Stratification and mixing in the Rhine region of fresh water influence; Analysing two parallel transects

Abstract

The Rhine River and the Meuse River discharge into the southern North Sea, and form the Rhine Region of Fresh Water Influence (ROFI) in front of the Dutch coast. A complex hydrodynamic system is generated, dominated by mixing from strong tides, wind and waves and the stratifying influence of the freshwater and solar heating. This influences the distribution and dynamics of fine sediments. A proper understanding of the system is required to evaluate the effects of human interferences, such as the dredging for Maasvlakte 2. Previous years more insight is gained into the interaction between the tide and the stratification by numerical modelling (de Boer, et al., 2008). Furthermore, various measurement campaigns are carried out to determine the effect of dredging activities for Maasvlakte 2. This Master Thesis presents the analysis of a unique in-situ dataset focused on the different cross- and alongshore processes which induce the onset and breakdown of stratification. The aim of this study is to gain more insight into the complex four dimensional nature of the Rhine ROFI, in order to generate a better understanding of the behaviour of Suspended Particluate Matter (SPM). In October 2011 the Port of Rotterdam Authority (PoR) and NIOZ (Royal Netherlands Institute for Sea Research) sailed simultaneously on two parallel cross-shore transects in order to conduct measurements, one near Egmond and one near Wijk aan Zee. Salinity, temperature, SPM and velocities were measured. The advantage of measuring along two transects is the extra dimension in alongshore direction. Therefore, a dataset is obtained allowing analysis in four dimensions. In the previous years the four dimensional nature of the ROFI has only been studied with numerical modelling. However, data analysis was only performed with data from a single mooring or a single transect, both of which do not give spatial information in cross- and alongshore direction. The contribution of alongshore advection and straining is investigated with a simplified form of the three-dimensional Potential Energy Anomaly Equation (PEA) (de Boer, et al., 2008), influenced by wind, waves and tide. Tidal analysis is used to investigate the presence of ellipses, which indicate the presence of the plume. After analysis it has become clear that the Northern transect is far more stratified than the expected. It was anticipated that the waters this far north were well-mixed. Instead it is shown that the Northern transect is even more stratified than the Southern transect. The simplified three-dimensional Potential Energy Anomaly analysis shows that cross-shore straining is the dominant process inducing stratification and mixing. Although the other cross- and alongshore processes are also observed and these prove significant as well. However, the alongshore advection works different than expected; it enhances stratification during ebb instead of during flood. This is the result of the more stratified Northern transect in comparison with the more homogeneous Southern transect. Tidal ellipses are observed in front of the coast. The ellipses show that the state of the water column is spring-like, stratification is yet observed. Using two vessels to obtain information in four dimensions is on the verge of what is currently possible with the ships available to the Dutch community. This first measurement in four dimensions with two vessels and using the three-dimensional Potential Energy Anomaly Equation has led to more insights. This study highlights the dynamic and unpredictable behaviour of the Rhine ROFI, as the measurements have led to surprising outcomes.