NUMERICAL MODELING
The task of numerical modeling is the computer-aided simulation of processes in flowing and standing bodies of water. This involves the investigation of currents, sediment transport, and morphodynamics, as well as ecological habitats using computer models.
These models can be used to conduct parameter studies to analyze the effects of interventions in water bodies. This allows planned hydraulic engineering measures in the respective study area to be optimized to achieve the desired effects as effectively as possible and to minimize negative impacts. Furthermore, we use spatially and temporally high-resolution models in basic research. In hybrid modeling, we combine the respective advantages of physical and numerical models together with physical modeling. The necessary data basis for the calibration and validation of the models – that is, the adaptation to nature and verification using natural data – is obtained through monitoring in the field. At the IWA, we not only apply numerical models but also develop them from scratch.
Hydrodynamics
The task of hydrodynamics is to determine the spatially distributed quantities of water depth (as a scalar) and flow velocity (as a vector quantity) for a section of a water body. Higher-dimensional models additionally determine various characteristics of turbulence. The models used range from simple 1D flow models for long sections of water bodies or periods of time, to 2D models focusing on river sections of approximately 20-50 km in length, up to 3D models that we have also successfully applied to river sections of over 20 km in length. At the IWA, 1D models are primarily created with the software HEC-RAS, 2D models using Hydro_AS or our own development RSim-2D, and 3D models mainly with our self-developed software RSim-3D.
Sediment Transport and Morphodynamics
The modeling of sediment transport includes the processes of bedload transport, suspended load transport, bed evolution, and sediment sorting. As a result, individual transport mechanisms of sediment in the water body can be analyzed independently of each other, or the morphodynamics of the water body’s bed resulting from all processes can be examined over short, medium, or even longer periods of time. As with hydrodynamics, different model dimensions are used for different questions. The majority of sediment transport modeling at the IWA is carried out with our own development, iSed, which can be coupled with various 2D and 3D hydrodynamic models (including Hydro_AS, RSim-2D, and RSim-3D).
Habitat Modeling
The task of habitat modeling is to evaluate ecological habitats in the water body for various species or guilds on a micro and meso-scale level, based on the results of 2D and 3D hydrodynamic models. The evaluation can be carried out either for a stationary state or over time (e.g., during the course of a flood wave). At the IWA, the Habitat Evaluation Model (HEM) was developed for this purpose. With the recent additions of HEM-PEAK and HEM-Impoundment, evaluations of surge and drawdown processes, as well as of impounded areas in water bodies, are also possible.
High-Resolution Models
In 3D models used for engineering questions, the RANS turbulence approach (Reynolds-averaged Navier-Stokes) is commonly applied, where the hydrodynamic conservation equations (Navier-Stokes equations) are averaged over the turbulence scales. In addition, there are approaches for more accurate and high-resolution calculation of turbulence, such as Large-Eddy Simulation (LES) or Direct Numerical Simulation (DNS). Both methods resolve the computational domain spatially and temporally at a very high resolution, are consequently computationally intensive, and are therefore mainly used in the field of basic research. At the IWA, the open-source model OpenFOAM or the Virtual Flow Simulator (VFS) from Stony Brook University are used for this purpose.