Suspended particulate matter response to extreme forcings in the Bay of Seine

The latest Intergovernmental Panel on Climate Change report of 2023 alerts about an increase in the occurrence and intensity of extreme hydro-meteorological events such as storms and extreme river flows, i.e. drought and floods. Investigating the occurrence of these extreme events in the past 15 years and their impacts on sediment dynamics will provide crucial knowledge for anticipating future trajectories of coastal ecosystems. Time series from in situ observations are analyzed to identify extreme events of river flows and waves and examine their impact on Suspended Particulate Matter (SPM) dynamics in a highly turbid coastal area equipped with a high frequency in situ monitoring station at the interface between the Seine Estuary and the Bay of Seine (northern coast of France). Extreme river flow and wave orbital velocity events are investigated because high river discharge contributes to deliver large amounts of SPM concentration to the bay and strong wave action within the bay can lead to erosion and resuspension of bottom sediments. An original detection method is proposed, based on high frequency in situ observations combined with satellite and model data from 2006 to 2022. Extreme forcings are examined through their specific characteristics (high intensity, long duration, season of occurrence, succession of events), their impact on SPM concentration in the coastal environment and the comparison to mean seasonal dynamics. A positive relationship exists between SPM concentration and high SPM spatial extent and forcing intensity. Extremes are more intense in winter for both forcings and generate larger SPM concentration anomalies. However, extreme events during late spring/summer, periods or mean low forcing intensity, are demonstrated to generate SPM concentration anomalies up to 4 times larger than the monthly mean value, hence possibly strongly impacting the system during these atypical periods. This is particularly important as analyzing the distribution of extreme river flow events over the last 60 years indicated an increase in their occurrence and more important the progressive occurrence of high intensity extreme events during spring/summer periods.