Weisscher Steven A H, Shimizu Yasuyuki, Kleinhans Maarten G
Faculty of Geosciences Utrecht University Princetonlaan 8A Utrecht 3584 CB The Netherlands.
Faculty of Engineering Hokkaido University North 13, West 8, Kitaku, Sapporo Hokkaido 080-8628 Japan.
Earth Surf Process Landf. 2019 Sep 15;44(11):2156-2169. doi: 10.1002/esp.4638. Epub 2019 Apr 23.
A sustained dynamic inflow perturbation and bar-floodplain conversion are considered crucial to dynamic meandering. Past experiments, one-dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self-formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute-cutoff-dominated river, which requires two-dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll-bar-related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin-up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self-formed meander patterns is evidence that scroll-bar-dominated and chute-cutoff-dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.
持续的动态入流扰动和河漫滩转换被认为是动态蜿蜒形成的关键因素。过去的实验、一维建模和线性理论表明，动态蜿蜒的起始和持续需要入流的周期性横向运动。然而，入流扰动的周期是否会影响自生蜿蜒动力学仍不清楚。在此，我们通过数值模拟研究了入流扰动周期对以 chute - cutoff 为主的河流发育和蜿蜒动力学的影响，这需要在二维模型中考虑沙洲上形成河漫滩的植被情况。我们将植被的生长和死亡规则扩展到形态动力学模型 Nays2D 中，以实现蜿蜒模拟。我们通过在典型模拟蜿蜒周期附近改变一个数量级的扰动周期，测试了横向移动的入流边界的影响。从直河道初始形成蜿蜒之后，随着截弯序列的发展，所有具有足够植被的模拟都显示出一系列不断增长的蜿蜒，最终以 chute 截弯结束。这产生了一个复杂的河道带地形，其中点沙洲复合体被 chute、牛轭湖和与滚动沙洲相关的植被年龄模式截断。尽管对于微弱和不存在的入流扰动，达到动态平衡的启动过程需要更长的时间和距离，但从固有的生物形态反馈回路中出现的弯曲度、辫状指数和蜿蜒周期与入流扰动周期无关。这解释了为什么在之前的实验研究中，只有在相对于蜿蜒波长较短的水槽中进行持续的上游扰动才能实现动态蜿蜒。我们对自生蜿蜒模式的模拟证明，以滚动沙洲为主和以 chute - cutoff 为主的蜿蜒是由下游对流不稳定性发展而来的。这一见解扩展到形态模型和实验中的更多河流、河口和海岸系统，这些系统需要持续的动态扰动来形成复杂模式并发展自然动力学。© 2019 作者。《地表过程与地貌》由 John Wiley & Sons Ltd 出版
