Mangrove Cover and Extent of Protection Influence Lateral Erosion Control at Hybrid Mangrove Living Shorelines

Erosion poses a significant threat to coastal and estuarine environments worldwide and is further exacerbated by anthropogenic activities and increasing coastal hazards. While conventional engineered structures, such as seawalls and revetments, are commonly employed to protect shorelines from wave impact and erosion, they can also cause detrimental environmental effects. By creating/restoring coastal habitats with engineered structures, hybrid living shorelines offer coastal protection and other co-benefits. Using aerial imagery, we studied the rates of shoreline change before and after living shoreline installation, and between living shorelines and adjacent bare shorelines in three estuaries in New South Wales, Australia. Mangroves had established behind most rock fillets and displayed a trend of increasing canopy cover with fillet age. In the first 3 years since installation, the rates of lateral shoreline change reduced from − 0.20, − 0.16, and − 0.10 m/year to − 0.03, − 0.01, and 0.06 m/year in living shorelines in Hunter, Manning, and Richmond Rivers, respectively. However, when compared to control shorelines, the effectiveness in reducing erosion varied among living shorelines with mean effect sizes of 0.04, − 0.28, and 1.74 across the three estuaries. A more positive rate of shoreline change was associated with an increasing percentage of mangrove canopy area and an increasing length of protected shoreline at wide channels. While hybrid mangrove living shorelines are a promising solution for mitigating erosion and creating habitats at an estuary-wide scale, they may also contribute to downdrift erosion, emphasising the importance of considering site-specific hydrogeomorphology and sediment movement when installing living shorelines.