H.C.M. Hendriks | TU Delft Repository

Fine Sediment Dynamics in the Southern North Sea

Doctoral thesis (2026) - H.C.M. Hendriks, B.C. van Prooijen, S.G.J. Aarninkhof

The North Sea is one of the world’s most intensively used coastal seas, supporting activities such as sand mining, fishing, offshore energy development, dredging, and shipping. These uses provide major economic and societal benefits. At the same time, the sea also functions as an essential habitat for diverse marine species. Understanding the effects of human activities requires insight into key physical processes, particularly the transport and exchange of fine sediment, as this affects turbidity, light availability, nutrient dynamics, and habitat conditions. Still, the mechanisms governing fine sediment exchange between the sandy seabed and the water column were insufficiently understood. This research addresses that gap by examining the spatial and temporal dynamics of fines, focusing on burial and remobilisation processes.
The central research question is: How does seabed–water column exchange of fine sediment influence the spatial and temporal distribution of fines in the southern North Sea? To answer this, we analyse extensive field data, including a large-scale sediment dataset, long-term in-situ measurements, conceptual and mechanistic models, and targeted field campaign observations. The North Sea’s comprehensive monitoring record provides a strong foundation for this investigation.
The distribution of fines in the seabed is layered across different spatial and temporal scales. At the macro scale (tens of kilometres), fines accumulate primarily in a ~20 km coastal band along the Dutch coast. At the meso scale (hundreds of metres to kilometres), higher fine fractions appear near the Rhine outflow, dredged disposal sites, and former estuarine channels, with concentrations up to 20%. At the micro scale (centimetres to metres), fines exhibit a patchy distribution due to burial processes that create vertical and horizontal heterogeneity. Human interventions such as sand mining and land reclamation influence meso-scale patterns by altering sources and accumulation potential, while macro-scale patterns shift only when sources or transport pathways change.
Short-term burial of fines occurs after storms on timescales of days to a week. This process is driven by interactions between storm-induced megaripples and current-induced ripples: as megaripples decay under calmer conditions, smaller ripples migrate into their troughs, burying fines to depths of 10–15 cm. A four-phase mechanistic model describes this dynamic, supported by sediment cores and seabed imagery. Burial efficiency depends on the balance between storm intensity and tidal current strength.
Remobilisation occurs when new megaripples form during storms, rapidly increasing suspended sediment concentration (SSC). SSC typically peaks one to two days after maximum wave height and returns to background levels within one to six days. A bedform release model was developed to quantify this process, showing good agreement with a 21 month in-situ dataset and clarifying the physical basis of erosion formulations used in numerical models.
Overall, fine sediment dynamics in the North Sea are governed by a tightly oupled seabed–water column system with strong memory effects. Understanding small-scale processes is essential for predicting fines exchange. The models developed in this research support improved monitoring strategies and informed, science-based management. This knowledge is crucial as the North Sea undergoes major energy, food, and nature transitions.
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The North Sea is one of the world’s most intensively used coastal seas, supporting activities such as sand mining, fishing, offshore energy development, dredging, and shipping. These uses provide major economic and societal benefits. At the same time, the sea also functions as an essential habitat for diverse marine species. Understanding the effects of human activities requires insight into key physical processes, particularly the transport and exchange of fine sediment, as this affects turbidity, light availability, nutrient dynamics, and habitat conditions. Still, the mechanisms governing fine sediment exchange between the sandy seabed and the water column were insufficiently understood. This research addresses that gap by examining the spatial and temporal dynamics of fines, focusing on burial and remobilisation processes.
The central research question is: How does seabed–water column exchange of fine sediment influence the spatial and temporal distribution of fines in the southern North Sea? To answer this, we analyse extensive field data, including a large-scale sediment dataset, long-term in-situ measurements, conceptual and mechanistic models, and targeted field campaign observations. The North Sea’s comprehensive monitoring record provides a strong foundation for this investigation.
The distribution of fines in the seabed is layered across different spatial and temporal scales. At the macro scale (tens of kilometres), fines accumulate primarily in a ~20 km coastal band along the Dutch coast. At the meso scale (hundreds of metres to kilometres), higher fine fractions appear near the Rhine outflow, dredged disposal sites, and former estuarine channels, with concentrations up to 20%. At the micro scale (centimetres to metres), fines exhibit a patchy distribution due to burial processes that create vertical and horizontal heterogeneity. Human interventions such as sand mining and land reclamation influence meso-scale patterns by altering sources and accumulation potential, while macro-scale patterns shift only when sources or transport pathways change.
Short-term burial of fines occurs after storms on timescales of days to a week. This process is driven by interactions between storm-induced megaripples and current-induced ripples: as megaripples decay under calmer conditions, smaller ripples migrate into their troughs, burying fines to depths of 10–15 cm. A four-phase mechanistic model describes this dynamic, supported by sediment cores and seabed imagery. Burial efficiency depends on the balance between storm intensity and tidal current strength.
Remobilisation occurs when new megaripples form during storms, rapidly increasing suspended sediment concentration (SSC). SSC typically peaks one to two days after maximum wave height and returns to background levels within one to six days. A bedform release model was developed to quantify this process, showing good agreement with a 21 month in-situ dataset and clarifying the physical basis of erosion formulations used in numerical models.
Overall, fine sediment dynamics in the North Sea are governed by a tightly oupled seabed–water column system with strong memory effects. Understanding small-scale processes is essential for predicting fines exchange. The models developed in this research support improved monitoring strategies and informed, science-based management. This knowledge is crucial as the North Sea undergoes major energy, food, and nature transitions.

An explanatory model for the burial of fines in the sandy seabed of the southern North Sea

Journal article (2022) - E. Hendriks, B. C. van Prooijen, C. H. Cheng, S. G.J. Aarninkhof, J. C. Winterwerp, K. E. Soetaert

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. ...

Characterizing the Composition of Sand and Mud Suspensions in Coastal and Estuarine Environments Using Combined Optical and Acoustic Measurements

Journal article (2021) - Stuart G. Pearson, Romaric Verney, Bram C. van Prooijen, Duc Tran, Erik C. M. Hendriks, Matthias Jacquet, Zheng Bing Wang

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. ...

How human activities affect the fine sediment distribution in the Dutch Coastal Zone seabed

Journal article (2020) - H. C.M. Hendriks, B. C. van Prooijen, S. G.J. Aarninkhof, J. C. Winterwerp

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. ...

Feedbacks between Fine-Grained Sediment Deposits and Bedforms in a Predominantly Sandy Seabed

Field Observations from the Southern North Sea

Poster (2019) - Erik Hendriks, Bram van Prooijen, Stefan Aarninkhof, Karline Soetaert, Han Winterwerp

SPM response to tide and river flow in the hyper-turbid Ems River

Journal article (2017) - Han Winterwerp, J Vroom, Zhengbing Wang, Martin Krebs, Erik Hendriks, Bas van Maren, Kerstin Schrottke, Christine Borgsmüller, Andreas Schöl

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. ...

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.

Response of SPM concentrations to storms in the North Sea

Investigating the water-bed exchange of fine sediments

Abstract (2017) - Erik Hendriks, Bram van Prooijen, Han Winterwerp, Stefan Aarninkhof, CM van der Hout, Rob Witbaard

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. ...