Differential sediment trapping abilities of mangrove and saltmarsh vegetation a subtropical estuary

Abstract

Saltmarsh and mangrove are common coastal wetland types and their ability to enhance deposition has been investigated extensively, but rarely compared directly. This study carried out in situ observations to compare the sediment transport processes between a bare mudflat, a mangrove stand and a saltmarsh stand within a subtropical estuary. Turbidity variations over the latter portion of a spring tide were recorded, alongside measurements of flow data, to estimate sediment trapping by hydraulic forces under similar hydroperiods. In addition, vegetation was transplanted to compare the direct sediment trapping by high- and short-standing seedlings. The suspended sediment concentration (SSC) time series showed an overall reduction between the bare mudflat and the vegetated flats. Suspended sediment flux estimates revealed that a considerable amount of sediment was trapped by the saltmarsh and the mangrove edges. The flux estimates find that the saltmarsh edge is more efficient than the mangrove edge in trapping sediments transported normal to the edge. The sediment trapping mechanisms were considered based on two approaches: the hydrodynamic related sediment settling and direct trapping by vegetation. The calculation of deposition tendency showed that the presence of vegetation altered the flow direction and the tidal asymmetry of the deposition process, resulting in a higher deposition tendency during the flood phase to enhance sediment settling. In addition to sediment settling, vegetation surfaces were found to trap sediments directly. In combination with rinsing by precipitation, these trapped sediments accumulated on the bed and contributed to the deposition. Against the background of similar inundation periods, the saltmarsh grass showed a greater sediment trapping ability than the mangrove trees, in terms of both the hydraulic sediment trapping and the direct trapping by vegetation surface.

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