The goal of this thesis is to create a theoreticalmodel that captures the influence of wind on the horizontal and vertical salinity distribution in an estuary, as well as the dynamics that arise within the estuary. A secondary, numerical model, that uses fewer assumptions, will be developed and analysed. This serves to determine whether the implications from the theoretical model hold only because of all the assumptions that are made, or whether the results are equally valid when some of these assumptions are relaxed. The focus of this thesis is on gaining conceptual insight in estuaries subject to wind, rather than on obtaining a high accuracy in describing a specific estuary. This thesis is focused on steady-state solution in partially orwell mixed estuaries. Moreover, the dynamics and salinity averaged over the width of the estuary are considered, such that only dynamics in the vertical and along channel direction are modelled. Estuaries are subject to periodical variations forced by tides. The variation within a tidal period is not studied in this thesis, but the dynamics and salinity averaged over a tidal period are considered. The effects of tides or variation along the width of the estuary are not neglected from this model, but are incorporated in the parametrisation of the model instead.

Cells of the most common organisms like plants and animals are filled with polymeric networks that fulfil important functions of the cell. There is however no analytically solvable model that describes diffusion in such a cell. This thesis presents a model for diffusion in polymeric environments, and some predictions about the behaviour of the model are made and confirmed by simulations.
Furthermore, the Fokker-Planck equation of this problem is studied, in order to solve the problem. Certain approximations are presented and solved, and it is investigated when the approximations are sound. Moreover, a method is described that can derive a solution to an equation with certain boundary conditions, from a solution to the same equation with different boundary conditions. This thesis also shows how this novel method can be applied to this model to find a non-approximated solution to the equation, where this was not
possible without this method. Finally, it is described how a multitude of partial differential equations that are linked to each other via the boundary conditions can be solved, can be solved using the method described.