Current 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.

中文翻译：

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沙丘对沙床辫状河河道水流及泥沙输送的影响

目前对沙丘在控制河道和河道尺度过程以及河道形态动力学中的作用的了解还不完整。我们提供了来自数值模拟和现场监测研究相结合的结果，该研究隔离了沙丘对深度平均和近床流动结构的影响，对形态动力学建模具有重要意义。使用三维计算流体动力学代码OpenFOAM进行了数值模拟，以使用数字高程模型（DEM）来量化400 m×100 m通道内时间平均的流动结构，其中（i）模型中存在沙丘和条形（ii）仅解决了条形比例尺的地形特征（沙丘已删除）。这两个模拟的比较表明，沙丘可增强横向流动，并降低钢筋顶面上的速度多达30％。沙丘由于影响近床流结构的地形转向，因此会在比单个床形更大的空间尺度上影响模拟泥沙运移的方向。我们表明，沙丘可以放大，衰减甚至逆转横向倾斜的泥沙偏转，这与3D和倾斜的沙丘密切相关。使用在深度平均形态动力学模型中应用的理论计算的泥沙输运模式表明，即使在沙丘存在的情况下，沉积物的重力偏转仍受条形地形控制。但是，需要在深度平均形态动力学模型中改进流和泥沙输运的参数化，以解释沙丘和条形形态对近床流和泥沙输运的影响。