Comparing the effects of mangroves versus salt marshes on delta morphodynamics

Salt marshes and mangrove wetlands provide crucial ecosystem services to deltaic areas. They also significantly modulate hydrological conditions (e.g., currents, tides, and waves), thereby altering sediment dynamics and morphology. However, how these vegetation types shape morphology at the delta scale remains a largely unresolved question.

In this presentation we will compare the hydro-morphodynamic impacts of salt marshes and mangroves in river deltas, using field observations of the Changjiang (Yangtze) River Estuary and Beibu Gulf in China, respectively. Additionally, numerical modeling using Delft3D is employed to analyze the interactions between vegetation and hydro-sedimentary processes.

Preliminary results from fieldwork reveal that both salt marshes and mangroves effectively attenuate waves and currents, promoting sediment deposition, particularly at the interface between bare flats and vegetated zones. In calm weather, salt marshes tend to accumulate sediment more readily than mangroves. However, during storm events, salt marshes are more susceptible to erosion, resulting in greater variability in sediment dynamics. There are also seasonal differences. In salt marshes, wave and current attenuation is more pronounced during summer than winter, whereas such seasonal variation is less significant in mangroves. Multi-year variability, on the other hand, may be greater in mangroves.

In ongoing numerical simulations, we find a strong nonlinear sedimentation effect as mangroves transition from small saplings to mature individuals. Future work will include modeling the role of salt marshes, with comparisons across different temporal scales (e.g., tidal cycles, seasons, years, and decades) and in direct competition with mangroves. Broadly, these findings will help us to explore potential river delta change as mangroves encroach on salt marshes in our warming planet.

 

Acknowledgements: This research has been supported by the National Key R&D Program of China (2023YFE0121200) and the National Natural Science Key Foundation of China (NSFC) (42430406).