The amount of suspended fines in the southern North Sea strongly depends on their exchange with the sandy seabed. This exchange is governed by resuspension of fines during storms, followed by burial in the week thereafter. Despite its importance for fine sediment dynamics, the burial of fines into a sandy seabed is currently not well understood. This paper presents a mechanistic conceptual model, explaining how the interaction of migrating small ripples and larger megaripples can bury fine sediment in a sandy seabed shortly after storms. The burial process consists of four phases forming a dynamic cycle. A storm stirs up the bed, remobilising fines, while forming megaripples. After the storm, fines can settle again depositing atop the sandy seabed. Interaction between bedforms of different scales is then crucial to bury fines 1–2 dm within the seabed. Megaripples formed during storms gradually adjust to calmer conditions in the waning of storms. During this adjustment period, fines are buried by current-induced ripples in the troughs of the former megaripples. Field measurements collected in 2017 corroborate this conceptual model, showing fines in distinct patches, both horizontally and vertically. Furthermore, fines are found up to 10–15 cm in the seabed shortly after storms. The data further reveal how fine sediment occurrences on and within the seabed vary strongly over multiple length scales. They both vary on the mega-scale (kilometres) and on the micro-scale (metres-centimetres). As the micro-scale is multiple orders of magnitude smaller than the scale on which hydro-morphological models operate, parameterisations are required to aggregate the effect of burial in numerical models.

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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 (<63 μm) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains (>63 μm). 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.

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The fine sediment distribution in the seabed is an important indicator for the ecological functioning of shallow coastal seas. In this paper, we investigate the processes and conditions that determine the fine sediment distribution in the Dutch coastal zone surficial seabed, while also assessing the response of the system to human interventions. An extensive sediment dataset, collected in the Dutch coastal zone from 2006 to 2014, is presented. These data are used to map the distribution of fines in the seabed of the DCZ at unique spatiotemporal scales. For the entire Dutch coastal zone, the distribution of fines generally agrees well with previous studies. The recent extension of the Port of Rotterdam, the Maasvlakte 2 reclamation, was found to locally change the distribution of fines. In the sand mining pit and directly south of the reclamation, fines percentages in the seabed increased by more than 10%. We developed a conceptual framework to analyse the distribution of fines and how it is affected by human interventions. Three components are distinguished within this framework: (1) sources of fines; (2) transport pathways; and (3) accumulation potential. These components are determined both qualitatively and quantitatively, based on high-resolution bathymetric and hydrodynamic model data. The distinction between the three components makes it possible to unravel the contributions of different human interventions to the changes in the fines distribution. In the case of Maasvlakte 2, the local increase of fines percentage in the seabed could thus be attributed to a temporary additional source of fines and enhanced accumulation potential. The high spatiotemporal resolution of the new sediment dataset proved crucial to enable development and testing of the framework to evaluate the impact of (large) engineering works on the spatial distribution of fines.

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In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The
data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit downestuary
of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state
in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured aswell by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPMdynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion. @en

Shallow coastal seas are subject to an increasing pressure by offshore operations. Further to a direct influence these operations impose on benthic and pelagic organisms, an indirect influence is caused by changes in sediment dynamics and morphodynamics. Temporal variations in SPM have a large effect on the timing and rate of primary production, thereby also affecting higher trophic levels. Field measurements along the Dutch coast indicate significant seasonal variations in concentrations of SPM (Suijlen and Duin, 2001; Witbaard et al., 2015). These seasonal variations originate from a marked seasonality in wind climate and the occurrence of storms. During storms, increases in SPM occur simultaneously in large parts of the Dutch coastal zone of the North Sea (Suijlen & Duin 2001), demonstrating that on short timescales, the vertical exchange between the sea bed and the water column is dominant. Model concepts with two discrete seabed layers (a fluffy top layer and a sandy lower layer) turned out to capture these fine sediment dynamics, see van Kessel et al. (2011). However, the underlying physical processes resulting in the water-bed exchange of fines are still to be unravelled. Therefore, this study aims to investigate the resuspension of fines from the bed during and after storms, accounting for the tidal variation due to the spring-neap tide cycle. This will lead to a more specific conceptualization and related parameterization of the water-bed exchange, thereby enabling to study both the direct and indirect impact of offshore operations.@en