MODELLING MANGROVE ECOSYSTEM ENGINEERING EFFECTS FOR PERSISTENT FLOOD RISK REDUCTION

Mangroves are recognized as a nature-based solution for coastal flood risk reduction. The salt-tolerant trees occur along sheltered coastlines in the tropics, subtropics and warm temperate regions globally. With their above-ground biomass mangroves reduce energy of long and short-period waves, while they increase soil binding and stability of intertidal flats with their below-ground biomass. This way, mangroves do not only provide functionality for flood risk reduction by attenuating waves, they also provide persistence by stabilizing shorelines and creating sheltered conditions for forest development (Gijsman et al., 2021). These ecosystem engineering effects contribute to the long-term persistence of mangroves for sustained flood risk reduction because they allow mangroves to adapt to, and recover from, impacts of a changing climate. This study aims to quantify mangrove ecosystem engineering effects with a new biophysical, process-based model. The model is setup, developed, calibrated and validated with a unique dataset of hydrodynamic, morphodynamic and vegetation parameters collected for over a decade in the Firth of Thames estuary in Aotearoa New Zealand. The mangrove forest in the Firth is characterized by distinct cross-shore zones with different elevation and vegetation characteristics, ranging from the most seaward intertidal flat, the seedling establishment zone, the dynamic forest fringe, the scrub forest and, most landward, the relict forest fringe (Swales et al., 2019). Manipulated simulations with and without mangroves’ effects on hydrodynamics and morphodynamics are used to quantify their influence on the profile morphology (i.e., surface accretion) and on forest recovery (i.e., seedling survival probability) in these different cross-shore zones.