Unravelling Salt Fluxes: Development of a Diagnostic Instrument for Online Decomposition of Salt Flux Components and Dispersion Rates from 3D Model Results

In many delta’s of the world salinity intrusion imposes limits to fresh water availability. Where globally the need for fresh water is increasing, at many places also salinity intrusion is expected to increase due to changes in river discharge, sea level and storm characteristics. In manipulated delta’s like e.g. the Rhine-Meuse delta, salt intrusion is impacted by human activities as well, like deepening of waterways and opening of delta-branches closed earlier. All these developments call for a better understanding of such a system (science), but also for means to explore and design salinity intrusion prevention measures based upon that (engineering).

In both disciplines, 3D numerical models play an important role. However, salinity intrusion in estuaries is a difficult modelling issue. Many 3D mechanisms contribute to the salt transport and interpretation of 3D model output in terms of processes is complex. To get a picture of the relative importance of the various physical mechanisms (like vertical shear dispersion, tidal pumping, profile straining, correlation fluxes, …) and lateral variations therein, actually large amounts of model output and a complex temporal and spatial averaging are needed.

In our presentation, we will discuss recent and present work on development of a new methodology for diagnostic decomposition of salt fluxes from 3D numerical model results. Firstly, we discuss the averaging and filtering procedures, the various flux components and their relation with the relevant physical processes. Next, we present salt flux decomposition for a branch of the Rhine-Meuse delta and discuss how this is used to develop salt intrusion prevention measures. Finally, we describe how the decomposition is being implemented in the numerical modelling system Delft3D. This reduces the need for large amounts of u and s model output and post-processing effort, and produces an ‘online’ diagnosis on sub-tidal fluxes, applicable for any estuary in the world.