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# OBSERVATIONS OF SUSPENDED PARTICLE SIZE DISTRIBUTION ON AN ENERGETIC EBB-TIDAL DELTA

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Sustainable management of barrier islands and tidal inlet systems requires a knowledge of sediment transport pathways throughout the system. This paper places in situ suspended sediment observations (obtained using a LISST) in context with seabed sediment samples and hydrodynamic measurements to identify such pathways. The results indicate two distinct populations of sediment in suspension on the ebb-tidal delta: locally resuspended fine sand and (largely flocculated) mud exported from the Wadden Sea on ebb tide. This reinforces the notion of the strong dependence of sediment pathways on particle size. Future work will combine additional lines of evidence to better distinguish suspended sand from sand-sized flocs and provide a more robust definition of these pathways.
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OBSERVATIONS OF SUSPENDED PARTICLE SIZE
DISTRIBUTION ON AN ENERGETIC EBB-TIDAL DELTA
S.G. PEARSON1,2, B.C. VAN PROOIJEN1, F.P. DE WIT1, H. MEIJER-
HOLZHAUER3,2, A.P. DE LOOFF4, Z.B. WANG2,1
1. Department of Hydraulic Engineering, Delft University of Technology, Delft, the
Netherlands. s.g.pearson@tudelft.nl, b.c.vanprooijen@tudelft.nl,
f.p.dewit@tudelft.nl
2. Deltares, Delft, the Netherlands. zheng.wang@deltares.nl
3. University of Twente, Enschede, the Netherlands. h.holzhauer@utwente.nl
4. Rijkswaterstaat, Ministry of Infrastructure and Water Management, the
Netherlands. harry.de.looff@rws.nl
Abstract: Sustainable management of barrier islands and tidal inlet systems
requires a knowledge of sediment transport pathways throughout the system. This
paper places in situ suspended sediment observations (obtained using a LISST) in
context with seabed sediment samples and hydrodynamic measurements to
identify such pathways. The results indicate two distinct populations of sediment
in suspension on the ebb-tidal delta: locally resuspended fine sand and (largely
flocculated) mud exported from the Wadden Sea on ebb tide. This reinforces the
notion of the strong dependence of sediment pathways on particle size. Future
work will combine additional lines of evidence to better distinguish suspended
sand from sand-sized flocs and provide a more robust definition of these
pathways.
Introduction
Sustainable management strategies for barrier island coasts and tidal inlets
require robust predictions of their morphological evolution. These systems play
a key role in flood safety, navigation, fisheries, and form a living environment
for numerous mammals, birds, fish, and benthic species. To predict the response
of such systems to human interventions (e.g. dredging or nourishments) or sea
level rise and other climate change effects, it is necessary to quantify the
pathways that sediment takes as it moves through the system. Sediment
transport pathways at tidal inlets are governed by complex interactions between
tides, waves, wind, and density-driven forcing, and may vary significantly as a
function of particle size.
The current coastal safety policy of the Netherlands hinges around maintaining
sufficient sediment supplies in the coastal zone. Understanding and predicting
the long-term infilling trends of the Wadden Sea and the consequent source or
sink from the adjacent coastline is thus of critical importance (Wang et al.,
2018). Furthermore, it is necessary to quantify this net import and export
behaviour of sediment as a function of grain size.
2
This paper links in situ observations of suspended sediment particle size to the
composition of the seabed and concurrent hydrodynamic conditions in order to
estimate sediment sources, pathways, and receptors across the Ameland Inlet
system.
Methodology
From August to October 2017, an extensive field measurement campaign was
carried out at Ameland Inlet in the Dutch Wadden Sea (Figure 1).
Hydrodynamics, suspended sediment, and water quality were measured at 11
stations across the inlet, ebb-tidal delta, and tidal watersheds of the basin. This
paper focuses on the measurements obtained by a frame located on the distal
end of the ebb-tidal delta (Frame FED). Suspended particle measurements were
contextualized with in situ measurements of hydrodynamic conditions and
seabed sediment.
Fig. 1. Site overview of Ameland Inlet, the Netherlands. The inlet sits between the islands of
Ameland and Terschelling, and connects the North Sea with the shallow Wadden Sea. The yellow
triangle indicates the location of Frame FED on the western part of the ebb-tidal delta (8 m depth).
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Hydrodynamic Analysis
Near-bed current velocities, water level, and wave heights during the monitoring
period were measured using a downward-facing Nortek Aquadopp HR, a high-
resolution Acoustic Doppler Current Profiler (ADCP). It was mounted 0.5 m
from the base of the frame, although actual height above the seabed varied due
to field conditions. The ADCP sampled at a rate of 4 Hz in 30 min bursts.
These measurements were first depth-averaged and then averaged over the 30
min burst intervals. Each burst was classified into four tidal stages (flood, high
water slack (HWS), ebb, and low water slack (LWS)) using the velocity
measurements (Figure 2). At Frame FED, the mean flood current is
approximately eastward-directed, and the mean ebb current approximately
westward.
Bed Sediment Analysis
In addition, 165 box cores were obtained from the seabed in order to
characterize the bed sediment composition (Figure 3). To obtain a
sedimentologically representative coverage of the entire ebb-tidal delta, the
locations of these cores were chosen based on a series of 16 benthic habitat
zones, defined by their depth, slope, orientation, and degree of recent
morphological change (Holzhauer et al., in prep.). Subsamples of 8 cm depth
were taken from the surface of the box cores and analyzed using a Malvern
Mastersizer to obtain particle size distributions. This dataset was supplemented
1999; TNO, 2017) to provide additional context and greater spatial coverage
Suspended Particle Analysis
Particle size distributions (PSD) of suspended sediment were obtained using a
Laser In-Situ Scattering and Transmissometry (LISST-100X) instrument
(Sequoia Scientific, 2015) mounted 0.6 m above the seabed on Frame FED.
Differently-sized spherical particles scatter laser light in characteristic patterns
across 32 detector rings, enabling the calculation of volumetric particle
concentration (μL/L) for 32 unique particle sizes ranging logarithmically from
2.5 to 500 μm. Bulk particle size statistics (i.e. d50 and sorting) were calculated
using the Logarithmic Folk and Ward graphical measures (Blott & Pye, 2001).
The quality of the LISST measurements is highly dependent on the strength of
the transmitted laser beam through the water column. If the optical transmission
dropped beneath 10% (indicating extremely turbid water) or exceeded 99.5%
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(indicating extremely clear water), then the measurements were removed from
consideration, as per Sequoia Scientific (2015). This amounted to 3% of the
time series at FED. The LISST sampled at 1 Hz for 15 seconds every minute;
these samples were also averaged over the same 30 min intervals as the ADCP.
Results
Hydrodynamic Forcing
The tide at Ameland Inlet is semidiurnal with a spring tidal range of
approximately 2.3 m at Frame FED (Figure 2a). During the monitoring period,
three storms were observed; two on August 31st and September 7th with
significant wave height Hm0 >1.5 m, and the much larger Storm Sebastian on
September 14th with Hm0 of approximately 5 m (Figure 2b). The storms also had
a significant influence on the water level and flow velocities. For instance, a
significantly longer flood period is found during Sebastian. Bed shear stress due
to the combined influence of waves and currents was calculated using the
method of Soulsby (1997) to give an indication of the potential for local bed
material to be resuspended at Frame FED (Figure 2c). The critical motion
threshold for the fine sand composing much of the local sediment is exceeded
during all three storms and at spring tide, which suggests that the seabed of the
ebb-tidal delta is highly mobile.
Bed Sediment Characteristics
The bed of the ebb-tidal delta primarily consists of well-sorted fine sand (mean
d50 = 211 μm, standard deviation d50 = 30 μm, n=165), while the deeper parts of
the inlet channel bed consist of medium sand (mean d50 = 289 μm) and shell
lags (Figure 3a). Mud content (< 63 μm) of the ebb-tidal delta areas is typically
<1% by volume, although a slightly muddier patch exists at its northeastern
edge. Conversely, the mud content is up to 20% in the bed at the landward edge
of the Wadden Sea and along the tidal watersheds separating Ameland Inlet
from its neighbouring basins.
5
Fig. 2. In situ measurements of 30 min burst-averaged (a) water level (η), near-bed velocity (U is
positive eastward and V is positive northward) and (b) significant wave height (Hm0). Vertical
stripes in (a-b) correspond to stages of the tidal cycle. (c) Maximum bed shear stress under waves
and currents is calculated using the method of Soulsby (1997). The dashed black line in indicates
the critical bed shear stress threshold for mobility of fine sand (125 μm) as calculated using Soulsby
(1997). Coloured dots in (a-c) indicate sample times for S1 and S2 in Figure 4d.
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Fig. 3. (a) Median bed sediment grain size (d50). Ebb-tidal delta sediment was obtained from box
cores for this study, and basin/offshore areas were obtained from the Wadden Sea Sediment Atlas
(Rijkswaterstaat, 1999). The yellow triangle indicates measurement Frame FED. (b) Particle size
distribution in the bed at key locations.
Suspended Sediment Characteristics
The total volumetric suspended particle concentration measured by the LISST
varies by several orders of magnitude during the measurement period, from a
base level of approximately 50 μL/L during calmer periods to approximately
1700 μL/L following Storm Sebastian (Figure 4). During periods with wave
heights < 1 m, a clear semidiurnal tidal signature is visible in the concentrations.
Under calm conditions at LWS, total concentrations can exceed 1000 μL/L.
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The size of suspended sediment particles varies with ebb and flood currents,
spring-neap cycles, and the impact of storms. The median suspended particle
size (d50) typically increases towards the end of flood, suggesting a dominance
of sand-sized particles. The d50 then decreases during ebb, reaching a minimum
at LWS, suggesting a greater contribution by mud-sized particles. Particles are
generally best-sorted at the end of flood under calm conditions, whereas they
tend to be poorly sorted after ebb or during storms.
Bed sediment closest to Frame FED is mainly composed of fine sand (d50 = 186
μm), and a sediment tracer study carried out on the site confirmed the transport
of such sand particles in suspension across the ebb tidal delta (Pearson et al.,
2018). The high concentrations of sand-sized suspended particles (63-500 μm)
observed by the LISST would seem to reflect this; however, many of these
particles appear at times when the bed shear stress is insufficient to suspend
sand particles (Figure 4a), or beyond expected settling timescales for sand.
However, such particle size distributions could be explained by the additional
presence of flocculated mud and organic particles advected from a remote
location, rather than solely locally-resuspended sand.
Suspended sand tends to be lognormally distributed and unimodal (Sengupta
(1979), whereas flocculated fine sediment and sand/silt/clay mixtures are often
characterized by multimodal PSDs (Lee et al, 2012). Suspended sediment in the
inlet and on the ebb-tidal delta are usually multimodal (89% of 30 min sample
bursts, n = 1035), with peaks suggesting a combination of fine and medium
sand, silt and clay particles (e.g. Figure 4d). Changes in the median particle size
and sorting reflect the hydrodynamic forcing, but the multimodal nature of the
PSDs mean that these statistics alone are insufficient to describe sediment
dynamics on Ameland ebb-tidal delta.
Flocs can be distinguished by examining concurrent hydrodynamic
measurements. High-concentration bursts of sediment (e.g. S1 in Figure 4d)
frequently coincide with calm conditions at LWS. This suggests that the fine
sediment has been ejected from the Wadden Sea during ebb tide past Frame FED
(e.g. Figure 5). Conversely, PSDs corresponding to flood tide and under high
waves are more likely to contain higher proportions of sand (e.g. S2 in Figure
4d). Thus, there are two distinct populations of sediment in suspension on
Ameland ebb-tidal delta: locally-resuspended sand, and mud originating from
within the Wadden Sea. Both may be present simultaneously, but the
dominance of a particular type depends on the hydrodynamic conditions.
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Fig. 4. In situ measurements of (a) suspended particle size distribution and concentration measured
using LISST at Frame FED. (b) Median particle size (d50) and (c) sorting coefficient (standard
deviation) from 0.5 (well-sorted) to 2.0 (poor) using the Logarithmic Folk and Ward graphical
measures (Blott & Pye, 2001). Vertical stripes in (b-c) correspond to stages of tidal cycle, see
Figure 3 for legend. (d) Example particle size distributions at key moments. Coloured dots in (a-c)
indicate sample times for S1 and S2 in (d)
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Fig. 5. Satellite image of Ameland Inlet on October 15th, 2017 (outside measurement period).
Taken towards the end of low water slack (10:40am), a highly turbid plume of suspended matter is
ejected from the Wadden Sea, across the ebb-tidal delta and several km into the North Sea. Fronts
are visible as white lines of foam and zones with sharp colour contrast. Sentinel-2 image courtesy of
Discussion
There are two distinct populations of sediment in suspension on Ameland ebb-
tidal delta, and their presence depends on the hydrodynamic conditions. Locally
resuspended sand at flood tide reflects the predominantly fine sand of the ebb-
tidal delta, whereas the presence of flocculated mud at ebb and LWS reflects the
Wadden Sea’s much higher mud content. This reinforces the notion of different
pathways and connectivity as a function of grain size. This study uses a unique
set of field observations of suspended sediment transport on an energetic ebb-
tidal delta in a mixed sediment environment. These findings demonstrate the
challenge of measuring and interpreting suspended sediment mixed sand/mud
environments.
Although we can derive detailed PSDs from the LISST data, the simultaneous
presence of both sand and sand-sized flocs makes it impossible to confidently
describe sand and mud transport using the LISST alone. Furthermore, a
spherical particle inversion method was used to interpret the LISST results, and
the anisotropy of flocs or other suspended organic matter may influence the
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measured PSDs. Furthermore, the LISST’s accuracy may be affected by
variations in particle composition (e.g. solid grains of sand vs. flocs),
The measurements used in this study are limited in their scope, both temporally
and spatially. The 21 day period captured here encompasses a full spring-neap
tidal cycle with a mix of calm and stormy conditions (including the largest
storm of 2017), but do not capture seasonal variations which may affect
suspended organic matter. Furthermore, Frame FED measured a single point in a
highly dynamic area, and as such may not be completely representative of the
entire ebb-tidal delta. In addition, the results from this study can be used to
calibrate and validate a multi-fraction sediment transport model and extend the
analysis over larger spatial extents and periods of time.
Outlook
To increase confidence in the classification of suspended sediment, additional
support is required. Ambiguity in the composition of sand-sized particles may
be resolved by analyzing the differential response of acoustic and optical signals
from ADCP and OBS measurements on the ebb-tidal delta as per Fugate &
Friedrichs (2002). Multimodal PSDs can also be broken down into constituent
distributions using Gaussian Mixture Models (e.g. Lee et al, 2012), and
measured chlorophyll levels can be used to estimate the effect of suspended
organic matter on flocculation (e.g. Shen et al., 2018).
Antecedent wind conditions may also be a predictor for high mud
concentrations on the ebb tidal delta, if mud is resuspended from intertidal areas
in the Wadden Sea by wind-driven waves and currents, then discharged on the
ebb tide. For instance, the turbid plume captured in Figure 5 was preceded by 5
days of persistent wind from S/SW directions (KNMI, 2018). Such trends in
local versus remote sources of suspended sediment may also be revealed by
examining hysteresis behaviour of sediment concentrations (e.g. Jalón-Rojas et
al., 2015).
Data collected from other instruments located around Ameland Inlet during the
field campaign should be used in the interpretation of the particle size
distributions to provide greater spatial context for the behaviour observed at
Frame FED. The results of a sediment tracer study carried out on the ebb-tidal
delta (Pearson et al., 2018) can also be incorporated to shed light on the
transport of sand particles there.
The last step will be to examine these measurements in the context of a
numerical model. This allows us to expand the scope of the present study from
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limited observations at a single point to larger spatial and temporal scales. The
suspended particle size distribution data obtained here can also be used to
improve the schematization of sediment in multi-fraction numerical models. By
combining additional lines of evidence, we will obtain a more robust description
of sediment pathways in Ameland Inlet.
Conclusions
Suspended particles on Ameland ebb-tidal delta are mainly fine sediment and
flocs during calm conditions, but locally resuspended sand dominates during
more energetic conditions. The western part of the ebb tidal delta functions as a
source, pathway, and receptor for fine sand, but merely as a pathway for mud.
Although there are large quantities of mud in suspension, they do not persist in
the seabed there. The results suggest a variation in sediment connectivity
between the ebb-tidal delta and other sources or receptors in the Ameland
system as a function of grain size and hydrodynamic forcing.
These findings are essential for the development of numerical models with
multiple sediment fractions, for predicting the evolution of nearby sand
nourishments, and for the description of ecological habitats. Future research will
focus on integrating additional measurements into the present analysis,
numerical modelling of sediment transport in Ameland Inlet, and predicting the
potential effects of nourishments and climate change on sediment pathways
there.
Acknowledgements
This work is part of the research programme ‘Collaboration Program Water’
with project number 14489 (SEAWAD), which is (partly) financed by NWO
Domain Applied and Engineering Sciences. Special thanks to the Dutch
Ministry of Infrastructure and Water Management (Rijkswaterstaat and
Rijksrederij) for organizing the field campaign and for their ongoing support as
part of the Kustgenese2.0 project. Thanks also to Rieneke van Noort, Erik
Hendriks, Ad Reniers, Marion Tissier, Alejandra Gijón, Claire Chassagne, and
Romaric Verney for their fruitful discussions.
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Ebb‐tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles. Among other bedforms of various scales, these hydrodynamics create small‐scale bedforms (ripples), which increase the bed roughness and will therefore affect hydrodynamics and sediment transport. In morphodynamic models, sediment transport predictions depend on the roughness height, but the accuracy of these predictors has not been tested for field conditions with strongly mixed (wave‐current dominated) forcing. In this study, small‐scale bedforms were observed in the field with a 3D Profiling Sonar at five locations on the Ameland ebb‐tidal delta, the Netherlands. Hydrodynamic conditions ranged from wave‐dominated to current‐dominated, but were mixed most of the time. Small‐scale ripples were found on all studied parts of the delta, superimposed on megaripples. Even though a large range of hydrodynamic conditions was encountered, the spatio‐temporal variations in small‐scale ripple dimensions were relatively small (height $\eta \approx$ 0.015 m, length $\lambda \approx$ 0.11 m). Also, the ripples were always highly three‐dimensional. These small dimensions are probably caused by the fact that the bed consists of relatively fine sediment. Five bedform height predictors were tested, but they all overestimated the ripple heights, partly because they were not created for small grain sizes. Furthermore, the predictors all have a strong dependence on wave‐ and current‐related velocities, whereas the ripples heights measured here were only related to the near‐bed orbital velocity. Therefore, ripple heights and lengths in wave‐current dominated, fine grained coastal areas (D50 \textless 0.25 mm), may be best estimated by constant values rather than values dependent on the hydrodynamics. In the case of the Ameland ebb‐tidal delta, these values were found to be $\eta$ = 0.015 m and $\lambda$ = 0.11 m.
Thesis
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In an era of rising seas and other challenges posed by climate change, coastal regions like the Netherlands are facing ever graver threats. Strategic sand nourishments could mitigate the threat of coastal erosion and sea level rise on barrier island coasts while limiting ecological impacts. However, insufficient knowledge of sediment transport pathways at tidal inlets and ebb-tidal deltas prevents an informed response in these areas. The main goal of this project was to describe and quantify the pathways that sediment takes on an ebb-tidal delta. To reach this goal, we focused our analyses on Ameland ebb-tidal delta in the Netherlands. Before we could begin to tackle this challenge, we needed to develop new tools and techniques for analyzing a combination of field measurements and numerical models. These include a method for analyzing the stratigraphy and mapping the morphodynamic evolution of ebb-tidal deltas, a new metric for characterizing suspended sediment composition, and innovative use of sediment tracers. We also established a quantitative approach for looking at and thinking about sediment pathways via the sediment connectivity framework, and developed a Lagrangian model to visualize and predict these pathways efficiently. The techniques developed here are useful in a wider range of coastal settings beyond Ameland, and are already being applied in practice. We foresee that the main impacts of this project will be to improve nourishment strategies, numerical modelling, and field data analysis. This dissertation also points forward to numerous opportunities for further investigation, including the continued development of the connectivity framework and SedTRAILS. By managing our coastal sediment more effectively, we will set the stage for a more sustainable future, in spite of the challenges that lie ahead.
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Multiple tidal inlet systems like the Wadden Sea have long been considered as separated basins, bordered by so-called tidal divides. Recently, it was however shown that fluxes of water and sediment occur over the borders of these basins, especially during wind events. In this paper, the wind-driven fluxes over these borders and the residual flow of water through the main inlet are studied. The study is based on flow measurements at the tidal divides and in the main inlet of the Ameland Inlet system in the Dutch Wadden Sea and on numerical modelling. The measurements were carried out during 40 days in the fall of 2017, including both calm conditions and storm events. Numerical simulations of a full year have been used for upscaling results from the measurements to system scale exchange flows, and to unravel the effects of several mechanisms. The wind-driven variability in exchange flows between back-barrier basins at tidal divides was measured in the field and reproduced by the numerical model. Water level set up increases the water depth and thus the conveyance capacity at tidal divides, such that the exchange flows increase in magnitude. The flow conditions due to wind forcing are similar for both tidal divides of the Ameland Basin. The conveyance capacity and therefore the total volume exchange are however different. This leads to a residual compensation flow through the main inlet, which is directed outward (i.e., in the ebb direction) during winds from the prevailing southwestern wind direction. The net discharge through the main inlet is therefore a consequence of the residual flows over the tidal divides.
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Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to mud particles ( < 63μm) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains ( > 63μm). It is thus challenging to estimate the relative proportion of sand or mud in environments where both types of sediment are present. The suspended sediment concentration measured by these devices depends on the composition of that sediment, thus it is also difficult to confidently measure concentration with a single instrument when the composition varies and extensive calibration is not possible. The objective of this paper is to develop a methodology for characterizing the relative proportions of sand and mud in mixed sediment suspensions by comparing the response of simultaneous optical and acoustic measurements. We derive a sediment composition index (SCI) that is used to directly predict the relative fraction of sand in suspension. Here we verify the theoretical response of these optical and acoustic instruments in laboratory experiments, and successfully apply this approach to field measurements from Ameland ebb-tidal delta (the Netherlands). Increasing sand content decreases SCI, which was verified in laboratory experiments. A reduction in SCI appears during more energetic conditions when sand resuspension is expected. Conversely, the SCI increases in calmer conditions when sand settles out, leaving behind mud. This approach provides crucial knowledge of suspended sediment composition in mixed sediment environments.
Preprint
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Abstract. A large-scale ﬁeld campaign has been carried out on the ebb-tidal delta (ETD) of Ameland Inlet, a basin of the Wadden Sea in the Netherlands, as well as on three transects along the Dutch lower shoreface. With this campaign, as part of KustGenese2.0 (Coastal Genesis 2.0) and SEAWAD, we aimed to gain new knowledge on the processes driving sediment transport and benthic species distribution in such a dynamic environment. These new insights will ultimately help the development of optimal strategies to nourish the Dutch coastal zone in order to prevent coastal erosion and keep up with sea level rise. The dataset obtained from the ﬁeld campaign consists of: (i) bathymetry data from single beam and multibeam measurements; (ii) ﬂow, waves, sediment concentration, conductivity and temperature, and bedforms at 10 locations on the delta; 7 stand-alone pressure sensors deployed on the ebb-tidal shoal; and 6 ADCPs on the watersheds; (iii) bed composition and macro benthic species from 166 (in 2017), 53 (in 2018) boxcores, 21 vibrocores; (iv) discharge measurements through the inlet; (v) X-band radar; (vi) meterological data. The combination of all these measurements at the same time makes this dataset unique and enables us to investigate the interactions between sediment transport, hydrodynamics, morphology and the benthic ecosystem in more detail. The data is publicly available at 4TU Centre for Research Data at https://doi.org/10.4121/collection:seawad (Delft University of Technology et al., 2019).
Poster
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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.
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The Wadden Sea is a unique coastal wetland containing an uninterrupted stretch of tidal flats that span a distance of nearly 500km along the North Sea coast from the Netherlands to Denmark. The development of this system is under pressure of climate change and especially the associated acceleration in sea-level rise (SLR). Sustainable management of the system to ensure safety against flooding of the hinterland, to protect the environmental value and to optimise the economic activities in the area requires predictions of the future morphological development. The Dutch Wadden Sea has been accreting by importing sediment from the ebb-tidal deltas and the North Sea coasts of the barrier islands. The average accretion rate since 1926 has been higher than that of the local relative SLR. The large sediment imports are predominantly caused by the damming of the Zuiderzee and Lauwerszee rather than due to response to this rise in sea level. The intertidal flats in all tidal basins increased in height to compensate for SLR. The barrier islands, the ebb-tidal deltas and the tidal basins that comprise tidal channels and flats together form a sediment-sharing system. The residual sediment transport between a tidal basin and its ebb-tidal delta through the tidal inlet is influenced by different processes and mechanisms. In the Dutch Wadden Sea, residual flow, tidal asymmetry and dispersion are dominant. The interaction between tidal channels and tidal flats is governed by both tides and waves. The height of the tidal flats is the result of the balance between sand supply by the tide and resuspension by waves. At present, long-term modelling for evaluating the effects of accelerated SLR mainly relies on aggregated models. These models are used to evaluate the maximum rates of sediment import into the tidal basins in the Dutch Wadden Sea. These maximum rates are compared to the combined scenarios of SLR and extraction-induced subsidence, in order to explore the future state of the Dutch Wadden Sea. For the near future, up to 2030, the effect of accelerated SLR will be limited and hardly noticeable. Over the long term, by the year 2100, the effect depends on the SLR scenarios. According to the low-end scenario, there will be hardly any effect due to SLR until 2100, whereas according to the high-end scenario the effect will be noticeable already in 2050.
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Climate change and human activities impact the volume and timing of freshwater input to estuaries. These modifications in fluvial discharges are expected to influence estuarine suspended sediment dynamics, and in particular the turbidity maximum zone (TMZ). Located in southwest France, the Gironde fluvial-estuarine system has an ideal context to address this issue. It is characterized by a very pronounced TMZ, a decrease in mean annual runoff in the last decade, and it is quite unique in having a long-term and high-frequency monitoring of turbidity. The effect of tide and river flow on turbidity in the fluvial estuary is detailed, focusing on dynamics related to changes in hydrological conditions (river floods, periods of low discharge, interannual changes). Turbidity shows hysteresis loops at different timescales: during river floods and over the transitional period between the installation and expulsion of the TMZ. These hysteresis patterns, that reveal the origin of sediment, locally resuspended or transported from the watershed, may be a tool to evaluate the presence of remained mud. Statistics on turbidity data bound the range of river flow that promotes the upstream migration of TMZ in the fluvial stations. Whereas the duration of the low discharge period mainly determines the TMZ persistence, the freshwater volume during high discharge periods explains the TMZ concentration at the following dry period. The evolution of these two hydrological indicators of TMZ persistence and turbidity level since 1960 confirms the effect of discharge decrease on the intensification of the TMZ in tidal rivers; both provide a tool to evaluate future scenarios.
Technical Report
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Particle size distributions (PSDs) of suspended particulate matters in a coastal zone are lognormal and multimodal in general. The multimodal PSD, which is caused by the mixing of multiple particle and aggregate size groups under flocculation and erosion/resuspension, is a record of the particle and aggregate dynamics in a coastal zone. Curve-fitting software was used to decompose the multimodal PSD into subordinate lognormal PSDs of primary particles, flocculi, microflocs, and macroflocs. The curve-fitting analysis for a time series of multimodal PSDs in the Belgian coastal zone showed the dependency of the multimodality on (1) shear-dependent flocculation in a flood and ebb tide, (2) breakage-resistant flocculation in the spring season, and (3) silt-sized particle erosion and advection in a storm surge. Also, for modeling and simulation purposes, the curve-fitting analysis and the settling flux estimation for the multimodal PSDs showed the possibility of using discrete groups of primary particles, flocculi, microflocs, and macroflocs as an approximation of a continuous multimodal PSD.
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The Floc Size Distributions (FSDs) of biomineral suspended particles are of great importance to understand the dynamics of bio-mediated Suspended Particulate Matters (SPMs). Field observations were investigated at Station MOW1 in Belgian coastal waters (southern North Sea) during two typical periods with abundant and reduced biomass. In addition, the Shen et al. (2018) [Water Res. Vol 145, pp 473-486] multi-class population balance flocculation model was extended to address the occurrence of suspended microflocs, macroflocs and megaflocs during these contrasting periods. The microflocs are treated as elementary particles that constitute macroflocs or megaflocs. The FSD is represented by the size and mass fraction of each particle group, which corresponds to a temporal and spatial varying mass weighted settling velocity. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. The growth of an aggregate because of bio-activities is allotted to each elementary particle. It is further hypothesized that the growth kinetics of biomineral particles due to biofilm coating follows the logistic equation. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. Within an intra-tide scale, the settling velocity (ws) is large during slack tides while it is small during maximum current velocities because of variations in turbulence intensities. Nonetheless, the ws may be largely underestimated when the biological effect is neglected. For a seasonal pattern, the ws is higher in biomass-rich periods in May than in biomass-poor periods in October. While the mean sizes of megaflocs are close during the two periods, the macroflocs during algae bloom periods are more abundant with a larger mean size. This study enhances our knowledge on the dynamics of SPMs, especially the biophysical influences on the fate and transport of estuarine aggregates.
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Grain-size frequency distributions of suspended loads at different flow velocities and over sand beds of four different grain-size patterns were studied in a laboratory flume. The proportion of bed material which went into suspension increased with decrease of grain-size in each case, but the modes of the suspended loads occurred in the size classes intermediate between the coarsest and the finest. With increase of flow velocity, as also with decrease of the bed's mean grain-size, the total amount of material in suspension markedly increased, mainly due to addition of particles to the medium size classes. The coarsest grains in the bed resisted erosion due to their weight, whereas the finest ones were either not available in sufficient quantities or resisted erosion due to their homogeneity. The finest of the erodible grains which were abundantly available in bed were therefore, lifted up in large quantities. This size sorting took place at or near the bed surface and was closely related to the process of bed form migration. Large accumulation of medium sized particles in suspension at high velocities led to lognormal grain-size distributions when the nature of the bed (source) material was suitable. At lower velocities, or over other types of bed materials, the phi (log)-probability plots of cumulative grain-size distributions of the suspended loads resolved into a number of straight lines. Mixtures of linear segments on phi-probability graphs therefore, need not necessarily indicate different modes of sediment transportation, as is commonly believed, but might reflect the conditions of flow and the nature of the source material.
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Grain size analysis is an essential tool for classifying sedimentary environments. The calculation of statistics for many samples can, however, be a laborious process. A computer program called GRADISTAT has been written for the rapid analysis of grain size statistics from any of the standard measuring techniques, such as sieving and laser granulometry. Mean, mode, sorting, skewness and other statistics are calculated arithmetically and geometrically (in metric units) and logarithmically (in phi units) using moment and Folk and Ward graphical methods. Method comparison has allowed Folk and Ward descriptive terms to be assigned to moments statistics. Results indicate that Folk and Ward measures, expressed in metric units, appear to provide the most robust basis for routine comparisons of compositionally variable sediments. The program runs within the Microsoft Excel spreadsheet package and is extremely versatile, accepting standard and non-standard size data, and producing a range of graphical outputs including frequency and ternary plots. Copyright © 2001 John Wiley & Sons, Ltd.
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Advances in technology have resulted in a new instrument that is designed for in-situ determination of particle size spectra. Such an instrument that can measure undisturbed particle size distributions is much needed for sediment transport studies. The LISST-100 (Laser In-Situ Scattering and Transmissometry) uses the principle of laser diffraction to obtain the size distribution and volume concentration of suspended material in 32 size classes logarithmically spaced between 1.25 and 250 μm. This paper describes a laboratory evaluation of the ability of LISST-100 to determine particle sizes using suspensions of single size, artificial particles. Findings show the instrument is able to determine particle size to within about 10% with increasing error as particle size increases. The instrument determines volume (or mass) concentration using a volume conversion factor Cv. This volume conversion factor is theoretically a constant. In the laboratory evaluation Cv is found to vary by a factor of about three over the particle size range between 5 and 200 μm. Results from field studies in South San Francisco Bay show that values of mass concentration of suspended marine sediments estimated by LISST-100 agree favorably with estimates from optical backscatterance sensors if an appropriate value of Cv, according to mean size, is used and the assumed average particle (aggregate) density is carefully chosen. Analyses of size distribution of suspended materials in South San Francisco Bay over multiple tide cycles suggest the likelihood of different sources of sediment because of different size characteristics during flood and ebb cycles.