MANGROVE INTEGRITY AND RESULTING IMPACTS OF HURRICANE OVERWASH

Mangroves are considered a key form of flood mitigation, yet case studies that document their response to extreme flood events are relatively limited. While mangroves aid in the protection of the coast during hurricanes, they are not immune to the damaging effects. Playa Grande is a lagoon located on the exposed, southern coast of Vieques, a small island to the southeast of mainland Puerto Rico. Previous work identified hurricane-induced overwash deposits in Playa Grande that showed periods of quiet and higher hurricane activity, which have been attributed to largescale climatic trends. Hurricane Maria, a category 5 hurricane in 2017, represents a theoretical worst-case scenario for Vieques. While climatic forcing may play a role in the pattern of overwash deposits in Playa Grande, observations after Maria indicate mangroves also play a predominant role in the spatial distribution of resulting deposition. Here we provide observations of mangrove impacts on the island of Vieques following Maria and connect mangrove integrity to patterns of resultant overwash. Despite the high intensity of wind and waves during the event, there were no overwash deposits in the western portion of the Playa Grande lagoon where previous studies identified hurricane-induced deposits. The mangrove forest fringing the southwestern coastline of the lagoon was destroyed and trapped extensive wracklines of debris, preventing overwash deposition further landward. Significant deposition observed in the smaller, eastern portion of the lagoon is associated with foot paths through the mangroves. This suggests that mangroves, if in healthy condition, can mitigate wave-induced flood damage. Here, we propose that centennial-to-millennial scale periods of reduced overwash, previously interpreted as climate-driven intervals of lower storm activity, may in part be attributed to periods of robust fringing mangrove forests. In contrast, more active overwash intervals may be due to prolonged periods of decreased local mangrove coverage. These results highlight the potential for geomorphic impacts due to mangrove mortality, and the resulting changes in coastline vulnerability that may result from catastrophic events like Maria.