Search

The Tonle Sap lake, located in the Cambodian floodplain of the Mekong River system, plays an important role for freshwater storage in Cambodia and the Vietnam Mekong Delta. The lake’s water level can rise by approximately 10 m and its water volume can increase 5 times during a year, showing various drying and wetting areas in the lake and its floodplains. Thus the accurate simulation of wetting - drying processes in this domain is essential for water management in the Mekong River system. Here, we present the extension of an existing wetting - drying algorithm in two space dimensions. The algorithm consists in applying a threshold value of fluid depth for a thin layer and a blending parameter in order to guarantee positive values of the water depth, while preserving local mass conservation and the well-balanced property at wet/dry interfaces. It is implemented in SLIM (Second-generation Louvain-la-Neuve Ice-ocean Model, www. slim-ocean.be), a discontinuous Galerkin finite element model (DG-FEM) solving the shallow water equations in a fully implicit way. Before applying in the Tonle Sap lake, the technique is validated against the standard analytical test cases (Balzano 1, Balzano 3 and Thacker).

This paper presents a fast and robust mesh generation procedure that is able to generate meshes of the earth system (ocean and continent) in matters of seconds. Our algorithm takes as input a standard shape-file i.e. geospatial vector data format for geographic information system (GIS) software. The input is initially coarsened in order to automatically remove unwanted channels that are under a desired resolution. A valid non-overlapping 1D mesh is then created on the sphere using the Euclidean coordinates system x, y, z. A modified Delaunay kernel is then proposed that enables to generate meshes on the sphere in a straightforward manner without parametrization. One of the main difficulty in dealing with geographical data is the over-sampled nature of coastline representations. We propose here an algorithm that automatically unrefines coastline data. Small features are automatically removed while always keeping a valid (non-overlapping) geometrical representation of the domain. A Delaunay refinement procedure is subsequently applied to the domain. The refinement scheme is also multi-threaded at a fine grain level, allowing to generate about a million points per second on 8 threads. Examples of meshes of the Baltic sea as well as of the global ocean are presented.

The extensive existing data set on the oceanography of Torres Strait (separating Australia and Papua New Guinea) is reviewed and is used to show that the long-term east to west water flux through Torres Strait is about 0.04 Sv. To explain this finding, a high resolution, finite element, depth-integrated (2D) model of Torres Strait is set-up. The model is driven by the wind and the open boundary conditions that are prescribed in the Coral Sea, the northern Great Barrier Reef continental shelf, the Gulf of Papua, and the Gulf of Carpentaria. The latter/open boundary conditions were taken from field data. The model suggests that the wind alone over Torres Strait generates negligible net currents; the net currents are instead generated by the wind setting up a water slope across Torres Strait through its opposing influence on the mean sea levels in the Gulf of Carpentaria and in the Coral Sea. The model predicts that the long-term, net east-west flow through Torres Strait is small to negligible, in agreement with field data. The model also reveals that large scale flow dynamics are largely restricted to reef passages and open waters, and that areas of shallow waters and areas densely populated with reefs and islands are poorly flushed. Finally, the model also reveals the prevalence of highly energetic tidal flows around shoals, reefs, islands and reef passages, and these swift tidal currents are responsible for the small net flow through Torres Strait through the sticky water effect. These findings enable us to propose a general water circulation model for the whole Great Barrier Reef during southeast winds.

The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, the rivers, the land surface and ground water as well as the sea ice - which, for the sake of simplicity, is considered herein to be part of the hydrosphere. The processes taking place in these domains are vastly different in nature and are characterized by a wide range of spaceand time-scales. The components of the hydrosphere interact with each other. For instance, the shallow marine and estuarine regions, though accounting for less than 1% of the volume of the oceans, have a biomass far from negligible as compared to that of the oceans, implying that they play a significant role in global biogeochemical cycles. This is one of the reasons why models are now needed that deal with most, if not all, of the components of the hydrospheric system.Numerical models of each of the components of the hydrosphere already exist. However, an integrated model of the whole hydrosphere has yet to be developed. Building such a model is a daunting task, requiring the development of multi-scale/physics simulation tools. Numerical methods for dealing with multi-scale problems are developing rapidly. Unstructured meshes offer an almost infinite geometrical flexibility, allowing the space resolution to be increased when and where necessary. In addition, time steppings for dealing with a wide spectrum of timescales while retaining a high order of accuracy have been developed over recent years (e.g. multi-rate schemes).In this talk we will present the current status of SLIM, as well as developments planned in the near future. We will highlight the efficiency issues related to the use of unstructured meshes and present possible strategies to overcome them. Applications will be reported on, in particular those pertaining to the land-sea continuum of the Scheldt River (France, Belgium, The Netherlands), the Great Barrier Reef (Australia) and the Mahakam River (Indonesia).

This paper provides a comparison of five finite element pairs for the shallow water equations. We consider continuous, discontinuous and partially discontinuous finite element formulations that are supposed to provide second-order spatial accuracy. All of them rely on the same weak formulation, using Riemann solver to evaluate interface integrals. We define several asymptotic limit cases of the shallow water equations within their space of parameters. The idea is to develop a comparison of these numerical schemes in several relevant regimes of the subcritical shallow water flow. Finally, a new pair, using non-conforming linear elements for both velocities and elevation (P-P), is presented, giving optimal rates of convergence in all test cases. P-P1 and P-P1 mixed formulations lack convergence for inviscid flows. P-P2 pair is more expensive but provides accurate results for all benchmarks. P-P provides an efficient option, except for inviscid Coriolis-dominated flows, where a small lack of convergence is observed.

We describe the space discretization of a three-dimensional baroclinic finite element model, based upon a discontinuous Galerkin method, while the companion paper (Comblen et al. 2010a) describes the discretization in time. We solve the hydrostatic Boussinesq equations governing marine flows on a mesh made up of triangles extruded from the surface toward the seabed to obtain prismatic three-dimensional elements. Diffusion is implemented using the symmetric interior penalty method. The tracer equation is consistent with the continuity equation. A Lax–Friedrichs flux is used to take into account internal wave propagation. By way of illustration, a flow exhibiting internal waves in the lee of an isolated seamount on the sphere is simulated. This enables us to show the advantages of using an unstructured mesh, where the resolution is higher in areas where the flow varies rapidly in space, the mesh being coarser far from the region of interest. The solution exhibits the expected wave structure. Linear and quadratic shape functions are used, and the extension to higher-order discretization is straightforward.

For the calibration of any model, measurements are necessary. As measurements are expensive, it is of interest to determine beforehand which kind of samples will provide the maximum of information. Using a criterion related to the Fisher information matrix, it is possible to design a sampling scheme that will enable the most precise model parameter estimates. This approach was applied to a reactive transport model (based on SLIM) of Escherichia coli in the Scheldt Estuary. As this estuary is highly influenced by the tide, it is expected that careful timing of the samples with respect to the tidal cycle will have an effect on the quality of the data. The timing and also the positioning of samples were optimised according to the proposed criterion. In the investigated case studies the precision of the estimated parameters could be improved by up to a factor of ten, confirming the usefulness of this approach to maximize the amount of information that can be retrieved from a fixed number of samples.

This review assesses and predicts the impacts that rapid climate change will have on population connectivity in coral reef ecosystems, using fishes as a model group. Increased ocean temperatures are expected to accelerate larval development, potentially leading to reduced pelagic durations and earlier reef-seeking behaviour. Depending on the spatial arrangement of reefs, the expectation would be a reduction in dispersal distances and the spatial scale of connectivity. Small increase in temperature might enhance the number of larvae surviving the pelagic phase, but larger increases are likely to reduce reproductive output and increase larval mortality. Changes to ocean currents could alter the dynamics of larval supply and changes to planktonic productivity could affect how many larvae survive the pelagic stage and their condition at settlement; however, these patterns are likely to vary greatly from place-to-place and projections of how oceanographic features will change in the future lack sufficient certainty and resolution to make robust predictions. Connectivity could also be compromised by the increased fragmentation of reef habitat due to the effects of coral bleaching and ocean acidification. Changes to the spatial and temporal scales of connectivity have implications for the management of coral reef ecosystems, especially the design and placement of marine-protected areas. The size and spacing of protected areas may need to be strategically adjusted if reserve networks are to retain their efficacy in the future.

ispartof: Sagalassos VI: geo- and bio-archaeology at Sagalassos and in its territory pages:189-208 ispartof: pages:189-208 status: published

An unstructured-mesh parallel hydrodynamic model of the whole Great Barrier Reef is presented. This model simultaneously simulates most scales of motion. It allows interactions between small- and large-scale processes. The depth-averaged equations of motion are discretized in space by means of a mixed finite element formulation while the time-marching procedure is based on a third order explicit Adams-Bashforth scheme. The mesh is made up of triangles. Their size and shape can be modified easily so as to resolve a wide range of scales of motion, from those of the regional flows to those of the eddies or tidal jets that develop in the vicinity of reefs and islands. The forcings are the surface wind stress, the tides and the inflow from the Coral Sea (East Australian Current, Coral Sea Coastal Current), the latter two forcings being applied along the open boundaries of the computational domain. The numerical results compare favourably with observations of both longshore currents and the local perturbations due to narrow reef passages. Comparisons are also performed with the simulations of a three-dimensional model applied to a small domain centered on Rattray Island, showing that both models produce similar horizontal flow fields. For a structured-mesh model to yield results of the same accuracy, there is no doubt that the computational cost would be much higher.