On the short- and long-term SPM variations in the Schelde estuary

Wouter Vandenbruwaene, Koen Wildemeersch, Yves Plancke, Joris Vanlede

Monday 29 june 2015

14:41 - 14:53h at South America (level 0)

Themes: (T) Special session, (ST) Scheldt Estuary physics and integrated management

Parallel session: 2J. Special session: Scheldt Estuary physics and integrated management

Recently there has been a growing interest in the question whether or not the Scheldt estuary may evolve towards a hyper-turbid system. Typical for such a regime shift is an increase in suspended particle matter (SPM) over a time scale of several decades. In this research we studied the historical evolution of SPM in the Scheldt estuary by the analysis of data over a period of 18 years (1995-2013). We looked at the effect of the tide, riverine discharge, seasons and sediment disposal (short-term) in order to understand the effect of these physical parameters on the SPM signal and its historical evolution (long-term). We quantified variations in SPM signal in function of the tide (spring-neap and ebb-flood), and found in the Sea Scheldt (i.e. the part of the Scheldt 58-160 km from the mouth) higher suspended sediment concentrations and corresponding sediment fluxes during ebb, suggesting an ebb-dominated sediment transport. Furthermore, we found that riverine discharge plays an important role in the formation of the estuarine turbidity maximum (ETM). If the riverine discharge is low (typical during summer and autumn), a distinct zone of elevated suspended sediment concentrations forms at a distance 100-140 km from the mouth. Although we could not distinguish a significant increase or decrease in SPM in the Sea Scheldt over the period 1995-2013, periods with higher and lower suspended sediment concentrations alternate with each other in time. Higher SPM values are hereby associated with periods of lower riverine discharge and vice versa. Riverine discharge thus not only affects the SPM signal on the daily or seasonal scale, but also has an effect in the longer term of years. Our study demonstrates the strong correlation between the physical parameters and the SPM signal, and emphasis that long-term trend analysis of SPM should always be interpreted with care, and in function of the historical changes of the forcing physical parameters.