Mohammad Daliri1*, Johan van der Molen1
1 Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, the Netherlands
* Corresponding author: mohammad.daliri@nioz.nl
Introduction
Sand is the second most extracted natural resource globally, surpassing fossil fuels and biomass (Torres et al., 2017). Its demand is driven by urbanization, infrastructure development, and coastal defense efforts to combat rising sea levels. However, Large-scale marine sand extraction has significant ecological consequences, including disruptions to hydrodynamic flows, sediment transport, and benthic habitats (De Jong et al., 2015), cascading through the food web and impacting essential ecosystem services. One critical concern is the effect of sand extraction on thermal stratification within the water column, which governs key ecological processes such as water quality, oxygen distribution, and nutrient cycling, directly influencing aquatic ecosystem health. This study focuses on how sandpits in the southern North Sea influence thermal stratification, examining factors such as excavation depth, heat exchange, and current interactions. The study aims to identify the critical depth at which sandpits cause stratification, providing insights for sustainable sand extraction and ecosystem-based management in the Dutch North Sea and similar environments worldwide.
Objective and Methods
To assess the impact of sandpits on temperature structure and the potential for thermal stratification, we used an energy-based model built on the concept of potential energy anomaly introduced by Simpson and Hunter (1974). This model examines the competition between buoyancy forces, driven by solar heating, which promotes stratification, and tidal mixing, which counteracts it. The method, originally designed to predict tidal mixing fronts, defines the boundary between stratified and mixed water regimes. Bowers and Simpson (1987) analyzed 13,000 temperature measurements of the European continental shelf and found that stratification occurs when the temperature difference between the surface and bottom (δT) exceeds 0.5° C. For shelf seas with relatively uniform solar radiation, tidal mixing fronts are defined by the condition log(h/u3M2)=2.7, where uM2 is the depth-averaged M2 tidal velocity and h is water depth. Applying these criteria to sandpit-excavated areas allowed the estimation of critical depths at which previously well-mixed waters may stratify due to temperature differences. To refine predictions, numerical simulations were conducted using the Delft3D model, incorporating boundary conditions derived from the GETM model for the Northwest European Shelf. These simulations provided a more detailed understanding of stratification dynamics in sandpit environments.
Results
The tidal mixing front condition was adapted for sand extraction areas by incorporating seabed deepening and hydrodynamic effects to calculate critical depths. Fig. 1a shows critical sandpit depths in the southern North Sea (summer 2019). Regions beyond the zero-contour critical depth, characterized by high depths and low tidal velocities, align with previously reported stratified areas (Van Leeuwen et al., 2015), while shallower areas with stronger tidal velocities indicate higher allowable excavation depths. Theoretical predictions were tested near the West Frisian Islands (yellow cross, Fig. 1a), where a 5 m deep sandpit calculated for the onset of stratification. Simulations of 16 square sandpits (5–20 km, 5–20 m depth) under mid-summer conditions revealed that stratification is strongly influenced by sandpit size and depth. Fig. 1b shows stratification initiating at δT > 0.5 for a 20 km sandpit with a depth of 5 m. Fig. 1c provides an overview of stratification across all sandpits: 5 km sandpits remain mixed, stratification begins at 10 m depth for 10 km sandpits, and all depths exhibit stratification in 15–20 km sandpits. These findings highlight the influence of sandpit dimensions on stratification and provide guidance for sustainable sand extraction.
Figure 1. (a) Spatial distribution of critical sandpit depths in the southern North Sea. (b) Onset of thermal stratification (δ𝑇>0.5 ) for a 20 km sandpit at 5 m depth. (c) Stratification states across sandpits of varying sizes and depths: mixed (M) and stratified (S) conditions.
References
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de Jong, M. F., Baptist, M. J., Lindeboom, H. J., & Hoekstra, P. (2015). Short-term impact of deep sand extraction and ecosystem-based landscaping on macrozoobenthos and sediment characteristics. Marine Pollution Bulletin, 97(1–2), 294–308.
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Bowers, D. G. and J. H. Simpson (1987). Mean position of tidal fronts in European-shelf seas. Continental Shelf Research 7(1): 35–44.
van Leeuwen, S., Tett, P., Mills, D., & van der Molen, J. (2015). Stratified and nonstratified areas in the North Sea: Long-term variability and biological and policy implications. Journal of Geophysical Research: Oceans, 120(7), 4670–4686.
