On modeling subgrid-scale macro-structures in narrow twisted channels

Abstract

Porosity-based subgrid topography models often fail to capture the effects of subgrid-scale topographic features in the interior of coarse grid cells. Existing approaches that modify bottom roughness or a drag coefficient are inadequate for macro-structures (large emergent or submerged obstacles) in subgrid-scale narrow twisted channels. Such structures partially block the cross-sectional area and provide enhanced topographic dissipation-effects that are not well represented by a drag coefficient that scales on a coarse-grid cell-averaged velocity and the cell volume. The relative alignment between mesh and flow further complicates this problem as it makes the subgrid model sensitive to mesh design. In the present study, three new approaches for simulating subgrid-scale macro-structures in narrow channels are proposed. The interior partial-blocking effect of structures is modeled as reduction of grid face-area. The sheltering of flow volumes around obstacles, which leads to topographic dissipation, is modeled by reducing the cell volume in the momentum equation (only). A mesh-shift procedure is designed to optimize mesh alignment for identifiable subgrid features. Combining the three subgrid methods improves the approximation of surface elevation and in-channel flow rate with a coarse-grid model. Tests are conducted for channelized flow using both synthetic domains and real marsh topography. The new methods reduce the overall mesh dependency of the subgrid model and provides stronger physical connection between effects of macro-structures and their geometry at coarse grid scales.

Porosity Models for Large-Scale Urban Flood Modelling: A Review

“…Özgen et al (2016) [12,22] proposed an extension of the IP model, in which the storage and conveyance porosities are depth-dependent. Approaches similar to that of Sanders et al (2008) [9] and Özgen et al (2016) [12] were adopted in multiple other studies, which account for obstruction-induced effects on storage and conveyance at the level of each computational cell [4,8,[24][25][26][45][46][47][48][49][50][51]. Various approximations of the shallow-water equations were used in these studies (e.g., diffusive wave), and the storage and conveyance properties were generally considered as depth-dependent.…”

Hydrodynamic modeling of a complex salt marsh system: Importance of channel shoreline and bathymetric resolution

“…For the fnite channels, wave properties can change in the presence of strong bottom friction, where the incident and refected wave amplitudes decay and their phase lag follows the friction force (Hunt, 1964). Beside channel bottom friction, the channel hypsometry change based on the tidal surface elevation can also affect the amplitude decay and phase lag by enhancing or reducing the topographic dissipation (Li and Hodges, 2020).…”

“…Several studies such as Sanders et al (2008), Volp et al (2013), andWu et al (2016) have modifed and enhanced the subgridding technique for different urban food and tidal hydrodynamic modeling applications in the last two decades. However, despite the recent progress in subgrid methods, getting the coarse grid model to refect the actual channel conveyance, surface area, and water volume is still challenging and can be time and labor-consuming (Li and Hodges, 2020). Especially in a complex wetland system with a sharp marsh-channel edge and rapid bathymetry change, we need to carefully represent the small-scale features to resolve the shifts in wetting and drying fronts and topographic dissipation.…”