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Monitoring sediment transport patterns on an energetic ebb-tidal delta using dual-signature tracers
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  • Stuart Pearson,
  • Bram van Prooijen,
  • Jack Poleykett,
  • Matthew Wright,
  • Kevin Black,
  • Zheng Bing Wang
Stuart Pearson
Delft University of Technology

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Bram van Prooijen
Delft University of Technology
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Jack Poleykett
Partrac Ltd.
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Matthew Wright
Partrac Ltd.
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Kevin Black
Partrac Ltd.
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Zheng Bing Wang
Technische Universiteit Delft, Deltares
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Abstract

The morphodynamic response of the Dutch Wadden Islands to the effects of climate change (e.g. sea level rise) or human interventions (e.g. nourishments) is closely tied to the evolution of the ebb-tidal deltas between them. To understand the fate of these ebb-tidal deltas, we must quantify the behaviour and transport patterns of sediment as it moves across them. In September 2017, 2000 kg of dual signature (fluorescent and ferrimagnetic) sediment tracer was deployed on the seabed at Ameland ebb-tidal delta in the Netherlands. The tracer’s physical characteristics (d50= 285 μm, ρ = 2628 kg/m3) closely matched those of the native sediment to ensure that it was eroded, transported and deposited in a similar manner. The tracer study was complemented by simultaneous measurements of hydrodynamics and suspended sediment at four locations across the ebb-tidal delta. Over the subsequent 41 days, the tracer’s dispersal was monitored via the collection of seabed grab samples and determination of tracer content and particle size within each sample. In addition, high-field magnets mounted on mooring lines 1, 2, and 5 m above the seabed at strategic locations around the deployment site were used to sample tracer particles travelling in suspension. Tracer particles were recovered from over 60 of approximately 200 samples, despite the occurrence of two significant storm events (Hs > 4 m). Although hydrodynamic measurements suggest an eastward tidal residual flow, the spatial pattern of the recovered tracer indicates that transport is highly dispersive, likely due to the storms. Furthermore, the samples recovered from the suspended magnets show an upward fining trend in grain size through the water column. The active sediment tracing approach provides useful insight into sediment transport patterns and sorting processes in energetic coastal environments. The study also demonstrated the potential of dual signature sediment tracers to monitor sand nourishment effectiveness. In particular, the use of magnets proved highly effective at sampling tracer travelling in suspension, enabling both bed load and suspended load transport processes to be investigated. The data obtained through this study will serve as a basis for future numerical model calibration and validation.