Geometric characteristics of subaqueous bedforms, such as height, length and leeside angle, are crucial for determining hydraulic form roughness and interpreting sedimentary records. Traditionally, bedform existence and geometry predictors are primarily based on uniform, cohesionless sediments. However, mixtures of sand, silt and clay are common in deltaic, estuarine, and lowland river environments, where bedforms are ubiquitous. Therefore, we investigate the impact of fine sand and silt in sand-silt mixtures on bedform geometry, based on laboratory experiments conducted in a recirculating flume. We systematically varied the fraction of sand and silt for different discharges, and utilized an acoustic Doppler velocimeter to measure flow velocity profiles. The final bed geometry was captured using a line laser scanner. Our findings reveal that the response of bedforms to an altered fine sediment percentage is ambiguous, and likely depends on, among others, bimodality-driven bed mobility and sediment cohesiveness. When fine, non-cohesive material (fine sand or coarse silt) is mixed with the base material (medium sand), an increased dune height and length is observed, possibly caused by the hiding-exposure effect, resulting in enhanced mobility of the coarser material. However, weakly cohesive fine silt suppresses dune height and length, possibly caused by reduced sediment mobility. Finally, in the transition from dunes to upper stage plane bed, there are indications that the bed becomes unstable and dune heights vary over time. The composition of the bed material does not significantly impact the hydraulic roughness, but mainly affects roughness via the bed morphology, especially the leeside angle.

中文翻译：

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混合沙-淤泥环境中的细沉积物通过改变沉积物流动性影响床形几何形状


水下地貌的几何特征，例如高度、长度和背风角，对于确定水力地貌粗糙度和解释沉积记录至关重要。传统上，床形存在和几何预测主要基于均匀、无粘性的沉积物。然而，沙子、淤泥和粘土的混合物在三角洲、河口和低地河流环境中很常见，这些地方的河床形态无处不在。因此，我们基于在循环水槽中进行的实验室实验，研究了砂-粉砂混合物中的细砂和粉砂对床形几何形状的影响。我们系统地改变了不同排放的沙子和淤泥的比例，并利用声学多普勒测速仪来测量流速剖面。使用线激光扫描仪捕获最终的床几何形状。我们的研究结果表明，河床形态对细泥沙百分比变化的响应是不明确的，并且可能取决于双峰驱动的河床流动性和沉积物粘性等。当细小的非粘性材料（细砂或粗粉砂）与基材（中砂）混合时，观察到沙丘高度和长度增加，这可能是由隐藏-暴露效应引起的，导致较粗的流动性增强材料。然而，弱粘性的细粉沙抑制了沙丘的高度和长度，这可能是由于沉积物流动性降低造成的。最后，在从沙丘到上层平面床的过渡过程中，有迹象表明床变得不稳定并且沙丘高度随着时间的推移而变化。床层材料的成分不会显着影响水力粗糙度，但主要通过床层形态，特别是背风角影响粗糙度。