Assessing Beach–Seawall Hybrid Systems: A Novel Metric-Based Approach for Robustness and Serviceability

Abstract

Coastal structures such as seawalls prevent hinterland erosion and flooding, making them an integral component in building resilient communities and infrastructure. However, these structures could fail to perform as intended during extreme events, given that their performance varies with environmental conditions such as incident wave characteristics. The probability of failure may be reduced using the flood mitigation benefits of natural or nature-based features, such as beaches. However, the performance of hybrid solutions is not yet fully understood. The objective of this study is to quantify the wave attenuation benefits of beaches for improving the performance of seawalls to reduce wave runup and prevent wave overtopping. To that end, this study first proposes novel metrics for the robustness and serviceability of such systems, as well as a new temporal metric to quantify the flooding time. Next, these metrics, as well as the volume of overtopping, are utilized to measure the performance of a hybrid beach–seawall system at a study site in the United States. A numerical modeling approach, using the phase-resolving wave model XBeach nonhydrostatic, is adopted to generate a comprehensive dataset of wave runup and overtopping. The results of numerical experiments are analyzed to investigate the value of the beach in reducing wave runup and overtopping on the seawall and, thus, enhance the robustness and flood serviceability of the structure during extreme events. The analyses are carried out for various scenarios of storm tides and wave characteristics and beaches of different widths and slopes. The results reveal that the influences of beach characteristics are significant in a hybrid beach–seawall coastal defense system, with a gentler (wider) slope reducing wave runup and overtopping, particularly for smaller storms and low storm tides. However, a steep (narrow) beach offered greater robustness against larger storms and higher storm tides, and the system’s robustness and serviceability were highly sensitive to the storm tide across all beach slopes, implying a nuanced approach to the design and management of the systems.