Regions of fresh water influence

The influence of depth on a ROFI

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The topic of this paper is the study of the tidal velocity profiles within a region of fresh water influence (referred to as ROFI in this paper). These regions of fresh water influence occur where rivers discharge fresh water into salty seas. The main research question is how depth influences the behaviour of the tidal velocity profile in a region of fresh water influence. The approach for this investigation is to apply a model to two different ROFI systems, and compare the results. The first system considered is the Rhine ROFI, for which numerous field data have been collected (Simpson, et al. 1993) and a numerical model has been created (de Boer, Pietrzak and Winterwerp 2006). The second system is the Fraser ROFI in Canada, having similar tidal characteristics as the Rhine ROFI (Foreman, et al. 1995), but a much larger depth. The model used for this comparison is an analytic model, based on Prandle (Prandle 1982a). In order to do so, it was first necessary to understand the complex behaviour of the tidal velocity profile of the Rhine ROFI. The big difference between the behaviour for neap and spring tide can be explained by decomposing the tidal velocity vector into two counter rotating phasors. Also, two parameters turned out to be important: the depth-averaged velocity and the eddy viscosity. As second goal, and main objective of this paper, was to study the impact of the depth on the behaviour of the tidal velocity profile in the region of fresh water influence. This was done by comparing the shallow Rhine ROFI with the much deeper Fraser ROFI. It turned out that finding realistic values for the eddy viscosity coefficient was the biggest challenge. The ability of predicting vertical current profiles may serve many applications, such as offshore construction or oil leaks. Being able to apply this model to other ROFI’s around the world with different depths, is therefore a very useful aim.