In deltas and estuaries throughout the world, a fluvial-to-tidal transition zone (FTTZ) exists where both the river discharge and the tidal motion drive the flow. It is unclear how dune characteristics are impacted by changes in tidal flow strength, and how this is reflected in the hydraulic roughness. To understand dune geometry and variability in the FTTZ and possible impacts on hydraulic roughness, we assess dune variability from multibeam bathymetric surveys, and we use a calibrated 2D hydrodynamic model (Delft3D-FM) of a sand-bedded lowland river (Fraser River, Canada). We focus on a period of low river discharge during which tidal impact is strong. We find that the fluvial-tidal to tidal regime change is not directly reflected in dune height, but local patterns of increasing and decreasing dune height are present. The fluvial-to-tidal regime change is reflected in dune shape, where dunes have lower leeside angles and are more symmetrical in the tidal regime. The calibrated model allows to estimate local patterns of dune heights using tidally averaged values of bed shear stress. However, the spatially variable dune morphology hampers local dune height estimation. Changes in dune shape do not significantly impact the reach-scale roughness, and estimated dune roughness using dune height and length is similar to the dune roughness inferred from model calibration. Hydraulic model performance with a calibrated, constant roughness is not improved by implementing dune-derived bed roughness. Instead, the data analysis revealed that large-scale river morphology may explain differences in model roughness and corresponding estimates from dune predictors.

中文翻译：

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低流量弗雷泽河河潮过渡带的沙丘几何形状和相关水力粗糙度

在世界各地的三角洲和河口，存在河流到潮汐过渡区 (FTTZ)，河流流量和潮汐运动共同驱动水流。目前尚不清楚潮汐流强度的变化如何影响沙丘特征，以及这如何反映在水力粗糙度中。为了了解 FTTZ 中沙丘的几何形状和变化以及对水力粗糙度可能产生的影响，我们通过多波束测深测量评估沙丘变化，并使用沙层低地河流（加拿大弗雷泽河）的校准二维水动力模型 (Delft3D-FM) ）。我们重点关注潮汐影响强烈的河流流量低的时期。我们发现河流潮汐到潮汐状态的变化并不直接反映在沙丘高度上，但存在沙丘高度增加和减少的局部模式。河流到潮汐状态的变化反映在沙丘形状上，沙丘的背风角较低，并且在潮汐状态下更加对称。校准模型允许使用床切应力的潮汐平均值来估计沙丘高度的局部模式。然而，空间变化的沙丘形态阻碍了局部沙丘高度​​的估计。沙丘形状的变化不会显着影响范围尺度粗糙度，并且使用沙丘高度和长度估计的沙丘粗糙度与从模型校准推断的沙丘粗糙度相似。通过实施源自沙丘的河床粗糙度并不能改善具有校准的恒定粗糙度的水力模型性能。相反，数据分析表明，大规模河流形态可以解释模型粗糙度和沙丘预测器相应估计值的差异。