Elsevier

Geomorphology

Volume 392, 1 November 2021, 107917
Geomorphology

The combined effect of discharge and tides on low-angle dune evolution at the tidal current limit of the Changjiang Estuary

https://doi.org/10.1016/j.geomorph.2021.107917Get rights and content

Highlights

  • LAD evolution at the tidal current limit of the Changjiang Estuary is investigated.

  • Seasonal variation of discharge leads to a significant change of dune size and shape.

  • The effect of tides enhances bedload transport and accelerates compound dunes' decay.

  • The size and evolution of active dunes are controlled by fortnightly tides.

Abstract

Low-angle dunes (LADs) are recognized as the most common roughness element in large fluvial rivers and estuaries. Comprehending their development is vital to understand the flow and sediment dynamics, and paleo-environmental reconstruction of the geological record. However, our understanding of LADs evolution under unsteady flows is poorly understood. Here we investigate dune adaptation under the combined effect of discharge and tides at the tidal current limit of the Changjiang Estuary. Our observations show that compound dunes exist in the late flood season, while only simple dunes are found in the late dry season after five-month decay. The effect of tides enhances bedload transport, thereby accelerating the decay of compound dunes. The increasing deformation rate of superimposed small dunes from the trough to the crest of primary dunes results in a decrease in dune size in the neap tide, as superimposed dunes over the higher part of primary dunes decay more significantly. We find that the size of superimposed small dunes in the late flood season and simple dunes in the late dry season are significantly controlled by fortnightly tides when discharge is between 20,000 and 30,000 m3/s. It implies that compound dunes decay with crest erosion via frequent migration and deformation of superimposed small dunes between neap and spring tides. This finding could potentially help identify and interpret paleo-hydraulic environments that were influenced by tides from sedimentary records. Additionally, given the discharge of the lower Changjiang River is increasingly regulated by dams, figuring out dune adaptation to the regulated hydrograph could be used to advance our prediction of the evolution of river morphology.

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 (Fr=U/gh); (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.

References (62)

  • R.P. de Almeida et al.

    Large barchanoid dunes in the Amazon River and the rock record: Implications for interpreting large river systems

    Earth Planet. Sci. Lett.

    (2016)
  • M.L. Amsler et al.

    Discussions and closure: sand-dune geometry of large rivers during floods

    J. Hydraul. Eng.

    (1997)
  • J.H. Baas et al.

    Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

    J. Geol. Soc.

    (2016)
  • J. Best

    The fluid dynamics of river dunes: a review and some future research directions

    J. Geophys. Res. Earth Surf.

    (2005)
  • J. Best et al.

    Describing fluvial systems: linking processes to deposits and stratigraphy

    Geol. Soc. Lond. Spec. Publ.

    (2019)
  • J. Best et al.

    Why do large, deep rivers have low-angle dune beds?: COMMENT

    Geology

    (2020)
  • J.S. Bridge et al.

    Interpreting the dimensions of ancient fluvial channel bars, channels, and channel belts from wireline-logs and cores

    AAPG Bull.

    (2000)
  • H. Cheng et al.

    Tidal currents, bed sediments, and bedforms at the South Branch and the South Channel of the Changjiang (Yangtze) estuary, China: Implications for the ripple-dune transition

    Estuaries

    (2004)
  • Cisneros, J., Best, J., Dijk, T. V., Almeida, R. P. D., and Zhang, Y., 2020, Dunes in the world's big rivers are...
  • W.R. Costello et al.

    Flume experiments on lower-flow-regime bed forms in coarse sand

    J. Sediment. Res.

    (1981)
  • CWRC, C. W. R. C

    Changjiang Sediment Bulletin

  • V.B. Ernstsen et al.

    Quantification of dune dynamics during a tidal cycle in an inlet channel of the Danish Wadden Sea

    Geo-Mar. Lett.

    (2006)
  • V.B. Ernstsen et al.

    Tide-controlled variations of primary-and secondary-bedform height: Innenjade tidal channel (Jade Bay, German Bight)

  • D. Fan et al.

    Rhythmic deposition on mudflats in the mesotidal Changjiang Estuary, China

    J. Sediment. Res.

    (2002)
  • R. Fernandez et al.

    Mean flow, turbulence structure, and bed form superimposition across the ripple‐dune transition

    Water Resour. Res.

    (2006)
  • B.W. Flemming et al.

    Dimensional adjustment of subaqueous dunes in the course of a spring-neap semicycle in a mesotidal backbarrier channel environment (German Wadden Sea, southern North Sea)

    Tidal Clastics

    (1992)
  • S.L. Gabel

    Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA

    Sedimentology

    (1993)
  • R.R. Gutierrez et al.

    Discrimination of bed form scales using robust spline filters and wavelet transforms: methods and application to synthetic signals and bed forms of the Río Paraná, Argentina

    J. Geophys. Res. Earth Surf.

    (2013)
  • A. Gyr et al.

    Bed forms in turbulent channel flow

    Appl. Mech. Rev.

    (2004)
  • M.L. Hendershot et al.

    Response of low-angle dunes to variable flow

    Sedimentology

    (2016)
  • R.G. Jackson

    Hierarchical attributes and a unifying model of bed forms composed of cohesionless material and produced by shearing flow

    Geol. Soc. Am. Bull.

    (1975)
  • View full text