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Flexible Disposal Strategy: monitoring as a key to understanding and steering environmental responses to dredging and disposal in the Scheldt Estuary.

D. Depreiter, M. Sas, K. Beirinckx, G.A. Liek

The Scheldt Estuary is an intensely shipped area, holding the fairway to the port of Antwerp (Belgium).  This is one of the 20 largest ports worldwide although it’s located 80 km from the mouth estuary’s mouth. Since the 3rd deepening of the fairway (2010-2011), the shoals and sills along the main shipping channel are kept to a depth of 14.5 m below LAT allowing for tide independent access toAntwerp for ocean ships up to 13.1 m draft.

The sediment volume involved in the dredging activities amounts to 12.6 Mm³ and 10.1 Mm³ for the first two years of the deepening project (including maintenance). This volume is expected to further decrease due to the end of deepening activities in 2011 and the expected system’s re-equilibration.

The Flexible Disposal Strategy, aimed at creating ecologically valuable area, was set up in this framework. A first approach in the strategy is the disposal of dredged sediments on the edges of tidal flats to create low-dynamic, shallow areas, beneficial for fauna development. A second approach is to dispose sediments in the deep parts of the fairway in the main shipping channel. This contrasts to the former disposal strategy that consisted of dumping into secondary channels.

The strategy is flexible through the use of monitoring data which is continuously acquired by the Department of Maritime Access and Rijkswaterstaat Zeeland (single- and multibeam bathymetry, RKT elevation measurements of the tidal flats, ADCP flow velocity measurements, dredging and disposal data, …) for assessment  of environmental (including morphodynamical) consequences. The assessment is then used to adapt the disposal strategy to reach the desired ecological goals and mitigate possible undesired effects. This process of monitoring is captured in monthly data reports and bi-yearly detailed analysis of which the results are tested to well-defined criteria.

This paper describes the process of monitoring, analysis and reporting as a decision-taking instrument in the approach of Flexible Disposal in the Scheldt Estuary.