Thermal Stratification Dynamics in Sandpits: Impacts of Marine Sand Extraction in the Southern North Sea

The increasing demand for marine sand, driven by urbanization, infrastructure development, and coastal defense against sea-level rise due to climate change intensifies environmental pressures on marine ecosystems. Large-scale sand extraction disrupts benthic habitats and alters hydrodynamics by modifying water depth and current velocities. These changes weaken natural tidal mixing processes, increasing susceptibility to thermal stratification. Such stratification limits oxygen and nutrient exchange between water layers, affecting local phytoplankton dynamics and benthic communities.

To investigate the potential occurrence of thermal stratification in sand pits, we applied the Simpson-Hunter method, originally developed for predicting tidal mixing fronts, to establish a theoretical framework for determining the critical depth at which well-mixed waters may stratify within sandpits in mid-summer. Using this method, we developed a map for the southern North Sea that identifies the maximum allowable sandpit depths before stratification occurs.

To further refine our findings, we conducted one-way nested, high-resolution numerical modeling of the hydrodynamics using the Delft3D model, incorporating boundary conditions derived from the existing GETM model of the Northwest European Shelf. Simulations were performed for various sandpit sizes and depths under realistic hydrodynamic conditions for mid-summer. The results agreed with the theoretical predictions but in addition revealed a strong dependence on sandpit size, showing that larger pits are more prone to stratification related to a relative reduction in mixing at the pit’s edges.

This research highlights the critical role of sandpit depth and size in influencing stratification dynamics. Understanding and preventing these processes is essential for minimizing ecological risks and ensuring the sustainable extraction of marine sand in dynamic shelf seas like the North Sea.