Search

We present a test case of river plume spreading to evaluate numerical methods used in ocean modeling. It includes an estuary-shelf system whose dynamics combine nonlinear flow regimes with sharp frontal boundaries and linear regimes with cross-shore geostrophic balance. This system is highly sensitive to physical or numerical dissipation and mixing. The main characteristics of the plume dynamics are predicted analytically, but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics.

The current study provides a new test case for the coastal numerical solutions dedicated to the plume spreading in the estuary and on the shelf. The suggested estuary-shelf system represents a mixture between pronounced nonlinear flow dynamics with sharp frontal boundaries and linear dynamics and across-shore geostrophic balance. The major aspects of the plume dynamics are analytically predicted, but are difficult to reproduce numerically. The test case manifests the level of numerical diffusion, ability of the model to reproduce the nonlinear processes and frontal zone dynamics. Numerical solutions were obtained with three unstructured mesh models SCHISM, THETIS and FESOM-C. The current study also suggests the plume spreading analysis based on numerical results, which can be useful for any intercomparison studies with focus on the plume behavior.

The Columbia River (CR) estuary is characterized by high river discharge and strong tides that generate high velocity flows and sharp density gradients. Its dynamics strongly affects the coastal ocean circulation. Tidal straining in turn modulates the stratification in the estuary. Simulating the hydrodynamics of the CR estuary and plume therefore requires a multi-scale model as both shelf and estuarine circulations are coupled. Such a model has to keep numerical dissipation as low as possible in order to correctly represent the plume propagation and the salinity intrusion in the estuary. Here, we show that the 3D baroclinic discontinuous Galerkin finite element model SLIM 3D is able to reproduce the main features of the CR estuary-to-ocean continuum. We introduce new vertical discretization and mode splitting that allow us to model a region characterized by complex bathymetry and sharp density and velocity gradients. Our model takes into account the major forcings, i.e. tides, surface wind stress and river discharge, on a single multi-scale grid. The simulation period covers the end of spring-early summer of 2006, a period of high river flow and strong changes in the wind regime. SLIM 3D is validated with in-situ data on the shelf and at multiple locations in the estuary and compared with an operational implementation of SELFE. The model skill in the estuary and on the shelf indicate that SLIM 3D is able to reproduce the key processes driving the river plume dynamics, such as the occurrence of bidirectional plumes or reversals of the inner shelf coastal currents., Accepted Author Manuscript, Mathematical Physics

Unstructured grid models allow highly flexible spatial discretization and therefore accurate representation of complex phenomena, such as the hydrodynamics in Regions Of Freshwater Influence (ROFI) of large rivers, characterized by large density variations, rapid flow dynamics and complex topography. The whole river-sea continuum can therefore be simulated seamlessly. The complex interactions between tides, river inflow and ocean dense waters in shallow areas lead to strong density gradients and rapid currents, often difficult to handle in a numerical model. Many models tend to stabilize the computations by adding diffusion, leading to overly-smoothed fronts. The aim of this study is to analyze the baroclinicity of the SLIM model. SLIM3D is a baroclinic discontinuous Galerkin finite element model. It has so far only been used to model very shallow areas, such as an idealised Rhine ROFI or subregions of the Great Barrier Reef. We analyze the ability of SLIM 3D to reproduce the main features of the Columbia River ROFI dynamics with correct salinity wedge and plume extension. Whereas the salt wedge is mainly driven by tides and discharges, the plume dynamics is strongly wind-dependent. The salt intrusion is validated against observations into the esuary and the key features of the simulated plume in different wind regimes are discussed.

Unstructured grid models allow highly flexible spatial discretization and therefore accurate representation of complex phenomena, such as the hydrodynamics in Regions Of Freshwater Influence (ROFI) of large rivers, characterized by large density variations, rapid flow dynamics and complex topography. The whole river- sea continuum can therefore be simulated seamlessly. The complex interactions between tides, river inflow and ocean dense waters in shallow areas lead to strong density gradients and rapid currents, often difficult to handle in a numerical model. Many models tend to stabilize the computations by adding diffusion, leading to overly-smoothed fronts. The aim of this study is to analyze the baroclinicity of the SLIM model. SLIM 3D is a baroclinic discontinuous Galerkin finite element model. It has so far only been used to model very shallow areas, such as an idealised Rhine ROFI or subregions of the Great Barrier Reef. The ability of stabilizing the density field by adding as less diffusion as possible is assessed through two different test cases. We first analyze the performance of the SLIM model on the lock-exchange test case. As coastal waters feature strong density gradients, the baroclinicity needs stabilization. Therefore, we study the use of different limiters. We then analyze the plume of the Columbia River as simulated with SLIM in two different regimes of the Columbia River ROFI. The horizontal structure is discussed and compared to another model of the region based on SELFE. The key features of the simulated plume in different regimes are also discussed.

Numerical modelling of coastal flows is a challenging topic due to complex topography of the coastal zone, rapid flow dynamics and large density variations. Such phenomena are best simulated with unstructured grid models due to their highly flexible spatial discretisation. This article presents a three-dimensional discontinuous Galerkin finite element marine model. Discontinuous Galerkin spatial discretisation is combined with an explicit mode splitting time integration scheme. Slope limiters are introduced to guarantee high quality of the tracer fields in the presence of strong gradients. Free surface movement is accounted for by means of an Arbitrary Lagrangian Eulerian (ALE) moving mesh method. Water volume and tracers are conserved. The conservation properties and baroclinic adjustment under gravity are tested with numerical benchmarks. Finally, the model is applied to the Rhine river plume in an idealised setting.

The paper deals with the classical ice engineering challenge of ice-induced vibrations (IIV) of offshore platforms. There is still a general industry concern with aspects of IIV and the load amplification that can arise in some situations resulting from ice interaction with a structure. It was thought important to re-visit current design methodology and practice, as well as the data that led to their formulation. A joint industry project (JIP) was organized, in which the main offshore oil companies joined together to sponsor development and validation of models for ice induced vibration. Broadly, the JIP: i) identified interesting aspects of the mechanisms behind IIV; ii) sponsored further development of three numerical modeling approaches chosen for the range of physics they captured; iii) and, provided calibrated models to an independent engineering company who used them to assess the model's accuracy in simulating five full-scale events whose data had been withheld from the model developers - ensuring an independent validation rather than a further fitting of models to data. This paper is a "briefing" and provides an overview of the background, progress, and some validation findings: a status report on the reliability of present procedures available to the offshore industry when designing platforms in moving ice where IIV can be expected. The "science" behind the models will be presented by the model developers themselves (e.g. Hendrikse & Metrikine, 2013) and in further publications once the JIP confidentiality conditions expire., Applied Mechanics