Effect of bottom stress formulation and tidal forcing on modeled flow and sediment trapping in cross-sections of tide-dominated estuaries

Field data collected in cross-sections of tide-dominated estuaries reveal that flow and suspended sedimentconcentration show pronounced spatial and temporal behavior, which depend on factors like tidal discharge,density gradients and the geometry of the cross-section. Models are capable of reproducing and explainingmany aspects of the observations, but also marked discrepancies occur between model results and data. Theobjective of the present study is to systematically investigate the sensitivity of model output to formulationsof physical processes. This is done by comparing two types of models. The first is a numerical model (NM)that solves the full shallow water equations with prognostic salt dynamics. The second is an IM that solves areduced set of equations for tidal water motion and uses a diagnostic salinity field. The IM can be used as atool to interpret the complex output of the NM. The NM, on the other hand, can be used to probe the limits ofapplicability of the IM and may give hints on further improvements of the IM

Tsunami's in de Noordzee: kan het?

A

Statistical Description of a Magnetized Corona above a Turbulent Accretion Disk

We present a physics-based statistical theory of a force-free magnetic field in the corona above a turbulent accretion disk. The field is represented by a statistical ensemble of loops tied to the disk. Each loop evolves under several physical processes: Keplerian shear, turbulent random walk of the disk footpoints, and reconnection with other loops. To build a statistical description, we introduce the distribution function of loops over their sizes and construct a kinetic equation that governs its evolution. This loop kinetic equation is formally analogous to Boltzmann's kinetic equation, with loop-loop reconnection described by a binary collision integral. A dimensionless parameter is introduced to scale the (unknown) overall rate of reconnection relative to Keplerian shear. After solving for the loop distribution function numerically, we calculate self-consistently the distribution of the mean magnetic pressure and dissipation rate with height, and the equilibrium shapes of loops of different sizes. We also compute the energy and torque associated with a given loop, as well as the total magnetic energy and torque in the corona. We explore the dependence of these quantities on the reconnection parameter and find that they can be greatly enhanced if reconnection between loops is suppressed.Comment: 22 pages, 15 figures. Submitted to the Astrophysical Journa

The iFlow modelling framework v2.4: a modular idealized process-based model for flow and transport in estuaries

The iFlow modelling framework is a width-averaged model for the systematic analysis of the water motion and sediment transport processes in estuaries and tidal rivers. The distinctive solution method, a mathematical perturbation method, used in the model allows for identification of the effect of individual physical processes on the water motion and sediment transport and study of the sensitivity of these processes to model parameters. This distinction between processes provides a unique tool for interpreting and explaining hydrodynamic interactions and sediment trapping. iFlow also includes a large number of options to configure the model geometry and multiple choices of turbulence and salinity models. Additionally, the model contains auxiliary components, including one that facilitates easy and fast sensitivity studies. iFlow has a modular structure, which makes it easy to include, exclude or change individual model components, called modules. Depending on the required functionality for the application at hand, modules can be selected to construct anything from very simple quasi-linear models to rather complex models involving multiple non-linear interactions. This way, the model complexity can be adjusted to the application. Once the modules containing the required functionality are selected, the underlying model structure automatically ensures modules are called in the correct order. The model inserts iteration loops over groups of modules that are mutually dependent. iFlow also ensures a smooth coupling of modules using analytical and numerical solution methods. This way the model combines the speed and accuracy of analytical solutions with the versatility of numerical solution methods. In this paper we present the modular structure, solution method and two examples of the use of iFlow. In the examples we present two case studies, of the Yangtze and Scheldt rivers, demonstrating how iFlow facilitates the analysis of model results, the understanding of the underlying physics and the testing of parameter sensitivity. A comparison of the model results to measurements shows a good qualitative agreement. iFlow is written in Python and is available as open source code under the LGPL license