Abstract
AbstractCurrent understanding of the role that dunes play in controlling bar and channel‐scale processes and river morphodynamics is incomplete. We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth‐averaged and near‐bed flow structure, with implications for morphodynamic modeling. Numerical simulations were conducted using the three‐dimensional computational fluid dynamics code OpenFOAM to quantify the time‐averaged flow structure within a 400 m × 100 m channel using digital elevation models (DEMs) for which (i) dunes and bars were present within the model and (ii) only bar‐scale topographic features were resolved (dunes were removed). Comparison of these two simulations shows that dunes enhance lateral flows and reduce velocities over bar tops by as much as 30%. Dunes influence the direction of modeled sediment transport at spatial scales larger than individual bedforms due to their effect on topographic steering of the near‐bed flow structure. We show that dunes can amplify, dampen, or even reverse the deflection of sediment down lateral bar slopes, and this is closely associated with 3‐D and obliquely orientated dunes. Sediment transport patterns calculated using theory implemented in depth‐averaged morphodynamic models suggest that gravitational deflection of sediment is still controlled by bar‐scale topography, even in the presence of dunes. However, improved parameterizations of flow and sediment transport in depth‐averaged morphodynamic models are needed that account for the effects of both dune‐ and bar‐scale morphology on near‐bed flow and sediment transport.
Highlights
We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth‐averaged and near‐bed flow structure, with implications for morphodynamic modeling
Sediment transport patterns calculated using theory implemented in depth‐averaged morphodynamic models suggest that gravitational deflection of sediment is still controlled by bar‐scale topography, even in the presence of dunes
Similar conclusions can be drawn in relation to the analysis presented in Figure 14 that indicates that the neglect of spatial variations in velocity associated with dunes will have a limited effect on sediment transport rates calculated in 2‐D morphodynamic models, where these transport rates are based on formulae that utilize depth‐averaged velocities
Summary
Over the past three decades, significant advances have been made in numerical modeling of river processes and evolution using both two‐dimensional “2‐D” depth‐averaged (Baar et al, 2019; Bridge, 1993; Chavarrías et al, 2019; Iwasaki et al, 2016; Lane & Richards, 1998; Langendoen et al, 2016; Nelson, 1990; Nicholas et al, 2013; Osada et al, 2020; Schuurman et al, 2013; Sloff & Mosselman, 2012) and three‐dimensional “3‐D” vertically resolved approaches (Bradbrook et al, 2000; Doré et al, 2016; Giri & Shimizu, 2006; Lane et al, 1999; Nabi et al, 2013; Wu et al, 2000; Yamaguchi et al, 2019; Zgheib & Balachandar, 2019). 2‐D depth‐averaged models must parameterize, rather than represent explicitly, phenomena such as secondary circulation, the effects of topographic UNSWORTH ET AL.
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