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

Manel Grifoll,Pablo Cerralbo,Manuel Espino,Lars Boye Hansen,Agustín Sánchez-Arcilla,Jorge Guillén

doi:10.5194/os-15-307-2019

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.

Highlights

Suspended sediment in the water column and subsequent deposition play a critical role in coastal ecosystems and the management of coastal environments
The observational set and the wave–current numerical results obtained for Alfacs Bay allowed for a thorough investigation of the resuspension mechanisms of fine sediment
The results indicate evidence of a clear mechanism of resuspension induced by eventual seiche events, which according to the bottom stress patterns may have a relevant spatial variability within the bay consistent with the node–antinode position

Summary

Introduction

Suspended sediment in the water column and subsequent deposition play a critical role in coastal ecosystems and the management of coastal environments. High levels of suspended sediment in the water column have substantial implications for aquatic ecosystems and natural habitats (Ellis et al, 2002), over longer exposure periods (Newcombe and Macdonald, 1991). Sediment supplied from rivers potentially transports significant amounts of organic matter, pollutants, and heavy metals that may be deposited on the seabed or even transported offshore (Palanques et al, 2017). The analysis and prevention of fine sediment within basins and channels plays an important role in port engineering and is used to examine and monitor siltation processes The growth of harmful species, such as dinoflagellate cysts, may be related to significant local resuspension via the mixing of the upper layers, resulting in more homogenous cyst profiles in the sediment (Giannakourou et al, 2005)

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