The combined effect of discharge and tides on low-angle dune evolution at the tidal current limit of the Changjiang Estuary
Introduction
Bedforms are ubiquitous features existing at various spatio-temporal scales and display a range of morphologies in environments ranging from deserts to oceans and even on Mars (Smith, 2014). Subaqueous dunes are the most prominent and dynamic bedforms in alluvial rivers and estuaries (Best, 2005; Parsons and Best, 2013; Reesink et al., 2018). Moreover, all fluvial and estuarine environments display temporal variations in flow discharge and water level, creating unsteadiness (Martin and Jerolmack, 2013). Thus, the subaqueous dunes change in size and shape (i.e., deformation) over time and in space. The migration and deformation of dunes, in turn, affects the flow and sediment-transport dynamics above in the water column (Reesink et al., 2018). Interactions between bedforms, flow and sediment transport are thus inextricably linked and recognized as “chicken-or-egg problems” (Costello and Southard, 1981) or “self-organisation” (Gyr and Kinzelbach, 2004).
Our understanding of subaqueous dune dynamics and dune adaptation in unsteady flows mainly focuses on flume-based results (e.g., Martin and Jerolmack, 2013; Warmink et al., 2014; Reesink et al., 2018) with flow depth less than 1 m (Venditti, 2013) or 2.5 m (Bradley and Venditti, 2017), called shallow dunes. However, recent research (Bradley and Venditti, 2017; Kostaschuk and Venditti, 2020) has highlighted that shallow and deep dunes are affected differently by changes in the dominant process of sediment transport. Asymmetric high-angle dunes (HADs, leeside slope > 24°) dominate in shallow flows while symmetric low-angle dunes (LADs, leeside slope < 24°; often <10°) commonly exist in deep flows, i.e., natural large rivers and estuaries (Cisneros et al., 2020; Kostaschuk and Venditti, 2020). Dominated by bedload transport, HADs migrate via granular avalanche, whereas LADs, commonly exist under suspended-load dominated conditions and maintain their shape due to suspension deposition (e.g., Kostaschuk et al., 2008, Kostaschuk et al., 2009), liquefied avalanches (e.g., Hendershot et al., 2016; Kostaschuk and Venditti, 2020) and downslope currents (Kwoll et al., 2016, Kwoll et al., 2017). Additionally, several contributing processes also affect LAD dynamics, such as flow unsteadiness, bedform superimposition, leeside fallout patterns and turbulence modulation by suspended sediment (Best et al., 2020; Cisneros et al., 2020). Thus, the results of flume experiments require greater analysis before being applied to natural systems. Growing evidence from field observations suggests that symmetrical LADs are the prominent bedforms in large rivers (Bradley and Venditti, 2017; Kostaschuk and Venditti, 2020; Cisneros et al., 2020). Therefore, advancing our understanding of LADs dynamics is the key to improving our ability to accurately predict the evolution of bedforms, channel roughness, and consequently simulation of water level, especially for the construction of flood protection measures (Paarlberg et al., 2010).
To date, most research has focused on investigating dune dynamics during flood events, as the growth and migration of large-scale dunes may lead to riverbank erosion and instability of subfluvial tunnel (Amsler et al., 1997; Ten Brinke et al., 1999; Julien et al., 2002). Dunes grow by amalgamation during the rising limb of a flood wave, but the different adaptation time of dune height and length leads to a hysteresis between dune size and flow condition (Martin and Jerolmack, 2013). Moreover, larger roughness during floods may be caused by the hysteresis differences between dune height and length, resulting in a higher water level than expected (Warmink, 2014). Limited research (e.g., Hendershot et al., 2016; Hu et al., 2018) has investigated the response of low-angle dunes to tides, but investigations are limited to few combinations of certain discharge and tide. The systematic dune response under different combined effects of discharge and tides has not been fully investigated to our knowledge.
The tidal current limit, i.e., the position in an estuary where the flood tidal current is zero, defines the critical region for the conversion between discharge and tidal current. It moves upstream and downstream, as it is highly sensitive to variations in discharge and tide. Thus, under this variable environment, the bed strives to maintain balance with the changing flow strength by frequently adjusting the roughness elements, i.e., bedforms (Reesink et al., 2013; Reesink et al., 2018). Additionally, compared with unidirectional rivers, the grain size of bed material in estuaries influenced by tides is finer. Recent research has highlighted that the effect of cohesive material (mud, clay and microorganisms) on bedform geometry and dynamics is considerable (Malarkey et al., 2015; Baas et al., 2016). Besides, previous research has proved that LADs pervasively exist in this area and developed relationships between channel morphology, flow strength and bedform morphology (Cheng et al., 2004; Chen et al., 2012; Zheng et al., 2017). However, in channels under the combined effect of discharge and tides, results from single measurements are not robust enough to reveal dune characters and dynamics. Thus, long-term measurements under different discharges and tides are the key to further understandinggela how roughness elements respond to changing flow conditions (Hu et al., 2018).
Herein, we take high-resolution repeat measurements at the tidal current limit of the Changjiang Estuary, where LADs exist. We investigate how LADs respond to both seasonal and tidal variations and identify their distinct effects. These findings will be valuable for improving the accuracy of numerical modeling and also for advancing paleo-environmental reconstruction applied to the geological record.
Section snippets
Field setting
The tidal reach of the Changjiang River is called the Changjiang Estuary and starts from the tidal limit Datong located nearly 600 km upstream from the entrance to the East China Sea (Fig. 1a). The Datong hydrology station is generally recognized as the controlling station for measuring hydrodynamics and sediment discharge into the sea (Yang et al., 2010). The Changjiang Estuary is characterized as a meso-tidal estuary in terms of tidal range (Fan and Li, 2002; Wu et al., 2009), varying from
Dune variation between the late flood and dry season
Fig. 1c and d display how the bed looks like during the late flood and late dry season (more details could be found in the supplementary material). In general, dunes generated in the late dry season (March 2017) are relatively larger and more regular, compared with those detected in the late flood season (October 2016). Furthermore, the dune crestlines in the late flood season are straighter, while those in the late dry season are relatively sinuous. In order to detail the changes of dunes
The impact of discharge on dune evolution throughout the year
Previous research on dune morphodynamics is summarized in Table 5: (1) compound dunes were observed in Lefebvre et al. (2011), Lefebvre et al. (2013) and Zheng et al. (2017) that primary dunes are nearly one order of magnitude larger than the superimposed ones, and compound dunes commonly generated under circumstances with smaller Froude number (); (2) only simple dunes were detected in Kostaschuk and Best (2005) and Hendershot et al. (2016) with a relatively higher Fr. Therefore,
Conclusions
It is critical to understand dune dynamics under the combined effect of discharge and tides in order to evaluate the sediment transport and morphodynamical evolution in tidally influenced areas. Our observations from the Changjiang estuary show that near the tidal current limit, the seasonal variation of discharge could result in a significant change of dune size and shape. Compound dunes generate during the falling limb of the flood season, as flow changes faster than dunes can adjust. During
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was supported by the China Scholarship Council (granted to Hao Hu). This research was also supported by grant 41606109 from National Natural Science Foundation of China, grant NE/I014101/1 from the UK Natural Environment Research Council (NERC) and grant 51761135023 from Cooperative Research Project between National Science Foundation of China, Dutch Research Council and UK Research Councils.
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