Decomposing the intra-annual variability of flushing characteristics in a tidal bay along the North Sea

Abstract

Flushing timescales in estuaries and coastal bays largely shape the distribution of dissolved and particulate matter and therefore have important environmental implications. Here we investigate the spatiotemporal variability of turnover times in a semi-enclosed tidal bay and examine the potential underlying causes. The basin considered here is the Dutch coastal bay Oosterschelde (Eastern Scheldt in English) on the east coast of the North Sea. Using a calibrated and validated 3D hydrodynamic model, conservative tracer experiments were conducted to estimate turnover times. Sensitivity scenarios were run to examine the relative roles of tides, winds, and gravitational flow in driving the intra-annual variability. Results indicate that gravitational flow and winds contribute most to the intra-annual variability of turnover times in the Oosterschelde. Freshets from the Westerschelde and Rhine Rivers extending to the mouth of the bay can reduce the axial density gradient and gravitational flow, increasing turnover times of the basin. The prevailing southwesterly winds, when of sufficient magnitude (>5 m/s), modify the spatial patterns of turnover times especially in shallow waters. Tides force flushing of the bay but do not induce significant intra-annual variability. Findings in this study underline the importance of intra-annual variability of turnover times in coastal bays and methodology proposed here facilitates intercomparison studies for other systems.