Simultaneous measurements of tidal straining and advection at two parallel transects far downstream in the Rhine ROFI

Sabine Rijnsburger,Julie D Pietrzak,Bram C Van Prooijen,Wil G Borst,Gerben J De Boer,Carola M Van Der Hout,Onno Van Tongeren

doi:10.1007/s10236-016-0947-x

Abstract

This study identifies and unravels the processes that lead to stratification and destratification in the far field of a Region of Freshwater Influence (ROFI). We present measurements that are novel for two reasons: (1) measurements were carried out with two vessels that sailed simultaneously over two cross-shore transects; (2) the measurements were carried out in the far field of the Rhine ROFI, 80 km downstream from the river mouth. This unique four dimensional dataset allows the application of the 3D potential energy anomaly equation for one of the first times on field data. With this equation, the relative importance of the depth mean advection, straining and nonlinear processes over one tidal cycle is assessed. The data shows that the Rhine ROFI extends 80 km downstream and periodic stratification is observed. The analysis not only shows the important role of cross-shore tidal straining but also the significance of along-shore straining and depth mean advection. In addition, the nonlinear terms seem to be small. The presence of all the terms influences the timing of maximum stratification. The analysis also shows that the importance of each term varies in the cross-shore direction. One of the most interesting findings is that the data are not inline with several hypotheses on the functioning of straining and advection in ROFIs. This highlights the dynamic behaviour of the Rhine ROFI, which is valuable for understanding the distribution of fine sediments, contaminants and the protection of coasts.

Highlights

Worldwide, river plumes are formed due to a large freshwater outflow of rivers into coastal seas and oceans
River plumes are referred to as Regions of Freshwater Influence (ROFI), a term introduced by Simpson et al (1993) to demarcate this distinctive region in coastal seas and oceans
Two parallel transects were simultaneously measured with the BRA-7 hired by the Port of Rotterdam Authority (PoR) and the Navicula from NIOZ

Summary

Introduction

River plumes are formed due to a large freshwater outflow of rivers into coastal seas and oceans. River plumes are referred to as Regions of Freshwater Influence (ROFI), a term introduced by Simpson et al (1993) to demarcate this distinctive region in coastal seas and oceans. The freshwater outflow leads to stratification, while other forces (tide, wind and waves) result in mixing the water column over the vertical. The days before were windy, with wind velocity peaking around 12 m/s and blowing from south-west to north-west (Fig. 3, second panel). The waves were coming from the north and had a significant wave height of ∼1 m and a wave period varying between 5.5 and 6.5 s (Fig. 3, third panel).

Methods

Results

Discussion

Conclusion