Morphodynamic stability of river and tidal bifurcations around bars tested in the Fast Flow Facility

Multithread rivers such as the Jamuna and Mekong have networks of channels and bars that change with every flood. Tidal systems such as the Scheldt, Humber and Columbia estuaries and short tidal basins in the Wadden Sea and in Florida, have perpetually changing and interacting channels and shoals formed by ebb and flood currents. Current models fail to forecast these natural dynamics, yet main channels are economically important shipping fairways, whilst shoal areas that emerge and submerge daily are ecologically valuable habitats. Human interference, changing river discharge and sealevel rise threaten all functions. Furthermore, there are strong indications that fairway deepening leads to reduced urban safety due to enhanced flow resistance by groynes in rivers and enhanced tidal range in estuaries (e.g. Bolla Pittaluga et al. 2015 in AWR, Seminara et al., in EH 2011). This enhances dike failure risk during low water level and flooding during high water level. We urgently need dynamic forecasting models to optimise management strategies for these multiple functions (Wang et al. 2012 in Ocean Coastal Manage., Coco et al. 2013 in Mar. Geol.). Here we target firstly river bifurcations and secondly the mutually evasive ebb- or flood-dominated channels that form around bars and are found in all sandy tidal systems in the world (van Veen 1950/2002 in J. R. Dutch Geograph. Soc.). The cause for the mutual evasion is still incompletely understood despite the fact that they also appear in our numerical model results and experiments (Canestrelli et al., in JGR 2010; Kleinhans et al. 2015 in JGR). The nodes where ebb and flood channels connect can be seen as asymmetric bifurcations where one channel is preferred during ebb and the other during flood. Such bifurcations are critical elements that partition flow and sediment through the channel network, govern bar merging and splitting and are locations where bed steps form in shipping lanes, as in river bifurcations. Stability and equilibrium configurations are mostly unknown for tidal bifurcations except for one recent theory (Wang et al in prep.). In particular, we have a fair understanding of the tidal dynamics, but this is incomplete for the morphodynamics, especially related to understanding the sediment division at the bifurcation. We take advantage of the better but yet incomplete understanding of river bifurcations. The stability of river bifurcations has been studied for two decades in fieldwork, experimentation, linear stability theory and numerical modelling (e.g. Wang et al. 1995, JHR, see review in Kleinhans et al. 2013, ESPL) and our recent theory (Bolla Pittaluga et al. 2015 in GRL) synthesises many of the earlier results as follows: In bedload-dominated rivers, symmetrical bifurcations are unstable and develop towards a highly asymmetrical division of discharge and sediment. The same is the case for suspended sediment-dominated rivers, but the theory predicts stable bifurcations for intermediate sediment mobility. However, there is very little data for conditions intermediate between low and high mobility rivers. Moreover, we have no idea whether bifurcations in reversing tidal flow are unstable for similar configurations and conditions as in rivers. Here we mean configurations that are entirely free of topographic forcings on the flow: straight channels split into two channels over some length and depth. Our objective was therefore to experimentally investigate bifurcation stability in a range of sediment mobilities in unidirectional flow and reversing tidal flow ceteris paribus.以亚穆纳河（Jamuna）、湄公河（Mekong）为代表的多汊河道（Multithread rivers）拥有随洪水事件频繁变迁的汉道与沙洲网络。以斯海尔德河（Scheldt）、亨伯河（Humber）、哥伦比亚河口湾（Columbia estuaries）以及瓦登海（Wadden Sea）、佛罗里达州小型潮汐盆地为代表的潮汐系统，则存在由落潮流与涨潮流塑造、持续动态演化且相互作用的汉道与浅滩。现有模型难以准确预报此类自然动态，而主汉道是兼具重要经济价值的通航航道，每日出露与淹没的浅滩区域同时也是生态关键栖息地。人类活动扰动、径流量变化与海平面上升正威胁着上述系统的各项功能。此外，多项研究表明，航道疏浚加深会降低城市安全水平：河道内丁坝会加剧水流阻力，河口湾的潮差亦会被进一步放大（例如Bolla Pittaluga等2015年发表于《AWR》的研究、Seminara等于2011年发表于《EH》的研究），进而引发低水位时段堤防溃决风险上升、高水位时段洪涝灾害加剧的问题。我们亟需开发动态预报模型，以优化这类兼具多重功能的系统的管理策略（Wang等2012年发表于《Ocean Coastal Manage.》的研究、Coco等2013年发表于《Mar. Geol.》的研究）。本研究首先聚焦河道分汊（river bifurcations），其次关注环绕沙洲形成、广泛分布于全球所有砂质潮汐系统中的相互避让的落潮主导汉道与涨潮主导汉道（范·文（van Veen）1950/2002年发表于《J. R. Dutch Geograph. Soc.》的研究）。尽管这类汉道在我们的数值模拟结果与物理实验中均有出现（Canestrelli等发表于《JGR》2010年的研究、Kleinhans等2015年发表于《JGR》的研究），但其相互避让的成因至今尚未完全阐明。落潮汉道与涨潮汉道的交汇节点可被视为不对称分汊结构：一条汉道在落潮时段占据主导，另一条则在涨潮时段更具优势。这类分汊结构是调控河道网络内水流与泥沙分配、决定沙洲合并与分裂过程的关键要素，同时也是航道内床面台阶形成的核心位置，与河道分汊的情形一致。除了一项新近提出的理论（Wang等待发表）外，潮汐分汊（tidal bifurcations）的稳定状态与平衡构型几乎尚未被探明。尽管我们对潮汐动力学已有较为充分的认知，但对其地貌动力学的理解仍存在显著不足，尤其是在分汊点的泥沙分配机制方面。我们将借鉴对河道分汊更为成熟但仍不完整的既有认知。河道分汊的稳定性已通过二十年的野外调查、物理实验、线性稳定理论与数值模拟得到了广泛研究（例如Wang等1995年发表于《JHR》的研究、Kleinhans等2013年发表于《ESPL》的综述研究），而我们近期提出的理论（Bolla Pittaluga等2015年发表于《GRL》的研究）可将诸多早期研究成果整合如下：在以推移质输沙为主的河道中，对称分汊结构是不稳定的，最终会演变为水沙分配高度不对称的分汊模式。以悬移质输沙为主的河道也存在同样的规律，但该理论预测，当泥沙活动性处于中等水平时，分汊结构可保持稳定。然而，针对泥沙活动性介于高低水平之间的河道的相关实测数据极为匮乏。此外，我们尚不明确，在反向潮汐流环境中，与河道分汊类似的构型与条件下，分汊结构是否同样不稳定。此处我们所指的构型是完全不受地形强迫影响的水流结构：平直河道在一定长度与深度范围内被分为两条独立汉道。因此，本研究的目标是在其余条件恒定不变的前提下，通过实验探究单向流与反向潮汐流环境中，不同泥沙活动性条件下的分汊结构稳定性。