Wave and slope thresholds for mangrove restoration and nature-based solution design

Abstract Globally, shorelines are experiencing widespread recession, promoting greater interest in nature-based solutions (NbSs) for coastal protection. Along tropical and sub-tropical coastlines, mangroves are among the habitat-forming species increasingly targeted for nature-based coastal protection. However, practical implementation requires tools that identify shorelines where hydrodynamic and geomorphic conditions are likely to support successful mangrove establishment and long-term persistence. This study quantifies first-order suitability envelopes (hereafter thresholds) for mangrove presence by linking wave height and bathymetric slope to observed mangrove distributions in subtropical wave-dominated barrier estuarine systems. Mangroves were most likely to be present where the 80 th percentile significant wave height ( H s ) was below 0.14 m and bathymetric slope was gentler than 0.01. Above these thresholds, the probability of mangrove occurrence decreased from approximately 23%–25% to 13%–15%, indicating strong but non-exclusive physical controls. Importantly, the probability did not decline to zero beyond these limits, suggesting that mangrove distribution is better described as a suitability threshold rather than a fixed ecological threshold. Spatial analysis indicated that approximately 26% of currently unvegetated shoreline areas in the study fell within the identified wave-slope thresholds, representing potentially suitable but unrealized mangrove habitat. These findings demonstrate that mangrove distribution is partly governed by hydrodynamic and geomorphic processes. While these potential restoration areas are identified based strictly on hydrodynamic and bathymetric thresholds, other critical drivers, such as tidal elevation, sediment supply, and salinity gradients, may further restrict suitable locations. The proposed methods of deriving thresholds in this study provide transferable, process-based criteria for identifying restoration opportunity areas and informing the design of NbSs for coastal resilience in estuarine environments.