Research article
Long-term morphodynamics of a large estuary subject to decreasing sediment supply and sea level rise

https://doi.org/10.1016/j.gloplacha.2020.103212Get rights and content

Highlights

  • Equilibrium shift occurs in an estuary with reduced sediment supply and SLR.

  • With SLR a significant amount of sediment supply is trapped in the estuary.

  • Transitional zone adjusts slowest and shifts upstream till equilibrium.

  • Under present conditions the Yangtze Estuary would experience erosion for centuries.

Abstract

Globally the sediment supply from rivers to estuaries is decreasing and the mean sea level is rising, while the effects of these changes on the long-term estuarine morphodynamics have not been fully explored. An idealized one-dimensional model was utilized to investigate the long-term morphodynamics of a large estuary subject to the changes in the sediment supply and sea level. Simplifications involved the use of a 560 km long funnel-shaped channel with fixed banks, constant input water and sediment fluxes, a single grain size and a semi-diurnal tide. A range of values of changes in the sediment supply (50–90% reduction) and sea level (1–5 mm/yr increase) were considered. Starting from an equilibrium state for an initial sediment supply, the estuary shifts to a new equilibrium for the considered changes. The shift is completed on a timescale of millennia, while 50% of the bed level change occurs within several hundreds of years. A larger decrease in the sediment supply results in a stronger bed erosion, while the corresponding adjustment time has minor changes in its range for the considered sediment reduction. In addition to the reduced sediment supply, under sea level rise the erosion is weakened and the adjustment time is shortened. The equilibrium under the considered rates of sea level rise is characterized by a bed level keeping pace with the sea level and a significant amount of sediment being trapped in the estuary. Additional numerical experiments that use the real geometry and more realistic forcing of the Yangtze Estuary show that overall erosion of the estuary is expected in the coming centuries.

Introduction

The global construction of dams on rivers has led to decreases in the riverine sediment flux to seas (Syvitski et al., 2005, and references therein). Examples of large rivers with significant decreases in sediment flux are the Nile and Orange in Africa and the Indus and Yangtze in Asia, where the sediment fluxes have decreased by more than 50% with respect to the pre-dam period (Vörösmarty et al., 2003; Yang et al., 2011). Meanwhile, the global mean sea level is rising, with a rate of almost 2 mm/yr averaged over the 20th century, and the rise is expected to accelerate through the 21st century (Church et al., 2013; Ablain et al., 2017, and references therein). Estuaries, the key pathway for water and sediment transport that connects rivers and seas, would be affected by these changes in the riverine sediment flux and sea level (e.g. Ganju and Schoellhamer, 2010; Luan et al., 2017). However, uncertainties in the response of estuaries to these changes still remain (Nicholls and Cazenave, 2010).

The long-term (centuries to millennia) morphodynamic evolution of estuaries is controlled by both external factors, e.g., riverine water and sediment fluxes, tides and sea level change, and internal factors, e.g., geometry and erodibility of the bed. The focus of this study is on the external factors, especially the riverine sediment supply and sea level rise (SLR). During the last few decades, a few studies have been conducted to investigate the long-term morphodynamic evolution of estuaries. Different focuses have been observed. Some studies (Todeschini et al., 2008; van der Wegen and Roelvink, 2008, and references therein) focused on the long-term evolution of tide-dominated estuaries, in which the riverine water flux and sediment supply were neglected. Others (e.g. Guo et al., 2014, Guo et al., 2015a, Guo et al., 2015b, Guo et al., 2016) focused on the effect of the river flow and tides on the long-term morphodynamics of tide-and river-influenced estuaries. In most of these studies, the riverine sediment supply was assumed to be either zero or in equilibrium according to the local flow condition.

However, in reality the riverine sediment supply does not necessarily fit the local flow condition. Exceptions that considered specified values of sediment supply (not in equilibrium with respect to the local flow condition) were Canestrelli et al. (2014) and Bolla Pittaluga et al. (2015). Canestrelli et al. (2014) studied the long-term morphodynamic evolution of a 400 km long tide-dominated estuary of the Fly River, and they showed that an increase in the sediment supply results in aggradation while a decrease causes erosion of the entire estuary. In Bolla Pittaluga et al. (2015), to indicate the tide-influenced region they employed a term ‘tidal length’, which was defined as the distance from the river mouth where the tidal amplitude is reduced to 5% of the forcing amplitude. It was found that the tidal length increases (decreases) as the upstream sediment supply decreases (increases). In both studies, when the specified values of sediment supply were used, the focus was either on the equilibrium bed profile or the change in the tidal length, while the transient morphodynamic evolution was not explicitly examined.

A classification of estuaries based on the balance between SLR and sedimentation during Holocene was given in Cooper et al. (2012), i.e., drowned estuary, and estuaries in which sedimentation either keeps pace with sea level or occurs after initial drowning. Regarding the effect of SLR on the long-term estuarine morphodynamics, both behavior-based models (Sampath et al., 2011, Sampath et al., 2015; Sampath and Boski, 2016, and references therein) and process-based models (Elmilady et al., 2019, and references therein) have been employed. The former are based on empirical rules from observations, which are relatively more efficient but are unable to represent the estuarine physical processes. Most of the process-based model studies focus on short tidal basins, tidal inlets or salt marshes (e.g. van der Wegen, 2013; Yin et al., 2019; Best et al., 2018), while studies of long estuaries (order of 100 km) are limited. Moreover, few of these studies have considered the decreasing sediment supply. In Ganju and Schoellhamer (2010), four scenarios were conducted to estimate future geomorphic changes in a short tidal bay (~40 km) subject to changes in the sea level, water discharge and sediment supply within a relatively short period (30 years). They found that the net deposition in the estuary was reduced when the sediment supply decreased, and the deposition did not keep pace with SLR for all the cases. In Guo et al. (2016), one case was conducted to check the effect of SLR on the equilibrium bed profile of the schematized Yangtze Estuary, in which no fundamental changes were observed. Again, in this study the input sediment flux was assumed to be in equilibrium. Taking both changes in the sediment supply and the sea level into account, Luan et al. (2017) modeled the morphological evolution of the Yangtze Estuary on a decadal timescale. They found that the mouth zone would shift from net deposition before 2010 to net erosion by 2030. However, their focused area was on the river mouth and the considered timescale was relatively small.

One-dimensional (1D) models with idealized geometries, i.e., rectangular or funnel-shaped channels, have been widely used to study long-term estuarine morphodynamics (e.g. Guo et al., 2014; Canestrelli et al., 2014; Bolla Pittaluga et al., 2015). It has been shown that for the 1D setting, tide-influenced estuaries reach a dynamic equilibrium, which is characterized by an unchanged bed profile. In those 1D model studies, usually the initial bed level was assumed to be a constant-sloping bottom, and the focus was on the final equilibrium. Although initial conditions have little effect on the final bed profile, they are important during the adjustment period (Todeschini et al., 2008; Canestrelli et al., 2014). Assuming that an estuary is in an equilibrium before the period of substantial reduction of sediment supply, how the estuary adjusts to the decreasing sediment supply and the rising sea level in time and space has not been fully explored yet.

The Yangtze Estuary is an example which has experienced a substantial decreasing sediment supply during the last few decades. Worldwide the Yangtze ranks fifth in average annual water flux (~900 km3/yr) and fourth historically in average annual sediment flux (~470 Mt./yr in the 1960s) (Milliman and Farnsworth, 2011). Fig. 1 shows the map of the Yangtze Estuary from the tidal limit Datong to the river mouth, the annual sediment and water fluxes from the 1950s at Datong and the width of the main channel. It is seen that the change in the annual water flux is small, which is mainly due to precipitation, while a substantial decrease in the annual sediment flux is observed since the 1980s, which is mainly related to extensive dam construction in the river basin (Yang et al., 2015).

The aim of this study is to explore the response of a large river- and tide-influenced estuary to the decreasing sediment supply and SLR in the long term. More specifically, the following questions will be addressed. First, what is the relationship between the decrease in the sediment supply and the morphodynamic adjustment, specifically the bed erosion and adjustment time? Second, in addition to the reduced sediment supply, what is the effect of SLR on the morphodynamic adjustment of the large estuary in the long term? To answer these questions, the Delft3D model (Lesser et al., 2004) is utilized with idealized 1D settings, which involves simplifications in the boundary conditions, geometry, sediment characteristics and sediment transport formulation. Waves are excluded as the attention here is more into the tide-influenced estuaries where the wave effect on the estuarine morphodynamics is relatively small. The reason to use the idealized settings is that the focus here is to explore the mechanism as a first step for further investigations that consider more complexities of the system, rather than to reproduce the morphology of a natural estuary as close as possible.

The manuscript is organized as follows. In Section 2, the model and its setup are introduced. Results are presented in Section 3, followed by discussion in Section 4. Finally, Section 5 presents the conclusions.

Section snippets

Model

Simplifications were made in the model geometry to explore the long-term morphodynamic evolution. A 1D model domain was used, in which the channel curvatures were neglected, the river banks were fixed and the cross sections were assumed to be rectangular (see e.g. Lanzoni and Seminara, 2002; Todeschini et al., 2008; Nicholls and Cazenave, 2010; Guo et al., 2014). The use of fixed river banks is essentially based on the assumption that the channel width changes on a larger time scale than the

Validation of the model

Fig. 3 shows the comparison between modeled results without tides and the analytical solutions for a range of values of sediment concentration. An excellent agreement is observed. For all the cases, CC is larger than 0.99 for the bed level zb, water depth D and velocity u, and RMAE is less than 0.01 for D and u and is less than 0.13 for zb.

Effect of decreasing sediment supply on long-term estuarine morphodynamics

Fig. 4 shows the bed level zb, water level ζ and water depth D at high tide at several times and the contour of bed level change Δzb as a function of

Equilibrium shift of a river- and tide-influenced 1D estuary

For an estuary influenced by both river flow and tides, it has been shown in several studies (e.g. Canestrelli et al., 2014; Guo et al., 2014; Bolla Pittaluga et al., 2015) that for a 1D setting the estuary will reach a dynamic equilibrium in a long time (millennia). For a constant water discharge, the equilibrium is characterized by a spatially uniform tidally averaged sediment flux in the estuary, i.e., the riverine sediment flux is flushed into the sea completely.

Fig. 9 shows the tidally

Conclusions

The role of the decreasing sediment supply (50%–90% decrease) and SLR (1–5 mm/yr) in the long-term morphodynamic evolution of a large river- and tide-influenced estuary was investigated by employing an idealized 1D model. Constant riverine water flux and sediment supply and a semi-diurnal tide were applied to a 560 km long funnel-shaped estuary with the fixed width, unlimited erodible thickness of the sandy bed and an equilibrium sediment flux at the mouth. Main findings are as follows.

For the

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.

Acknowledgments

The authors thank the three anonymous reviewers for their constructive comments. The research is funded by the Ministry of Science and Technology of China (2016YFE0133700 and 2016YFA0600901) and the National Natural Science Foundation of China (51779121 and 51679125).

References (60)

  • H.L. Luan et al.

    Process-based morphodynamic modeling of the Yangtze Estuary at a decadal timescale: Controls on estuarine evolution and future trends

    Geomorphology

    (2017)
  • X.X. Luo et al.

    The impact of the Three Gorges Dam on the downstream distribution and texture of sediments along the middle and lower Yangtze River (Changjiang) and its estuary, and subsequent sediment dispersal in the East China Sea

    Geomorphology

    (2012)
  • J.A. Roelvink

    Coastal morphodynamic evolution techniques

    Coast. Eng.

    (2006)
  • D.M. Sampath et al.

    Morphological response of the saltmarsh habitats of the Guadiana estuary due to flow regulation and sea-level rise

    Estuar. Coast. Shelf Sci.

    (2016)
  • D.M. Sampath et al.

    Modelling of estuarine response to sea-level rise during the Holocene: Application to the Guadiana Estuary-SW Iberia

    Geomorphology

    (2015)
  • J. Sutherland et al.

    Evaluation of coastal area modelling systems at an estuary mouth

    Coast. Eng.

    (2004)
  • B. Van Maanen et al.

    Modelling the effects of tidal range and initial bathymetry on the morphological evolution of tidal embayments

    Geomorphology

    (2013)
  • C.J. Vörösmarty et al.

    Anthropogenic sediment retention: Major global impact from registered river impoundments

    Glob. Planet. Chang.

    (2003)
  • S.L. Yang et al.

    Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next half-century

    Estuar. Coast. Shelf Sci.

    (2003)
  • S.L. Yang et al.

    50,000 dams later: erosion of the Yangtze River and its delta

    Glob. Planet. Chang.

    (2011)
  • S.L. Yang et al.

    Downstream sedimentary and geomorphic impacts of the Three Gorges Dam on the Yangtze River

    Earth Sci. Rev.

    (2014)
  • H. Yang et al.

    River-sea transitions of sediment dynamics: a case study of the tide-impacted Yangtze River estuary

    Estuar. Coast. Shelf Sci.

    (2017)
  • Y. Yin et al.

    Numerical modelling of hydrodynamic and morphodynamic response of a meso-tidal estuary inlet to the impacts of global climate variabilities

    Mar. Geol.

    (2019)
  • F. Zhang et al.

    Seasonal hydrodynamic interactions between tidal waves and river flows in the Yangtze Estuary

    J. Mar. Syst.

    (2018)
  • Z. Zhou et al.

    Modelling the role of self-weight consolidation on the morphodynamics of accretional mudflats

    Environ. Model. Softw.

    (2016)
  • L. Zhu et al.

    The influence of human activities on morphodynamics and alteration of sediment source and sink in the Changjiang Estuary

    Geomorphology

    (2016)
  • M. Ablain et al.

    Satellite altimetry-based sea level at global and regional scales

    Surv. Geophys.

    (2017)
  • S. Best et al.

    Do salt marshes survive sea level rise? Modelling wave action, morphodynamics and vegetation dynamics

    Environ. Model. Softw.

    (2018)
  • M. Bolla Pittaluga et al.

    Where river and tide meet: the morphodynamic equilibrium of alluvial estuaries

    J. Geophys. Res. Earth Surf.

    (2015)
  • J.A. Church et al.

    2013: sea level change

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