An investigation of bed armoring process and its implications for channel bed stability

泥沙粒度分布和河道水流水力对砾石床河流中装甲层形成的影响和相互作用尚未完全确定。覆盖物和微团簇作为装甲层的一部分发展起来，有助于增加装甲床的整体稳定性。最近对微团簇的研究主要集中在它们的存在对湍流流场的影响上，这一点还没有完全阐明。这项研究将把河床装甲、微团形成和河床稳定性的过程联系起来，并提供管理水流释放所需的量化，以便下游河道表面不会侵蚀和潜在地损害下游生态系统。水槽试验将创建装甲床，测试4种泥沙混合物在4种流速下的变化。微团簇密度和床层地形将通过对床面的激光轮廓扫描来测量。声学多普勒测速仪将记录点的速度(u，v，w)和波动(u？，v？，w？)在装甲表面和微团簇周围，计算了湍流描述符，如湍流动能(TKE)。流动数据将使用双平均Navier-Stokes(DANS)方法和象限分析，结合详细的床面地形，将提供对装甲床上方特定位置的流场中发生的动量传递和存在的相干流动结构的洞察。使用ADV测量，将有可能区分由于微团簇、地形表面、粘性应力或来自多个团簇相互作用的应力而产生的边界应力。在拟议的研究结论中，将量化装甲层形成过程中底板颗粒尺寸分布和流速的影响，并将其与装甲层地形结构的形成和槽床的稳定性联系起来。我们将对装甲层形成和床层稳定性的控制变量有所了解。这项研究将提供一种预测流量的方法，该流量将打破下游装甲床层作为渠道中的基质和现有的床微地形的函数，两者都可以在现场测量。

The influence and interaction of sediment grain size distributions and channel flow hydraulics over the formation of armor layers in gravel bed rivers has yet to be fully defined. Imbrications and microclusters develop as part of the armor layer and contribute to the overall increased stability of armored beds. Recent research on microclusters has focused on how the turbulent flow field is affected by their presence, which has not been fully elucidated. This research will connect the processes of bed armoring, microcluster formation, and bed stability and provide the quantification necessary to manage flow releases so that the downstream channel surface does not erode and potentially harm the downstream ecosystem.Flume experiments will create armored beds testing 4 sediment mixtures against 4 flow rates. Microcluster density and bed topography will be measured from laser profile scans of the bed surface. Acoustic Doppler Velocimeters (ADVs) will record point velocities (u,v,w) and fluctuations (u?,v?,w?) over the armored surface and around microclusters, from which turbulence descriptors, such as Turbulent Kinetic Energy (TKE), are calculated. Flow data will be analyzed using the Double Averaged Navier-Stokes (DANS) approach and quadrant analysis, which together with the detailed bed topography will provide insight into the momentum transfers occurring and the coherent flow structures present in the flow field at specific locations over the armored bed. Using ADV measurements, it will be possible to differentiate the boundary stresses as due to microcluster, topographic surface, viscous stress, or stress from the interaction of multiple clusters. At the conclusion of the proposed research, the effect of bed substrate grain size distributions and flow rate during armor layer formation will be quantified and linked to the creation of structure in the armor layer topography and the stability of the channel bed. An understanding of the controlling variables over armor layer formation and bed stability will be gained. This research will provide a means of predicting the flow rate that will break the downstream armored bed layer as a function of the substrate in the channel and the existing bed microtopography, both of which can be measured in the field.