Collaborative Research: Width Adjustment in Mixed-Load Channels

河流调整其形状以反映其流域的水和泥沙的供应。近四十年来的研究为预报河道断面的稳定形态提供了坚实的理论基础，也为了解下游水力几何的发展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。这项研究项目考察了混合负荷河流的宽度和深度调整过程，这些河流携带着细沙和粗沙的混合泥沙。将进行一系列适当规模的水槽试验，以检验在以粗沙为边界但主要携带细沙的河流中形成稳定的河道横断面的假设。这样的河流经常发生，但现有的理论很难解释它们，现有的理论往往将问题限制在一种类型的泥沙上，并施加了稳定、均匀流动的约束。本研究试图模拟水流和泥沙输移的时空变化对河道宽度的影响。将进行三组实验。第一组研究了一条没有携带细粒泥沙的直石河道，当它受到一系列漫滩水流时，它是如何演变的。这些资金流的规模将从略高于银行满额到远高于银行满额。第二组实验将研究同样的基本问题，尽管水流只携带适中数量的悬浮泥沙。由于沿河道边缘或洪泛区顶部沉积了细小的沉积物，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如何影响细颗粒泥沙沉积。这些实验将集中在平流和扩散作为将细颗粒泥沙输送到沙洲顶部和泛滥平原的机制的相对重要性。试验结果将最终用于检验和完善混合负荷河流准三维水流泥沙输移理论。这些实验将在明尼苏达大学的圣安东尼瀑布实验室进行。这项工程与以前的工程不同，以前的工程是基于“有代表性的”流量形成和维持均衡航道的概念。通过关注流量和泥沙负荷变化的影响，这项研究应该扩大对自然河流行为的理解。研究结果对受管理或经修改的河流系统具有潜在的额外影响，特别是在河流恢复工作方面。这项研究还将提供对河道-泛滥平原相互作用的更多洞察，从而有助于更全面地了解泛滥平原沉积过程。

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.