University of Twente Student Theses
The influence of a grid structure on hydraulic river modelling outcomes of river meanders
Bilgili, E. (2020) The influence of a grid structure on hydraulic river modelling outcomes of river meanders.
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| Abstract: | To evaluate the efficacy and impact of river interventions, a detailed insight in flow patterns in rivers is essential. A common approach to investigate such processes is by making use of hydrodynamic simulations, which solve the (depth-averaged) shallow water equations. In these models, fully triangular and fully curvilinear grids are commonly applied to discretise study areas. A combination of both grid shapes is also possible, which is known as a hybrid grid. Previous studies have shown that the accuracy and computation time of depth-averaged models is substantially influenced by the grid structure. It has been highlighted that in river models, grid coarsening and poor alignment between grid and the direction of the flow have a diffusive-like appearance, resulting in lower depth-averaged flow velocities and hence higher water depths. These generated numerical effects by a grid are referred to as respectively numerical diffusion and false diffusion. According to previous studies, the former depends on grid resolution, depth-averaged flow velocity and rapid flow changes, whereas the latter relies on the orientation of the grid lines with respect to the flow direction as well as grid resolution and depth-averaged flow velocity. Nonetheless, in previous studies, grid effects are interrelated with how well the bathymetry is captured by a grid. Consequently, it is unclear to what extent effects by grid generation choices influence hydraulic river modelling outcomes, especially in river bends. The objective of this study is to understand under which conditions effects by grid generation choices affect hydraulic river modelling outcomes of river meander bends. In this study, we performed simulations for hypothetical river meanders and the Grensmaas river, which is a section of the Meuse River, the Netherlands. The hypothetical rivers helped to isolate the effects by grid generation choices on hydraulic modelling outcomes in river meanders and are set up based on the characteristics of the Grensmaas river. The Grensmaas river consists of both mild and sharp bends with large local variations in floodplain width. To capture the extremes of these geometrical characteristics in the Grensmaas river, four hypothetical river schematisations are set up which can be differentiated by a mild or sharp bend and the presence/absence of floodplains. For the hypothetical cases with floodplains, a constant floodplain height is considered with respect to the bed level of the main channel. Except for the transition between main channel and floodplains, a constant bed level in transverse flow direction is used. All four rivers are forced at the upstream boundary with a constant discharge until similar water levels are obtained between the cases. Three flow scenarios are calculated with each lasting 10 days: (i) low; (ii) mid and (iii) high discharge range. The downstream boundary conditions are set by predefined rating curves based on steady uniform flow considerations. Curvilinear and triangular grids are considered with three different grid resolutions (high, medium and low) in the hypothetical cases with only a main channel. Regarding the resolution in the main channel of the curvilinear grids, 20, 10 and 5 grid cells are placed in the transverse flow direction for respectively the high, medium and low resolution. For the triangular grids, 8, 4 and 3 cells in the transverse flow direction are placed for respectively the high, medium and low resolution. In hypothetical cases where floodplains are present, curvilinear and triangular grids, as well as hybrid grids, are used with only a high and medium grid resolution. For the Grensmaas river, similar grid shapes are constructed as for the hypothetical cases with floodplains. Three levels of grid resolution are examined for the different grids shapes: (i) a high; (ii) a medium grid resolution; and (iii) a locally refined medium resolution grid. D-Flow FM is used as the software to perform the computations. In terms of the general flood patterns, it was found that an elevated water surface near the outer bank was simulated by all grids in both the hypothetical river meanders as well as in the Grensmaas river. Higher depth-averaged flow velocities are obtained close to the inner bank at the bend entrance and apex in the main channel of the hypothetical rivers. The opposite occurred at in the river bends of the Grensmaas river, where higher depth-averaged flow velocities are simulated close to the outer bank due to the bed topography. The latter is generally asymmetrical with a shallow (sometimes nearly flat) section extending from the centre of the channel towards the inner bank and a deep portion (pool) located at the outer bank. The analysis showed that lower depth-averaged flow velocities and hence higher water depths are obtained with coarser grids in the hypothetical river meanders in the absence of floodplains. Even larger deviations are simulated in the sharper bend, as rapid flow changes have to be captured by the grids. These differences are more evident at higher discharges. Regarding the differences between grid shapes, greater numerical effects are obtained with curvilinear grids at lower resolutions than triangular grids. The opposite is observed at highest resolution of both grids. In contrast to the cases without floodplains, negligible differences are obtained in terms of the water depth in the hypothetical cases with floodplains. This is a consequence of relatively less deviations in depth-averaged flow velocity differences throughout the spatial domain even though considerable differences are present in the main channel. The results showed that the generated numerical effects become larger in the case with higher discharges and hence higher depth-averaged flow velocities, and under circumstances in which rapid flow changes occur (i.e. for cases with sharp river bends). Furthermore, the results also indicated that numerical effects are proportional to grid resolution, as coarser grids generated lower depth-averaged flow velocities and higher water levels. In Grensmaas river, greater differences in simulated water levels and depth-averaged flow velocities are obtained compared to the hypothetical river meanders. This showed that the discretisation of the bathymetry plays a more dominant role than numerical effects. In order to simulate water levels and depth-averaged flow velocities accurately, executing a calibration is necessary. Thereby, it is important to address that coarse grids contain larger bed level discretisation errors and hence are more sensitive to calibration. The influence of the generated numerical effects and the bed level discretisation are dampened by the presence of large floodplains. This indicates that the numerical effects and discretisation errors are both proportional to the discharge per unit width due to relatively less deviations in depth-averaged flow velocity differences throughout the river bend. The use of a locally refined grid contributed to have water levels and depth-averaged flow velocities which converges towards those of a higher grid resolution. This was a result of better representation of the bathymetry. In terms of the calibration, it is preferable to first carry out a local grid refinement before executing a calibration procedure, as generated numerical and discretised bathymetry errors can differ for locally refined grids and the coarser grids. In practise, however, it can be time expensive to calibrate various grids after each local grid refinement and unnecessary if the locally refined region is small in comparison to the calibrated roughness trajectory. Nonetheless, if small hydrodynamic differences are expected between a coarse and a fine grid, calibrating after a local grid refinement might have minor influlences on the model accuracy. Yet, it is recommended that model results from a grid, which is locally refined after calibration, are analysed carefully. |
| Item Type: | Essay (Master) |
| Faculty: | ET: Engineering Technology |
| Programme: | Civil Engineering and Management MSc (60026) |
| Link to this item: | https://purl.utwente.nl/essays/85095 |
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