PhD defence: Suspended matter transport in the coastal turbidity maximum zone along the Holland coast

The turbid Dutch coastal waters: latent silt transport to the Wadden Sea

This research focuses on the elevated concentrations of suspended sediment along the Dutch coast. This suspended sediment consists of a mixture of fine minerals (silt and clay), microorganisms, algae and nutrients that cause natural turbidity in seawater. The Dutch coast forms a transit zone of suspended matter from the Strait of Dover to the Wadden Sea. This sediment is crucial for the Wadden Sea ecosystem because it creates nutrient-rich soils and is important for silting mudflats and the coast to compensate for sea level rise.

The study combines data from two measurement programs in 2010-2012 to map suspended matter transport and the influence of factors such as tidal currents, waves and biological influences. Measurements took place between IJmuiden and Callantsoog, where high concentrations were found. The results show a persistent turbidity zone with high suspended matter concentrations on average only 1.5 km from the coast.

The observed suspended matter concentrations show significant variations over time, influenced by tides, waves and seasonal biological activities. Storms lead to resuspension, after which, due to slow settling velocities, the highest concentrations exceeding 1000 mg/l occur during calm weather conditions in a post-storm period. Then a thin, concentrated layer of silt reappers near the seafloor. Seasonal fluctuations are also evident: high concentrations are usually measured in autumn and winter, while spring and summer show lower values.

In calm conditions, the tide provides a net northward transport in the turbidity zone. The influence of the wind (via resuspension by waves and amplification of currents) plays a larger role in the annual net transport. This causes a large variation in net transport during the year and between years. Thus, it is essential to integrate all time scales when calculating annual transports in order to capture the full dynamics of sediment transport. The measurements give an estimate of the average annual northward transport in the turbidity zone of 2.5 Mton.