Mud disposal and suspended sediment concentration in the Lower Sea Scheldt – towards a hyperturbid system?

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Davy Depreiter, Gijsbert van Holland, Thijs Lanckriet, Kirsten Beirinckx, Joris Vanlede, Tom Maris

Monday 29 june 2015

14:17 - 14:29h 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

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In this paper, information from different projects (OMES, MONEOS, Flexible Disposal and Marine-Fluvial mud ratio) will be combined to increase the insight in mud disposal and the increasing sediment concentrations (SSC) in the Lower Sea Scheldt. In the Scheldt Estuary, major projects have been carried out in the past decade, among which the third deepening of the navigation channel and the opening of the Deurganck dock. Maintenance dredging is carried out to guarantee a minimum navigation depth. A rising trend in the volume of mud dredged in the Lower Sea Scheldt is observed since 2006, the year after the opening of the Deurganck Dock. The trend is explained by increasing mud volumes dredged in this dock and on a nearby shoal. This volume culminated in 2011 (4.8 million m³) when the depth of this dock was increased to its design depth. The dredged mud is disposed upstream, quickly to be resuspended. Near the mud disposal location, yearly average SSC tripled between 2005 and 2011 (108 to 348 mg/L), and SSC peaks increased even stronger. A multivariate regression model indicated a strong correlation between mud disposal volumes and observed SSC. The SSC increase raises an alert with regard to the risk for a regime shift towards a hyperturbid system. Increasing SSC may indeed decrease the hydraulic resistance initiating a feedback mechanism that results in further increasing SSC values. It thus appears that more mud is being circulated: the Deurganck dock acts as mud sink, from which the mud is - after dredging and disposal - resuspended. The mud may have different sources: fluvial or marine influx, or resuspended from within the system (less mud stored in e.g. mudflats).