The effect of successive storm events and seagrass coverage on sediment suspension in a coastal lagoon

The effect of a succession of eleven storm events (hourly averaged wind speeds exceeding 10.8 m s−1) on the sediment suspension was investigated in a coastal lagoon through in situ measurements of hydro- and sediment dynamics from a mobile jack-up platform. Results showed that wave-driven bed shear stress (0.1-0.7 N m−2) was the main driver for sediment suspension in contrast to large-scale flushing, which did not trigger sediment suspension. The suspended particulate matter concentration (SPMC) reached a maximum of 200 mg l−1. A meteorologically-driven lagoonal seiche effect was identified, which could be the driver for advective sediment transport in the lagoon. Two major findings for the suspension of sediment can be drawn from the results. First, the energy applied to the bed during successive high-energy storm events caused a reduction in the vegetation cover during a particular strong storm event. This increased the SPMC relative to the bed shear stress, suggesting that the sediment availability increased. Second, successive high-energy storm events decreased the bed shear stress threshold for sediment suspension subsequent to initial consolidation of the bed, suggesting an increase in the bed erodibility. The average bed shear stress threshold for sediment suspension was 0.1 N m−2. Furthermore, successive high-energy storm events increased the SPMC relative to the bed shear stress subsequent to initial consolidation of the bed, before an ultimate decrease. This suggested that the sediment availability subsequent to initial consolidation of the bed increased, but ultimately decreased. The impacts were possibly caused by advective sorting processes during settling of the suspended sediment. The study helps improve numerical modelling of coupled physical and biological environments during successive storm events and thus contributes to advance coastal management of shallow coastal lagoons under a changing climate.