Sensitivity of tidal motion in well-mixed estuaries to cross-sectional shape, deepening and sea level rise: An analytical study
Estuaries are transition areas between rivers and seas, where fresh and salt water mix. The primary source of mixing is often the tide. Examples of estuaries in Northwestern Europe are those of the Elbe, Ems, Humber, Scheldt and Seine. Most of these estuaries are important for shipping, e.g. at the harbors of Antwerp, Emden and Hamburg, and for this purpose they have been subject to human interventions such as deepening of navigation channels. The tidal range and associated tidal current velocities in many estuaries have increased during the last decades, in response to a combination of these human interventions and sea level rise. This often has negative impacts on e.g. ecology, navigation and risk of flooding in these estuaries.
To gain further physical insight into the dynamics of estuarine tides, a three-dimensional analytical model was designed that computes the tidal elevation and the longitudinal and lateral tidal flow throughout an estuary. The default geometry and parameter values that were used are representative for the Ems estuary. With this model the potential effects of deepening of the navigation channel and sea level rise on the tidal elevation were studied. Also the effects of changing the lateral bottom profile shape (a possible consequence of dredging) on the lateral tidal velocities were investigated.
Results show that realistic amounts of deepening of the navigation channel (~1-2 m) and sea level rise over the coming 50 to 100 years (~30-60 cm) have a significant amplifying effect on the semidiurnal tidal sea surface elevation in the Ems estuary. Furthermore, it is found that there is a critical amount of sea level rise after which dampening of the tidal wave occurs. This critical value is smaller when flooding of adjacent banks is considered.
The lateral tidal velocities are driven by Coriolis deflection of longitudinal tidal flow, lateral density gradients and continuity effects. Its relevance lies in the fact that lateral tidal velocities transport longitudinal tidal momentum and thereby generate mean longitudinal currents that contribute to the formation of turbidity zones. It turns out that the lateral tidal flow due to continuity effects is amplified when using a more asymmetric lateral bottom profile. This results in stronger rightward (leftward) lateral tidal velocities at maximum flood for bottom profiles skewed towards the left (right) when looking landward.
Erik Ensing, PhD student in the Coastal and Shelf Sea Dynamics research group at IMAU
This study by E. Ensing, H.E. de Swart, and H.M. Schuttelaars, was published in Ocean Dynamics.