Collaborative Research: Width Adjustment in Mixed-Load Channels

河流调整其形状以反映其流域的水和沉积物的供应。过去四十年的研究为预测河道截面的稳定形式提供了强有力的理论基础，也为了解水力几何的下游趋势提供了经验基础。然而，关于河流形态的许多重要问题仍然存在。本研究项目考察了细泥沙和粗泥沙混合携带的混载河流的宽度和深度调整过程。将进行一系列适当比例的水槽实验，以测试有关在以粗质沉积物为界但主要携带细质沉积物的河流中形成稳定通道横截面的假设。这样的河流经常发生，但它们不容易被现有的理论解释，这些理论倾向于将问题限制在一种沉积物上，并施加稳定、均匀流动的约束。本研究试图模拟受水流和泥沙输运时空变化影响的河道宽度调整。将进行三组实验。第一组研究了一条不携带细沉积物的直砾石河道在受到一系列河岸水流的影响时是如何演变的。这些资金流的规模从略高于足额到远高于足额不等。第二组实验将检验同样的基本问题，尽管水流只会携带适量的悬浮沉积物。由于细泥沙沿河道边缘或漫滩顶部沉积，预计河道的最终形态将不同于清水情况。第三组实验将研究河床地形和河道曲率的空间变化对细泥沙沉积的影响。这些实验将集中在平流与扩散的相对重要性上，作为一种将细沉积物输送到沙洲顶部和洪泛平原的机制。实验结果最终将用于测试和完善混合负载河流准三维水流和泥沙运输理论。实验将在明尼苏达大学的圣安东尼福尔斯实验室进行。该项目与以前的项目不同，以前的项目基于平衡通道由“代表性”排放形成和维持的概念。通过关注流量和泥沙负荷变化的影响，这项研究应该拓宽对自然河流行为的理解。研究结果对管理或修改河流系统具有潜在的附加意义，特别是在河流恢复工作方面。这项研究还将为河道与洪泛区的相互作用提供更多的见解，从而有助于更全面地了解洪泛区沉积过程。

A river adjusts its shape to reflect the supply of water and sediment from its drainage basin. Research conducted over the last four decades has provided a strong theoretical basis for predicting the stable form of channel cross sections and an empirical basis for understanding downstream trends in hydraulic geometry. Many important questions about river morphology remain, however. This research project examines processes of width and depth adjustment in mixed-load rivers, which carry a mixture of fine and coarse sediment. A series of appropriately scaled flume experiments will be conducted to test hypotheses regarding the formation of a stable channel cross-section in rivers that are bounded by coarse sediment but that carry predominantly fine sediment. Such rivers commonly occur, but they are not easily explained by existing theories, which tend to restrict the problem to one type of sediment and to impose the constraint of steady, uniform flow. The present study attempts to model width adjustment in channels subject to temporal and spatial variations in flow and sediment transport. Three sets of experiments will be conducted. The first set examines how a straight gravel channel, carrying no fine sediment, evolves when it is subjected to a series of overbank flows. These flows will range in magnitude from slightly above to well above bankfull. The second set of experiments will examine the same basic question, although the flow will carry only moderate amounts of suspended sediment. The final form of the channel is expected to be different from the clear-water case owing to deposition of fine sediment along the channel margin or on top of the floodplain. The third set of experiments will examine how spatial variations in riverbed topography and channel curvature affect fine sediment deposition. These experiments will focus on the relative importance of advection versus diffusion as a mechanism for transporting fine sediment onto bar tops and floodplains. The results of the experiments will ultimately be used to test and refine a theory for quasi three-dimensional flow and sediment transport in mixed-load rivers. The experiments will be conducted at the St. Anthony Falls Laboratory at the University of Minnesota. This project represents a departure from previous projects that have been based on the concept that equilibrium channels are formed and maintained by "representative" discharges. By focusing on the effects of variations in discharge and sediment load, this study should broaden understandings of how natural rivers behave. The results have potential additional implications for managed or modified river systems, especially with respect to river restoration efforts. The study also will provide added insight into channel-floodplain interactions, thereby helping lead to fuller understandings of the processes involved in floodplain sedimentation.