResilientMass Action Team (RMAT) Massachusetts Climate Resilience Design Standards Tool and provides a visual representation of coastal storm flooding for 2050- and 2070-time horizon. The tool also provides recommended storm return-periods based on the user’s selection of site-specific data including infrastructure type, criticality, and structure service life and is intended to be used for planning purposes, but is not a substitution for site-specific investigation and modeling.

This paper provides an alternative method for the assessment of coastal vulnerability and development of resilient design solutions through site-specific means. Initially constructed by the United States Army Corps of Engineers in 1874, the Point Allerton seawall in Hull stretches approximately 1,200-feet and consists primarily of a granite block wall and stone revetment. With approximately 50% of the site exposed to open ocean, the seawall was used a strategic vantage point during World War II. Currently owned by the Massachusetts Department of Conservation and Recreation (DCR), the site has undergone significant damage from coastal storms and has been unsuccessfully repaired between 1978 and 2007.
Due to the structure type, location adjacent to privately owned homes, and generally low immediate risk to human life, the structure is considered Low Criticality through the RMAT Tool which corresponds to a recommended return period of 50-years (2% annual chance flood) for the 2050- and 2070-time horizon. However, due to the project’s unique exposure to open water, a traditional probabilistic method of modeling prior storm events for a 50-year return period may not accurately represent actual site conditions that inform recommended resiliency measures. This paper will take a comprehensive look at how site-specific investigations and modeling can impact the climate change risk assessment and recommendations. The Point Allerton model includes projected sea level rise, storm surge scenarios from multiple wind directions, expanded bathymetric survey up to 4,000 feet from shore, and review of wave breaking and reflection effects on the structure, and will be validated through correlation of historic damage to actual wave incidence.

How does this project / topic contribute to the advancement of the industry and profession?: The project will review current industry practice for resiliency planning efforts in Massachusetts and emphasizes the importance of site-specific investigations/modeling for coastal storm vulnerability. The paper discusses alternative approaches to projected flood elevations not currently provided in Massachusetts guidance tools and documents.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: Yes, the project will evaluate alternative approaches to a site-specific coastal flood model which will be validated through correlation of historic damage to actual wave incidence.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: After an extended period of band-aid repair approaches, several stakeholders have been adamant that the only viable solution for the site is a raised concrete seawall. We expect that the conceptual design phase of the project will continue to be difficult, as alternative approaches to address coastal storm vulnerability are proposed including offshore breakwaters.

Who is the target audience for this paper?: The target audience for this paper are technical consultants and owners seeking guidance for alternative approaches to assess coastal vulnerability and implementation of resiliency measures at their site.