Erosion and accretion of salt marsh in extremely shallow water stages

Abstract

Salt marshes, which commonly exist on the upper tidal flat, provide a natural barrier against sea level rise and coastal storm. The extremely shallow water stages (water depth< 0.2 m), including the initial stage of flood tides and the last stage of ebb tides, can induce a significant impact on sediment dynamics of saltmarshes and associated tidal flats, despite lasting for only a short time (around 10 min), which has been less studied. In this study, two parallel field sites were established to quantify erosion-accretion processes and morphological changes during extremely shallow water stages in salt marshes within Doulonggang tidal flat along the Jiangsu coast. Our results revealed that obvious accretion occurred during extremely shallow water stages, with a total deposition amount of +33.8 mm in vegetated areas and +20.8 mm in unvegetated areas. In contrast, erosion dominated during deep water stages, with a total erosion amount of -22.3 mm at the vegetated site and -32.7 mm at the unvegetated site. The magnitude of bed-level change during extremely shallow water stages was 7~8 times greater than that during deep water stages, even though the duration of extremely shallow water stages was only about 14~15% of the entire tidal cycle. Furthermore, strong winds significantly impacted deposition during extremely shallow water stages compared to calm weather. During the strong wind period, the average bed level change rate reached +0.15 mm/min and +0.12 mm/min in the vegetated and unvegetated areas, respectively. This is significantly higher than the +0.05 mm/min and +0.01 mm/min during the calm weather period. These results reveal that extremely shallow water stages have substantial impacts on sedimentary processes, which are vital for the maintenance of tidal flat systems.

Tidal Flat Erosion Processes and Their Dynamic Mechanisms on the South Side of Sheyang River Estuary, Jiangsu Province

“…According to the instrument parameters and installation methods used in this study, the observation data are incorrect when the inundation height is less than 0.5 m. Therefore, the calculated fluxes of sediment transport and erosion-deposition in this study were obtained for periods when the submerged inundation height exceeded 0.5 m, which does not cover the entire tidal cycle, especially during the early stage of flood tide and late stage of ebb tide, when the inundation height is very shallow and wave action is more significant. The observation results also show that in extremely shallow-water environments (water depth less than 0.2 m), there are very evident variations in tidal flat erosion and accretion, with significant erosion caused by wave-current interaction during the early stage of flood tide [23] and substantial sediment deposition during the late stage of ebb tide [24], which were not included in this study. In addition, the intertidal flat is rich in algae and abundant in benthic biological activity, which significantly influences the critical shear stress for erosion of sediments [65,66].…”

“…(2) using time-series remote sensing images to study coastline changes [17,18]; (3) using remote sensing image interpretation or drone measurements to obtain data for the distribution of seabed elevation in different periods and analyzing the spatiotemporal variations in coastal erosion and accumulation [21]; (4) using in situ observation of hydrodynamics to calculate the erosion and accumulation processes and analyze their control mechanisms [22][23][24]; (5) using numerical modeling methods to carry out modeling research on tidal currents, waves, and sediment transport to simulate the evolution processes of coastal morphodynamics and their control mechanisms [25,26].…”