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Effect of advective and diffusive sediment transport on the formation of local and global bottom patterns in tidal embayments

机译：平流和扩散性泥沙输送对潮汐港湾局部和整体底部形态形成的影响

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摘要

Bathymetric field data of tidal basins reveal two main classes of bottom patterns: (1) tidal bars, located near the entrance of the basin (length scale determined by the embayment width) and (2) global channel-shoal patterns which scale with the basin length. Previous models were able to describe only either one of these patterns. In this paper it is shown that both of them can be investigated within the framework of an idealised model of a rectangular tidal embayment, with fixed side walls and an erodible bed. The water motion is described by the depth-averaged shallow-water equations and is forced by a prescribed vertical tide at the seaward entrance. Sediment is transported as suspended load and only realistic values of the bottom friction parameter are considered. By assuming the ratio of embayment length over tidal wave length to be small, the model allows for a morphodynamic equilibrium, characterised by a spatially uniform tide moving over a bottom which slopes upwards toward the landward boundary. This equilibrium is unstable for a range of values of the model parameters, such that growth of bedforms occurs. Both global and local bottom patterns are found. In this study particular emphasis is laid on the mechanism governing the growth of a new type of localised bottom pattern. These patterns consist of small bars located near the entrance of the basin, resembling multiple row bars, and are found when advective sediment fluxes prevail over diffusive sediment fluxes. The formation process of these new bedforms is discussed in detail. The results agree well with field data. Comparison of the results with those obtained with a process-based, numerical model shows that, although the idealised model is strongly simplified, it is capable of producing the essential morphodynamics. Therefore, the idealised model is a useful tool to investigate mechanisms of bottom pattern growth.
机译：潮汐盆地的测深场数据揭示了两类主要的底部模式：（1）位于盆地入口附近的潮汐条（长度尺度由浮标宽度决定），以及（2）随盆地尺度变化的整体河道-浅滩模式。长度。以前的模型只能描述这些模式中的任何一个。在本文中，表明可以在具有固定侧壁和易蚀床的矩形潮汐掩埋的理想化模型的框架内研究这两种情况。水运动由深度平均的浅水方程式描述，并由指定的垂直潮汐向海入口强迫。泥沙作为悬浮物运输，仅考虑底部摩擦参数的实际值。通过假设潮差长度与潮汐波长度之比较小，该模型可实现形态动力学平衡，其特征在于空间均匀的潮汐在底部向上移动，该底部向上倾斜向着陆边界。对于一系列模型参数值，此平衡不稳定，因此会出现床形的增长。找到全局和局部底部模式。在这项研究中，重点放在控制新型局部底部模式的增长的机制上。这些模式由位于盆地入口附近的小条组成，类似于多排条，是在对流沉积物通量高于扩散性沉积物通量时发现的。详细讨论了这些新床形的形成过程。结果与现场数据非常吻合。将结果与使用基于过程的数值模型获得的结果进行比较，结果表明，尽管理想化的模型得到了大大简化，但它能够产生基本的形态动力学。因此，理想化模型是研究底部模式增长机制的有用工具。

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