Fine Sediment Dynamics in the Southern North Sea

Abstract

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.