Modeling the infilling process of an abandoned fluvial-deltaic distributary channel: An example from the Yellow River delta, China

Highlights

• A model was proposed to quantify the in-filling process of an abandoned channel.

• Landward flux and channel depth are dominant variables for the in-filling process.

• Shore-parallel flow shapes the mudflat morphology at the late stage of abandonment.

• The abandoned channel acts as a sediment sink in the deltaic depositional system.

Abstract

As an inevitable result of fluvial-deltaic avulsions, abandoned distributary channels and associated lobes form an important component of coastal marine systems, serving as a sediment source and sink for the adjacent delta region. The distinct differences between normal tidal flats and abandoned channel systems restrict the application of classic intertidal mud flat morphological theory to the study of the evolution of abandoned channels. Herein, a quantitative model based on the concept of mass conservation was proposed to describe the infilling processes for an abandoned channel. Sediment cores and elevation profiles from an abandoned channel of the Yellow River delta (China), along with hydrodynamic observational data, are used to constrain a numerical approach and test model performance. The modeled accumulation thicknesses are satisfyingly consistent with those derived from sediment cores. The model results indicate that the tide-delivered sediment flux and channel water depth are the dominant variables governing the variation in accumulation thickness in the abandoned channel, whereas the channel shortening rate largely impacts the depositional area rather than depositional thickness. By contributing to steady deposition on the mud flat, tide-associated landward sediment deposition formed a steep profile near the estuary, while the final four years of deposition represent only a quarter of the total deposition compared to that from the first five years owing to a decreasing depositional rate. The model-derived sediment volume indicates that shore-parallel flow is significant in shaping mudflat morphology, especially in the late stages, while the shortening of tidal channels leads to shrinking of the mud flat and sediment volumes. Despite its limitation to residual channels, our model presents a new approach for quantitative interpretation of the geomorphic evolution of an abandoned river channel with finite boundary conditions, providing a good reference to apply to other large deltaic systems.

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.