Mangrove forest drag and bed stabilisation effects on intertidal flat morphology

Mangrove trees influence their physical environment by exerting drag on tidal flows and waves while also stabilising the sediment bed of intertidal flats. These processes influence sediment accretion, the mangrove habitat and their resilience to sea level rise. However, little is known about the magnitude and spatial extent of the effects of mangrove forests on sediment transport and the morphology of the intertidal flat. We use manipulated simulations with an extended process-based numerical model, to study the influence of mangrove forests on intertidal flat morphology on a yearly timescale. The model includes the influence of mangrove trees on tidal flows, waves and sediment dynamics. The model is calibrated and validated with a comprehensive set of measurement data including hydrodynamics, sediment transport and morphological processes from an expanding mangrove forest in the sediment-rich Firth of Thames estuary in Aotearoa New Zealand.Sediment accretion on the upper intertidal flat is predominantly influenced by the characteristic morphology of the established mangrove forest, with increased bed stability at higher mudflat elevations related to prolonged aerial exposure and drying of the bed. Our results show that, in comparison to the situation without mangroves, sediment accretion increases in the most seaward fringe area of the forest. The unvegetated intertidal flat fronting the mangrove forest captures less sediment compared to the situation without mangroves. The mangrove forest drag triggers the development of a steeper, convex-up-shaped, upper intertidal flat profile, especially during periods with higher water levels and waves. These effects are expected to influence the development and storm-recovery of natural and restored mangrove forests and may contribute to the resilience and persistence of mangrove-vegetated intertidal flats for coastal flood risk reduction. Sediment accretion of mangrove-vegetated intertidal mudflats is enhanced by the existing morphology, with increased bed stability in higher-elevated areas due to prolonged aerial exposure. Mangrove drag and bed stabilisation effects further increase accretion rates in the mangrove forest fringe, but reduce accretion rates in the area fronting the forest. Mangroves thereby actively contribute to upper intertidal flat development and play a role in the resilience of upper intertidal flats and their persistence to reduce coastal flood risk.image