Using Reliability Metrics to Evaluate Vulnerability of a Coastal Dike Subjected to Climate Change

This paper describes how the vulnerability of a coastal dike was evaluated within the context of several future sea level rise (SLR) scenarios. The Westcott dike is part of a hurricane barrier system that protects low-laying urban areas of Stamford, Connecticut (USA) from hurricane storm surge along the open coast of Long Island Sound. The Stamford hurricane barrier was designed in 1962 and constructed in 1969. The paper discusses the use of reliability as an objective metric for evaluating the vulnerability of the coastal dike. Reliability is well-suited for application in the coastal zone, where significant uncertainty in stochastic loading conditions (water levels and waves) can render deterministic evaluations of project performance as speculative. The non-linear effect of SLR on the vulnerability of the coastal dike is presented along with potential measures for adapting the coastal dike to mitigate SLR. Results presented in this paper also highlight the effect that evolving engineering-design conventions may have on project performance, and demonstrate why it is important to consider changes in design practice when evaluating project vulnerability. Based on present-day design methods, results of this vulnerability evaluation indicate that the Westcott coastal dike will become vulnerable for sustaining damage from direct wave action and excessive wave overtopping by 2020 to 2040, depending upon the rate of SLR realized at the project. Functional performance of the Westcott dike could also be compromised. Several adaptive measures can be implemented to address present-future dike vulnerability: A) Place additional armour stone on the ocean-side of the dike to increase median armour unit weight from 30 Kg to 2000 Kg. B) Increase crest elevation by 1.2 meters. C) Increase median armour unit weight along crest and lee-side from 14 Kg to 1400 Kg. These adaptive measures would mitigate project vulnerability though 2050, based on a high SLR scenario.