The effect of a local mesh refinement on hydraulic modelling of river meanders

Abstract

Large-scale river models are generally discretized by relatively large mesh cells resulting in bathymetry discretization errors and numerical effects. These hydraulic models are generally calibrated by altering the bed roughness to compensate for these errors and effects. Consequently, the calibrated roughness values are mesh-dependent while generally local mesh refinements are executed after model calibration to study the effects of river interventions. This study shows both the errors caused by bathymetry discretization and numerical effects for locally refined meshes. First, schematised river meanders with a flat bed in the transverse flow direction are analysed to isolate the induced numerical effects by the mesh. Afterwards, a case study is considered to verify if similar mesh influences are found in natural river meanders.Curvilinear, triangular and hybrid (combination of curvilinear and triangular cells) meshes are used with different resolutions. The analysis shows that in the schematised river meanders lower depth-averaged flow velocities and larger water depths are simulated with coarser meshes. In the case study, substantial differences in hydrodynamics between the meshes are obtained suggesting that the bathymetry discretization is more influential than the numerical effects. Finally, it was found that triangular meshes, and rivers with narrow meander bends, are most sensitive to mesh resolution. Especially in these cases, it is desirable to refine the mesh at the desired locations before model calibration.

Lidar DEM and Computational Mesh Grid Resolutions Modify Roughness in 2D Hydrodynamic Models

“…One potential parameter that could be used to guide modelers on selecting a DEM or mesh grid resolution is the ratio of channel width to resolution. It has been shown that accurate representation of the channel is a dominant factor in hydrodynamic modeling, with regard to resolution and roughness (Bilgili et al., 2023; Caviedes‐Voullième et al., 2012; Savage, Bates, et al., 2016). Because of this, modelers should consider a range of ratios of channel width to resolution that would work for their study sites.…”

“…There are several modeling aspects that need to be considered when choosing DEM and mesh grid resolutions. Features in the floodplain that could alter or influence flow and wetting, such as negative relief features (Fivash et al., 2023; Lewin & Ashworth, 2014) like floodplain channels (Czuba et al., 2019; Sumaiya et al., 2021), should be spatially represented in the model with sufficient resolution (Adams & Zampiron, 2020; Bilgili et al., 2023; Dottori et al., 2013; Kasprak et al., 2019; Mazdeh et al., 2023; Yu & Lane, 2006). Additionally, increasing roughness in these areas might also be an important consideration (Caviedes‐Voullième et al., 2012; Mazdeh et al., 2023).…”

“…In order to address this, mesh resolution should be selected so that hydraulically important flow features, landcovers, and structures are properly represented (Bilgili et al., 2023; Dottori et al., 2013; Horritt et al., 2006; Mazdeh et al., 2023; Wright et al., 2022). If the mesh is too coarse, erroneous estimates may create a resultant outflow hydrograph that is dampened or smoothed (Bilgili et al., 2023; Yu & Lane, 2006), while too fine a mesh would result in increased model run times and a decreased inundation extent (Hardy et al., 1999; Mandelbrot, 1967). Discretization can be in rectangular grids, triangular meshes, or other unstructured formats, which can be done in many 2D hydrodynamic models, such as Delft3D, SRH‐2D, Fast‐Mech and HEC‐RAS.…”

“…Averaging occurs within each mesh cell, thus the model does not fully represent the true hydraulic variability of the river and floodplain (Bradley, 2023). In order to address this, mesh resolution should be selected so that hydraulically important flow features, landcovers, and structures are properly represented (Bilgili et al., 2023; Dottori et al., 2013; Horritt et al., 2006; Mazdeh et al., 2023; Wright et al., 2022). If the mesh is too coarse, erroneous estimates may create a resultant outflow hydrograph that is dampened or smoothed (Bilgili et al., 2023; Yu & Lane, 2006), while too fine a mesh would result in increased model run times and a decreased inundation extent (Hardy et al., 1999; Mandelbrot, 1967).…”

“…This is done by retaining details like hydraulic radius, volume, and cross sectional area to be used for coarser numerical modeling (Brunner, 2016; Casulli, 2009; Casulli & Stelling, 2011). Refinement of the mesh in the main channel and in other hydraulically influential areas has been shown to have an overall effect on modeled results since coarse resolution meshes misrepresent the channel cross‐section, thus altering modeled velocities and total inundated area (Bilgili et al., 2023; Bomers et al., 2019; Bradley, 2023; Yu & Lane, 2006). Despite the overall effects that mesh and DEM resolutions have on models, they are usually not altered because calibration is typically done by altering the roughness values (Attari & Hosseini, 2019; Ballesteros et al., 2011; Bilgili et al., 2023; Ferreira et al., 2021).…”