Modeling the infilling process of an abandoned fluvial-deltaic distributary channel: An example from the Yellow River delta, China
Introduction
River avulsion near the marine terminus is a fundamental process that controls coastal sediment deposition, forming numerous morphological patterns and environmental settings including abandoned deltaic lobes and channels (Chatanantavet et al., 2012) and occurring frequently on large rivers with high sediment flux such as the Mississippi and Yellow Rivers. By acting as sediment repositories for fluvial-marine sediments, abandoned deltaic channels are important to the development of coastal morphology. Sediment deposits in these settings record hydrodynamic variations such as tides and waves. Various publications have focused on delta lobe erosion after fluvial abandonment (Frazier, 1967; Penland et al., 1988; Wang et al., 2006; Nienhuis et al., 2013; Zhou et al., 2014), but the depositional processes within abandoned channels rarely have been examined (Carlson et al., 2020).
Upon losing a direct water-sediment supply from the main river, the hydrodynamical environment of an abandoned deltaic channel is primarily influenced by tides and waves, often leading to the evolution of a tidal flat (Carlson et al., 2020). Within the intertidal zone, the linkage between subaerial and submarine environments is a significant component of coastal and estuarine systems; these environments are characterized by a shallow gradient and are therefore susceptible to repeated inundations that impact sediment dynamics. Over the last few decades, tidal flat morphology has been studied extensively, and several numerical models for the evolution of intertidal landforms have been proposed. Friedrichs and Aubrey (1996) advanced a theory of intertidal flat morphology (referred to herein as “FA96”). According to FA96, the maximum tidal velocity is assumed to be stable in subtidal and lower flat regions and to decrease in the upper flat with distance from the low water line. This provides a useful analytical model for summarizing morphodynamical processes. More recently, multiple mechanisms have been suggested to describe the sedimentation over tidal flats to shape the shore-normal profile (Lee and Mehta, 1997; Le Hir et al., 2000; Roberts et al., 2000; Pritchard et al., 2002; H. J. Lee et al., 2004;). The core concept of muddy profile progradation is primarily derived from mass conservation, yielding a dynamic equilibrium theory (DET) proposed by Friedrichs (2011) with spatial and local asymmetries (Hsu et al., 2013; Hu et al., 2015; Hu et al., 2018). Mariotti and Fagherazzi (2010) developed a comprehensive numerical model for long-term evolution of salt marshes and tidal flats and discussed the morphological evolution of tidal channels and unvegetated tidal flats (Mariotti and Fagherazzi, 2013). Xu et al. (2019) investigated a two-dimensional model for the development of tidal channels using flow and sedimentation models. Vegetation and ecogeomorphic factors significantly influence tidal flat and tidal channel evolution (Fagherazzi et al., 2012; Belliard et al., 2015; Kearney and Fagherazzi, 2016).
These classical models have several limitations when applied to the evaluation of the morphological development of an abandoned deltaic channel. First, abandoned channels, especially those in mud flats (the object of study here), are complex sedimentary environments that are very different from the classic tidal flat. Compared with an open and broad coastal zone tidal flat, tide water intrusion into an abandoned channel is restricted by identifiable levees from the antecedent river channel, which affects the hydrodynamic environment. Concurrently, an abandoned channel maintains a morphological structure that is highly variable, inheriting the form of the previously active channel, and thus can differ from a traditional tidal channel formed by ocean dynamics. For example, channel depth is initially set by the antecedent bed before abandonment, and as the estuary develops, this morphology naturally changes as a consequence of sediment deposition and erosion (Carlson et al., 2020). In turn, this produces a feedback effect on the maximum/minimum flood and ebb water surface elevations (respectively). Second, classical tidal flat models are developed based on the hypothesis that sediment is transported exclusively by periodic tides and waves and deposited on a relatively flat surface (Friedrichs, 2011), and thus the flat is a net sediment sink. While an abandoned lobe may behave similarly, it can also be eroded as sediment is recycled and transported landward before deposition on a flat. Thus, the sediment sources for abandoned channels are nonunique. A new approach is therefore required to model this dynamic in-filling regime for an abandoned deltaic channel while preserving sediment mass conservation.
This study establishes a quantitative model to describe sediment depositional processes and morphological development associated with the infilling processes of an abandoned fluvial-deltaic channel, with the core concepts of the model grounded in mass conservation. In-situ observational data collected from an abandoned distributary channel of the Yellow River delta (China) are used to constrain a filling model and test its accuracy when using different boundary conditions to explore the model sensitivity. Additionally, sediment deposition rates and infilling volume are evaluated by analysis of sediment samples, and the model results are inverted to constrain filling processes from 1996 to 2015.
Section snippets
Model description
The establishment of a depositional model with appropriate boundary conditions describing the environmental situation is required. According to previous work (Carlson et al., 2020), the abandoned channel is influenced by sedimentation resulting from tide-induced overbank deposition, which is especially enhanced under spring tide conditions. This creates a gentle channel-normal slope as sediment accumulates over time onto the adjacent tidal channel. Additionally, the tidal channel possesses a
Study area and data source
As the second largest river in the world, with sediment load maximums of up to 1.08 × 109 tons per year (Milliman and Meade, 1983; Milliman and Syvitski, 1992), the Yellow River is regarded as an excellent natural laboratory for investigating the healing processes of abandoned channels. Frequent channel avulsions (approximately once every decade) and the resulting lobate landforms built by each event comprise the modern Yellow River delta complex (Pang and Si, 1979; Wang et al., 2006; Xue, 1993
Channel landform after abandonment
As supported by the above analyses, the present model effectively simulates the infilling processes in the abandoned channel, particularly for parameters p1 = − 0.087, p2 = 0.584, a = 2.55 × 10−10, and i = 0.8. The model produces a predicted sediment distribution pattern within the Qingshuigou channel after 20 years of filling (Fig. 9) based on parameter values including model unknown parameters (a, i, p1, p2) and environmental parameters including the sediment net flux, sediment density,
Conclusion
As composite landforms at the ocean interface, the morphological evolution of abandoned channels with multiple sediment sources differs importantly from that of general tidal flats. The main body of the abandoned channel is influenced by the morphological structure of the antecedent channel, with identifiable levees that constrain tidal flows and result in a large tidal flat with a shore-normal slope and a shore-parallel slope that provides a sediment source for mud flat development.
A
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.
Acknowledgements
We appreciate the constructive comments from both editors and reviewers that greatly improved the science and quality of the original manuscript. This work was primarily funded by National Natural Science Foundation of China (NSFC, grants No. 41525021, 41806101), Ministry of Science and Technology of China (2016YFA0600903 and 2017YFC0405502), the Shandong Provincial Natural Science Foundation (No. ZR2018BD028) and the Taishan Scholar Project (No. TS20190913). J.A.N. acknowledges support from
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