The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary
The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary
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DOI：
10.1002/esp.5170
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发表时间：
2021-06
影响因子：
3.3
通讯作者：
G. Herrling;M. Becker;A. Lefebvre;Anna C. Zorndt;K. Krämer;C. Winter
中科院分区：
文献类型：
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作者：
G. Herrling;M. Becker;A. Lefebvre;Anna C. Zorndt;K. Krämer;C. Winter
河口河床的特点是有大小不一的波纹和沙丘。较小的底形使其形态形状适应于潮汐流的振荡，而大型复合沙丘（此处为不对称潮汐沙丘）保持稳定的时间长于一个潮汐周期。底形构成了一种形式粗糙度，即水力流动阻力，它对潮汐不对称性有很大的影响，因此，对河口和近海的水动力学，沉积物输运和形态动力学有很大的影响。沙丘顶部后面的流动分离和陡峭下游侧的再循环导致湍流和能量损失。由于能量耗散可以与沙丘背风坡角，不对称的沙丘形状诱导在落潮和洪水阶段可变的流动阻力。在这里，一个未校准的数值模型已被应用于分析的大尺度效应的对称和不对称的沙丘形状对河口潮汐不对称的剩余推移质输沙评估在威悉河口，德国。情景模拟进行了参数化的床面粗糙度的对称和不对称的沙丘形状和没有沙丘粗糙度。不同的沙丘形状与河口沉积物和形态动力学的主要驱动因素，即河流流量和潮汐能的时空相互作用，是复杂的，但实质性的。洪水和落潮导向的不对称沙丘对剩余推移质输沙率和方向的对比效应估计与下威悉河口流量季节变化对这些净沉积物通量的控制具有相似的重要性。这证实了在河口和潮汐环境的数值模型中需要考虑沙丘诱导的方向性河床粗糙度。
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.