The Effects of Intra-Annual Variability of River Discharge on the Spatio-Temporal Dynamics of Saltmarsh Vegetation at River Mouth Bar: Insights from an Ecogeomorphological Model

Natural or human-induced intra-annual variation of river discharge alters estuarine hydrological regimes and further affects habitat conditions for saltmarsh vegetation, particularly at the river mouth bar. In this study, numerical experiments were performed in Delft3D to simulate the evolution of a schematized river mouth bar under prototypical unsteady river discharge scenarios. The simulated hydrodynamic and morphodynamic changes were used to drive a vegetation dynamics model developed based on Spartina alterniflora in real time to model the resultant vegetation responses throughout the plant life history. Our results show that the imposed seasonal high flow can create more potential suitable habitat for the vegetation expansion and at the same time, cause marsh erosion through flood-induced drag force and substrate erosion. The overall effect of the trade-off between expansion and erosion depends on the timing, magnitude and duration of the high flow as well as its carried sediment concentration, leading to three vegetation response regimes, namely, minimal impact with small flood, erosion with big flood and low sediment supply, and expansion with big flood and high sediment supply. Besides, the timing of the high flow determines whether the vegetation has enough time to occupy the newly created subaerial area after the high flow and thereby affects the overall saltmarsh extent. The proposed vegetation response regimes are verified in principle in real cases such as Yellow River Estuary, Wax Lake Delta and Yangtze River Estuary. Our findings can help inform water diversion projects in river deltas to restore coastal wetlands in terms of suitable sediment supply and timing, etc.