Hydro-sediment dynamics in an abandoned estuarine pond under artificial mangrove restoration

Abstract

Mangrove wetlands are highly productive and provide essential services to coastal communities. However, global mangrove extent has declined, underscoring the need for restoration, particularly in abandoned aquaculture ponds. In this study, we conducted in-situ vegetation, hydro-sediment, and morphological observations, coupled with an idealized numerical model, in a former aquaculture pond in an estuary. Measurements revealed that tidal asymmetry shifted from ebb dominance to flood dominance along the inflow path, with the flood-to-ebb water flux ratio increasing from 0.4 in the outer channel to 1.5 in the inner pond. Currents exhibited a vertical structure characterized by opposite directions between upper and lower layers in the pond region. From outside in, bed shear stress decreased sharply from a peak of 1.4 Pa to 0.2 Pa, remaining below the critical threshold for deposition throughout the entire tidal cycle within the pond. Additionally, near-bed SSC during the flood phase (0.1 kg/m3) was nearly 1.5 times that during the ebb phase (0.07 kg/m3), with suspended sediment flux showing a clear decline from the inlet to the pond's interior. Planted mangrove heights and canopy widths increased by 17 % and 40 %, respectively, with an overall mineral accretion of 5 cm observed in the pond over a nine-month period. The numerical simulation results show that both the increase in pond dike closure and the early growth of mangroves exhibited a non-linear effect (initially increasing and then decreasing) on flood-dominated asymmetry and bed shear stress ratio. Moreover, increasing planting density reinforced these effects from the scenario analysis. Our study highlights a shift in hydro-sediment dynamics within a mangrove-restored pond, offering insights into optimizing topographic closure and planting density in abandoned ponds to promote sustainable mangrove restoration.