Physical oceanographer Theo Gerkema investigates the sea level in the Wadden Sea and the Zeeland delta. ‘Global mean sea level is measured to an accuracy of a few millimetres with the help of satellites. Climate change and sea level rise are clearly measurable in those data. Nevertheless, that does not say a lot about regional values; considerable differences exist across the world. Furthermore, the annual mean sea level along the Dutch coast can vary by ten or even twenty centimetres from year to year. That is mainly due to the mean direction and strength of the wind in that year. Was the prevailing wind more easterly than normal? Then the mean sea level on the Dutch coast can easily be ten centimetres lower!’
‘Those variations become relatively less important when we take a very long time series. The sea level along our coast has been accurately measured for more than a century. Over that period, trends can therefore be recognized. However, in relation to climate change and sea level rise, we also want to know what has happened to the sea level over the past ten years, for example. We need to be able to account for the effects of the wind to determine that. So that is an important part of our research too.'
‘Just how large an influence the wind has on water movements in the Wadden Sea is something we have only recently become aware of. These water movements are important for almost everything that happens in the Wadden Sea. What comes through the inlets between the islands twice per day is not just a lot of water but also sand, sediment and nutrients. These nutrients, in turn, ensure that the algae can grow, as a result of which shellfish, fish and birds can find food.’
‘Freshwater also regularly comes in from the sluices in the Afsluitdijk, the dyke that separates the IJsselmeer lake from the Wadden Sea. How that freshwater subsequently spreads through the channels and over the mudflats strongly depends on the wind. This way, the variable wind leaves a strong mark on the Wadden Sea and makes its dynamics very much event-driven.’Read more +
Theo Gerkema, An introduction to tides. Cambridge University Press, 2019.
Contains the following chapters: 1. Introductory Concepts; 2. Tidal Forcing; 3. Celestial Motions; 4. Tidal Constituents and the Harmonic Method; 5. Tidal Wave Propagation; 6. Tides in Coastal Seas and Basins; 7. Internal Tides.
Every tidal period, millions of cubic meters of water and tons of suspended sediment flow back and forth between tidal basins or estuaries and the adjacent sea. But what is, in the long run, the cumulative effect of this exchange? Where does the sediment end up? Is there sufficient import of sediment in tidal basins for the intertidal flats to keep pace with sea level rise?
These questions provide a challenge because the transports are also driven by the wind, which is highly variable, not only from day to day, but also from year to year. As a result, annual mean transports are for a large part determined by wind events: tidal basins are event-driven systems.
Realistic numerical models are an indispensable tool to analyse this variability, because they capture many more places and moments than one possibly can with measurements at sea. Still, observational work is crucial to validate models and to assess how realistic they really are.
Two types of measurement are particularly useful: long-term time series of currents, sediment concentration, salinity and other quantities, collected by instruments on a lander deployed in the sea, or at a jetty. To cover more places within a tidal cycle, we use the NIOZ research vessel Navicula; this enables us to calculate the transport of water and suspended sediment in a tidal channel.
Presently, our focus is on the transports through the Kimstergat near Harlingen, as part of the STW Mudmotor project, and on transports in the Delta in Zeeland.
Annual mean sea level varies in the order of a decimeter from year to year. This inter-annual variability is also in large part due to the prevailing wind climate in different years. Regional sea level variability forms a core topic at the department of Estuarine & Delta Systems. Therefore I am also part of the NIOZ Sea Level Centre of expertise.
I am a member of the department of Estuarine & Delta Systems (NIOZ-Yerseke) since 2016. Before I worked as senior researcher on coastal dynamics at NIOZ-Texel, after having been postdoctoral researcher at NIOZ, LEGI Grenoble (France) and Utrecht University. Until 2010, I worked for many years on internal tides, solitons, and the effects of Earth rotation.