Characterization of bottom sediment resuspension events observed in a micro-tidal bay

Abstract

Abstract. In this study we investigate the variability in near-bottom turbidity in Alfacs Bay (in the northwestern Mediterranean Sea). The bay is characterized by a micro-tidal environment and seiching activity that may lead to flow velocities of more than 50 cm s−1. A set of current meters and optical sensors were mounted near the sea bottom to acquire synchronous hydrodynamic and optical information from the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node–antinode location of the fundamental and first resonance periods of the bay. The implementation of a coupled wave–current numerical model shows strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible for the resuspension of fine sediment within the bay. This is confirmed by suspended sediment concentration maps derived from Sentinel-2 satellite imagery. We suggest that the sequence of resuspension events plays an important role in the suspended sediment concentration, meaning that the occurrence of sediment resuspension events may increase the suspended sediment in subsequent events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process over a long-term period.

Short-Term Response of Chlorophyll a Concentration Due to Intense Wind and Freshwater Peak Episodes in Estuaries: The Case of Fangar Bay (Ebro Delta)

“…Recently, Díez-Minguito and de Swart [37] demonstrated that as suspended sediment concentration increased, so did Chl a concentrations at spring and neap tidal scales due to simultaneous resuspension of sediment and Chl a containing biomass induced by tidal shear stress. In this sense, Grifoll et al [41] showed evident relations in time-series observations in Alfacs Bay between strong wind events and an increase in near-bottom turbidity. Turbidity measurements in Fangar Bay (Figure 2g) have shown a relationship between the increase in near-bottom turbidity and intense wind episodes making plausible the incorporation of Chl a containing biomass in the water column during the wind episodes.…”

“…The larger impact of NW and E-NE wind episodes (in comparison of sea-breeze) on Chl a concentration was due to the characteristics of the bay, which is very shallow and microtidal. In this sense, winds > 10 m•s −1 (above typical sea-breezes intensities) immediately cause mixing in the water column and likely resuspension events [4,9,41]. Also, the duration of these strong winds associated with E-NE and NW events was a few days, compared to the strong winds that may occur during sea breeze periods, which are a few hours in length.…”

“…Also, satellite data during the FANGAR-III campaign tended to overestimate in situ data, likely due to the satellite sensor capturing more suspended matter during autumn in comparison during summer. The Chl a concentration increasing was presumably linked to the increase in turbidity within the bay during the autumn due to the intense wind episodes (mainly NW wind episodes) increasing resuspension events [41]. Fangar Bay, as an example of a coastal and very shallow estuary, shows the limitations of the remote sensor to distinguish between Chl a concentration and suspended matter; in addition, the increased reflectance might also be related to bottom reflectance [11].…”

“…autumn due to the intense wind episodes (mainly NW wind episodes) increasing resuspension events [41]. Fangar Bay, as an example of a coastal and very shallow estuary, shows the limitations of the remote sensor to distinguish between Chl a concentration and suspended matter; in addition, the increased reflectance might also be related to bottom reflectance [11].…”

Wind-Driven Hydrodynamics in the Shallow, Micro-Tidal Estuary at the Fangar Bay (Ebro Delta, NW Mediterranean Sea)

“…Wong et al [4] showed how the impact of the wind-induced coastal sea level at the entrance of an inlet exerts a huge impact on the subtidal variability within the interior of the bay. The influence of wind can also have effects on salinity structure or resuspension events in shallow estuaries [5][6][7][8]. These studies indicate that spatial scales and water depth determine the flow response to wind forcing.…”