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.

中文翻译：

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不对称沙丘粗糙度对威悉河口潮汐不对称性的影响

河口河床通常以大小不一的波纹和沙丘为特征。较小的河床使它们的形态形状适应振荡的潮汐流，而大型复合沙丘（此处：不对称潮汐沙丘）在比潮汐周期更长的时间内保持稳定。床型构成了一种形式粗糙度，即水力流动阻力，它对潮汐不对称性具有大规模影响，因此对河口和沿海的流体动力学、沉积物输送和地貌动力学有很大影响。沙丘顶后的流动分离和陡峭下游侧的再循环导致湍流和能量损失。由于能量耗散可能与沙丘背风角有关，不对称的沙丘形状会在退潮和洪水阶段引起可变的流动阻力。这里，非校准数值模型已被应用于分析对称和非对称沙丘形状对河口潮汐不对称性的大规模影响，该影响通过德国威悉河河口的残余床荷载沉积物输送进行评估。情景模拟是使用对称和非对称沙丘形状的参数化床粗糙度进行的，没有沙丘粗糙度。不同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）的时空相互作用被证明是复杂但重要的。据估计，洪水和退潮不对称沙丘对残余床荷载传输速率和方向的对比影响与控制下威悉河河口这些净沉积物通量的季节性流量变化具有相似的重要性。