Combining gray and green infrastructure to improve coastal resilience: lessons learnt from hybrid flood defenses

Shoreline erosion and storm tide inundation increasingly threaten coastal populations, infrastructure and economies. Hard infrastructure, known as gray infrastructure (e.g. concrete seawalls), has commonly been used to protect coastal communities but is expensive to build, maintain, and deteriorates coastal vegetation. Green infrastructure (e.g. restored or conserved mangrove and marsh ecosystems) delivers nature-based coastal protection but in comparatively lower density coastal areas. Nowadays a more popularized approach to coastal protection is hybrid eco-engineering. In this study, we discuss lessons-learnt on how the hybridization of engineered structures and wetland restoration practices compared with traditional gray and green approaches. We contrast hybrid applications in mangrove and tidal marsh areas in the UK and south-east Asia. The majority (70%) of successful hybrid infrastructure cases were underpinned by understanding of ecological and hydrological changes in response to infrastructure and involved a wide range of stakeholders. In terms of construction and maintenance cost, limited data suggested that hybrid infrastructure may be more cost effective than gray infrastructure, but more expensive than green; however, data were very limited and therefore we suggest the need for further cost-benefit analyses to inform a robust comparison. Development of new technologies should see growing efficacy of future hybrid infrastructure in mitigating coastal flood risks.