Hydro-Sedimentary Dynamics and Morphological Evolution of Intertidal Sandbar–Mangrove Systems in Mayotte

Sediment mobility in mangrove–sandbar coupled systems plays a key role in the evolution of coastal ecosystems through interactions between sediment dynamics, hydrodynamic forcing, and vegetation. Here, we present an integrated assessment of the hydro-sedimentary processes controlling the dynamics of intertidal mangrove–sandbar systems in the tropical context of Mayotte (Indian Ocean). Short-term in situ monitoring of morphological changes was combined with seasonal hydrodynamic measurements to investigate the respective roles of intertidal sandbars and mangroves in modulating local hydrodynamic processes. Two pilot sites, Tsingoni and Bandrele, were investigated using high-precision GNSS-RTK surveys and drone-derived digital elevation models (DEMs) to quantify sandbar morphology and volumetric sediment budgets, combined with hydrodynamic measurements from two acoustic Doppler current profilers (ADCPs) and eight RBR pressure sensors to characterize coastal morphodynamic processes. Results reveal contrasting dynamics between the two sites. At Tsingoni, intertidal sandbars exhibit rapid landward migration (10–61 m in 5 months) and significant vertical accretion (0.2–0.5 m), locally affecting young mangrove stands through sediment burial. In contrast, Bandrele shows more limited morphological changes, with minor sandbar migration and adjustment (0.2–0.3 m) at the mangrove edge at the mangrove edge and in adjacent troughs. Hydrodynamic analyses further indicate that under wave heights of ~0.68 m and current velocities of ~0.5 m s⁻¹, sandbars dissipate approximately 30% of the incident energy, while mangroves provide an additional 60–95% attenuation. Together, these results highlight the dynamic and complementary roles of sandbars and mangroves in shaping the evolution of tropical intertidal coastal systems.

Keywords: Mangrove, Intertidal sandbar, Sediment mobility, Morphodynamics, Coastal resilience, Hydro-sedimentary processes