Monitoring sediment transport patterns on an energetic ebb-tidal delta
using dual-signature tracers
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.