Linking marsh sustainability to event-based sedimentary processes: Large river floods initiated lateral erosion of deltaic marshes

Salt marshes providing valuable services in buffering flooding risks are regarded as cost-effective coastal defense solutions. Given that eroding marsh cliffs will threaten the sustainability of these protective functions, there is a need for mechanistic understanding of cliff formation. Based on short-term field measurements together with satellite-derived dataset and a morphodynamic model, we determined the response of marsh lateral dynamics to event-based sedimentary processes in Yellow River Delta, China. It was found that the Eastern Marsh with a cliff of ∼1.2 m in height significantly differed from the gentle Western Marsh in accretion pattern and lateral dynamics. Attributed to an artificial flood lasting ∼20 days in 2022, the Eastern Marsh platform experienced average vertical accretion of 23.3 mm, ∼5 times higher than the Western Marsh barely impacted by fluvial sediment supply. Although the accretion pattern ensured the overall vertical adaptability, Eastern Marsh has translated from a phase of rapid expansion to lateral retreat of ∼60 m/yr since 2018, whereas Western Marsh was relatively stable in recent decade. When Spartina alterniflora initially colonized, the validated morphodynamic model indicated the trapping efficiency of past Eastern Marsh could rise by ∼3 times than the present, due to greater inundation frequency with relatively lower elevations. Integrating with ten-fold increases in sediment loads, it was estimated that the large artificial floods in 2018 could contribute to a vertical growth approximating 30 cm at Eastern Marsh. Such episodic but rapid marsh accretion plays a crucial role in initiating the development of marsh cliff. Driven by positive feedbacks between platform elevations and cumulative wave thrust, a series of event-based marsh sedimentation will promote long-term marsh loss caused by lateral retreat at sediment-starved systems. This study emphasizing the significance of event-based sedimentary processes provides insights into increasing the predictability and reliability of coastal defense and marsh restorations.