Research article
The morphodynamic difference in the western and southern coasts of Laizhou Bay: Responses to the Yellow River Estuary evolution in the recent 60 years

https://doi.org/10.1016/j.gloplacha.2020.103138Get rights and content

Highlights

  • An arc high-velocity zone of tidal currents formed in the front area of the Yellow River (YR) delta lobe.

  • The western and southern coasts of Laizhou Bay (LZB) have shown significant differences in the geomorphic evolution.

  • The tidal shear frontal zone could have hindered the suspended sediment of the YR from transporting into southern LZB.

  • The YR estuary evolution had a significant geomorphic impact on the coast of western LZB.

Abstract

The Yellow River Estuary (YRE), which is famous for its abundant sediment, rapid accretion, and frequent migration, has experienced a remarkable decrease of sediment transported by the Yellow River (YR) during the past 60 years. By taking the west coast (Yellow River Delta, YRD) and the south coast of Laizhou Bay (LZB) as a unified coastal unit, this paper has analyzed the evolution of shorelines, isobaths, and geomorphic erosion/accretion during the last 60 years based on the remote sensing images and nautical charts; these data were also used to reveal the depositional dynamic mechanisms controlling the coastal geomorphic responses to YRE evolution in a hydrodynamic numerical simulation. Then, the coastal dynamics, subaqueous slope accretion, and geomorphic evolution were integrated spatially. Finally, the impacts of YRE evolution, including changes of the position and amount of sediment loading into the sea, on the coastal geomorphic evolution of western–southern LZB during the recent 60 year period were elucidated. Results showed that outside of the YRE delta lobes, there has always been an arc high-velocity zone, outside of which there has existed a northeast arc tidal shear frontal zone during the last 60 years. As the estuary delta lobes extended into the sea, the high-velocity zone moved toward the southeast, where its range and velocity also increased. As an overall trend, the duration of the tidal shear frontal zone (TSFZ) has grown longer and longer over a whole tidal cycle, while the interval time has grown shorter and shorter; moreover, the span between the southern and northern endpoints has grown larger and larger, while the southern end always remained confined to a fixed position in the northeast sea area of Qingtuozi. Under the control of TSFZ, the suspended sediment of the Yellow River deposits nearly entirely in the arc area in the vicinity of the estuary delta lobes on the west coast, and thus, the sediment cannot cross TSFZ and move into southern LZB. As a result, the western and southern coasts of LZB have shown significant differences in the evolution of shorelines, isobaths, and geomorphic erosion/accretion during the last 60 years. That is, these features have changed dramatically in the west coast but have remained relatively stable in the southern coast, and sediment deposition is now greatly reduced in the southern LZB. Therefore, the YRE evolution had a significant geomorphic impact on the coast of western LZB where the YRD is located, while it had only a slight impact on southern LZB.

Introduction

Physical environments and ecological systems are fragile along coastlines, which form the boundaries between landmasses and oceans around the world (Yu et al., 2015). However, large portions of the population and numerous economic activities are concentrated in narrow regions of less than 100 km along the coast (Airoldi and Beck, 2007; Xia, 2009). Under the background of global change and the increasing intensity of human activities, serious environmental and ecological problems have been manifested in the coastal zone and in river basins far from the coast. One such problem is the remarkable decrease in the runoff and sediment load of many large rivers that empty into the sea around the world, and the geomorphic effects on coasts caused by these changes in estuary-delta regions have become an important obstacle to the sustainable development process of prospective coastal zones since the 20th century (Wang et al., 2005; Wang et al., 2006a, Wang et al., 2006b; Ericson, 2006).

The Yellow River Estuary (YRE) is famous for its abundant sediment, rapid accretion, and frequent migration because of the depositional characteristics of the Yellow River and the small tide range of LZB. As a result of the rapid accretion and frequent migration of the YRE, the Yellow River Delta (YRD) has formed into a typical fan shape that is composed of several sub-deltas. The YRE migration together with the sudden disappearance of the sediment supply along the old sub-delta coast and the sudden appearance of it along the new sub-delta coast has occurred repeatedly over time-scales ranging from decades to centuries, and the erosional coast made up of coarse sediments alternates with the depositional coast made up of clay and silt along the YRD (Chen, 1982; Cai, 1995; Saito et al., 2000). Additionally, this region has been influenced by human activities and global climate change, and the sediment of the YRE has significantly decreased or even disappeared temporarily over the past 60 years (Wang et al., 2007, Wang et al., 2011). So, the decrease of sediment and the change of the estuarine location are intertwined.

Research on the coastal morphodynamic effects of the decrease of the YRE sediment into the sea has aroused great interest, which has been focused on the estuarine dynamic processes of accretion (Jiang et al., 2000; Chu et al., 2006; Fan et al., 2006; Liang et al., 2007; Wang et al., 2007; Qiao et al., 2008, Qiao et al., 2010; Bi et al., 2011; Yang et al., 2011a, Yang et al., 2011b), the transport and dispersal paths of the suspended sediment into LZB (Jiang et al., 2000; Qiao et al., 2010), and the geomorphic erosion/accretion evolution of the abandoned deltas (Li et al., 2000; Chu et al., 2006; Wang et al., 2006b; Wang et al., 2007; Wang et al., 2011; Ma and Li, 2010; Cui and Li, 2011; Qiao et al., 2011). However, there has been little research on the spatial scope of the coastal geomorphic change in response to the YRE evolution (including its decrease in sediment and migration of the estuarine location during the last 60 years) with a focus on the geomorphic responses of the southern coast of LZB adjacent to the YRE and the western coast of LZB along the YRD.

Studies on some large rivers have shown that delta recession/progradation is the most significant geomorphic effect that has resulted from the decrease of river sediment loads into the sea around the world (Yang et al., 2003; Wang et al., 2005; Wang et al., 2006a, Wang et al., 2006b; Ericson, 2006). In regard to such changes, coastal erosion, apart from shoreline erosion, includes the accretion and erosion of subaqueous deltas (Yang et al., 2003; Yang et al., 2011a, Yang et al., 2011b; Dallas and Barnard, 2011; Gao et al., 2011). Recently, some researchers have pointed out that there is a time lag between the disruption of the sediment supply by dams and coastal erosion (Huang, 2011). One reason for these observations might be that the hydrologic stations are located far away from the coast or in the upstream area of the dam, while the scouring of the channel in the downstream portion of the reservoir adds a new sediment source that can offset partially or entirely the reservoir's impact on fluvial sediment transport (Phillips et al., 2004; Wang et al., 2008; Slattery and Phillips, 2011; Jeffrey and Enrica, 2014).

By taking the southern and western coasts of LZB as a unified coastal unit, we analyzed the depositional dynamic processes, subaqueous slope erosion/accretion, and shoreline evolution of western–southern LZB and the differences in responses between western and southern LZB during a recent 60 year period; then, the results were used to reveal the dynamic mechanisms of Yellow River sediment transport and deposition as well as the spatial scope of the impacts of YRE evolution on coastal geomorphic change. These results will not only deepen the knowledge about the geomorphic evolutionary impacts of the estuary evolvement on the delta and adjacent coast over a decadal time-scale, but also enrich the studies of the relationship between human activities in the river basin and the coastal geomorphic evolution, and thus provide coastal geomorphologic support for engineering, disaster management and sustainable development activities in the coastal zone.

Section snippets

Regional background

LZB, which is located in the southern part of the Bohai Sea, was separated from adjacent Bohai Bay by the modern YRD after the course of the Yellow River changed as it flowed into the Bohai Sea in 1855 CE (Zhuang et al., 1991; Edition Committee of the Bay Chorography in China, 1993). With a dividing line from the Diaolongzui delta lobes to the YRE delta lobes, the water depth is 10–17 m in northeastern LZB and less than 10 m in southwestern LZB. In terms of landforms, along the western coast of

Data processing

The nautical charts and remote sensing images covering both western and southern LZB at different time points were chosen as the data sources for this study in accordance with the mutation times of the Yellow River sediment load and the history of YRE evolution and coastal land use over the past 60 years. Specifically, the nautical charts with a scale of 1:150,000 were surveyed in 1959, 1984, 2002, and 2009 based on Mercator projections and theoretical depth datum (TDD) information. The remote

Migration of the shoreline and isobaths

The results of the digital shoreline analysis showed that, when taking the area east of Qingtuozi as the dividing point, the changes of both the shoreline and isobaths between western and southern LZB exhibited remarkable diversities, in which great differences existed not only in terms of the time but also in regard to the magnitudes of the migration during the past 60 years (Fig. 4). From 1959 to 1976, the coast along the southern bay retreated with a very small amplitude of change on the

The modern sedimentary pattern in the western and southern coasts of LZB

Previous research has shown that complex depositional dynamic processes exist around the modern YRE, and these processes include a buoyant plume (Jiang et al., 2000; Liang et al., 2007; Qiao et al., 2010), tidal shear front (Li et al., 1998; Li et al., 2001; Wang et al., 2007; Qiao et al., 2008), hyperpycnal underflows (Li et al., 1998; Qiao et al., 2008), and the subaqueous gravity-driven depositional process (Li et al., 1998; Chu et al., 2006; Fan et al., 2006). Besides, all these processes

Conclusion

With the loose accumulation headland of the YRE delta lobes as the center, the depositional dynamic system of western and southern LZB has developed an arc high-velocity zone. The buoyant plume, hyperpycnal underflows, and subaqueous gravity-driven depositional process, among others, all exist inside of the arc high-velocity zone. Outside of the arc high-velocity zone, there is a northeast arc tidal shear frontal zone that can be subdivided into the IFOE type and the IEOF type. In conjunction

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to this work.

Acknowledgments

We are grateful for support from the Joint Funds of the National Natural Science Foundation of China (U1706220), National Natural Science Foundation of China (41901006; 41471005; 41271016)and Natural Science Foundation of Shandong Province (ZR2019BD005; ZR2019MD040).

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