Quantifying Overtopping Performance Of Green-Gray Hybrid Infrastructure

Abstract

Wave overtopping of shoreline infrastructure can lead to significant flooding and consequent loss of life, impairment of transportation systems, and ecological damage. Coastal defenses against overtopping traditionally include hard structures, such as seawalls and revetments, and design guidelines for these structures, e.g., the EurOtop manual (Van der Meer et al., 2018), have been developed from empirical studies of overtopping. Recently, natural and nature-based features (NNBF) including mangroves, wetlands, reefs, and other systems have gained attention as alternatives to conventional engineered coastal protection systems. Field observations have identified the potential of emergent vegetation, particularly mangrove forests, to mitigate damage during extreme coastal flood events (Alongi, 2008; Tomiczek et al., 2020). However, there is a lack of research on engineering NNBF systems to achieve specific design requirements for overtopping protection.

Hybrid or multi-tiered approaches to shoreline protection have also been proposed, where natural (“green”) features are combined with hardened (“gray”) infrastructure to protect coastlines and near-coast assets from erosion and/or flood-based hazards. For overtopping mitigation, hybrid designs can add the performance provided by emergent vegetation to the services of a revetment or a wall. It is unknown whether the green and gray features in a hybrid system perform independently and can be considered as separate design elements, or if the inclusion of one feature affects the performance of the other such that the hybrid system must be considered as a single, complex design element. This study constructed a large-scale physical model to investigate the overtopping performance of a hybrid system with an idealized Rhizophora mangrove forest seaward of a revetment abutting a vertical wall compared to that performance of the wall fronted by the revetment only, the wall fronted by vegetation only, and the wall alone.