Two subsequent presentations on the 13th UnTRIM Workshop 2016 with the same title Efficient river modelling applying subgrid technology (UnTRIM2), the first part concerning concept and grids, presented by Regina Patzwahl, and the second part concerning results, presented by Jacek A. Jankowski.
The first presentation (by Regina Patzwahl and Jacek A. Jankowski) summarizes the investigations on the subgrid-based river modeling made so far and introduces another approach for the representing the crucial river training structures with a special focus on groynes. After the discussion of different methods for forecasting the river training structures influence and taking the groynes into account in mesh-based numerical models, the applied methodology is introduced: representing these structures through leading edges in the mesh generation, switching to triangular meshes to guarantee the orthogonality requirement also by very variable leading edge configurations, and applying a fine subgrid for all computational mesh resolutions. The overall concept for the investigation is sketched: two discharges with groynes emerged and submerged, as well as forecasting for both discharges the impact of a measure of lowering and/or shortening the groynes in a given stretch of the River Elbe by Klöden in Germany.
The second presentation (by Jacek A. Jankowski, Regina Patzwahl and Frank Platzek) confirms the already well known properties of the subgrid technology implemented in UnTRIM2 for practical modelling of rivers with two-dimensional, vertically integrated models: overall improvement of accuracy, consistency of results over different computational mesh resolutions, and speedup of computation compared to the traditional meshes. The forecast ability of models applying subgrid meshes and the calibration methodology analog to Telemac-2D is demonstrated. A remaining source of insecurity is the presently prevalent calibration method of two-dimensional river models at BAW with Nikuradse roughness law and Elder turbulence model with turbulent viscosity values large enough to cause subiterations in the explicit diffusion operator step of UnTRIM2 due to exceeded stability criteria. (This has been discussed later during the workshop.)