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

This study identifies and unravels the processesthat lead to stratification and destratification in the far fieldof a Region of Freshwater Influence (ROFI). We presentmeasurements that are novel for two reasons: (1) measurementswere carried out with two vessels that sailedsimultaneously over two cross-shore transects; (2) the measurementswere carried out in the far field of the RhineROFI, 80 km downstream from the river mouth. This uniquefour dimensional dataset allows the application of the 3Dpotential energy anomaly equation for one of the first timeson field data. With this equation, the relative importanceof the depth mean advection, straining and nonlinear processesover one tidal cycle is assessed. The data shows thatthe Rhine ROFI extends 80 km downstream and periodicstratification is observed. The analysis not only shows theimportant role of cross-shore tidal straining but also thesignificance of along-shore straining and depth mean advection.In addition, the nonlinear terms seem to be small. Thepresence of all the terms influences the timing of maximumstratification. The analysis also shows that the importanceof each term varies in the cross-shore direction. One ofthe most interesting findings is that the data are not inlinewith several hypotheses on the functioning of straining andadvection in ROFIs. This highlights the dynamic behaviourof the Rhine ROFI, which is valuable for understandingthe distribution of fine sediments, contaminants and theprotection of coasts