Morphodynamic adaptation of a tidal basin to centennial sea-level rise: The importance of lateral expansion
Introduction
Coastal areas and wetlands provide important habitats for human beings and ecosystems (Craft et al., 2009; Muis et al., 2016). However, rising sea levels are posing a threat to populated or protected areas, leading to coastal erosion, shoreline retreat, loss of salt-marshes, and increasing risk of flooding (Nicholls et al., 1999). The global mean sea-level rise (SLR) rate has been estimated at 1.8 ± 0.1 mm yr−1 between 1880 and 1980 (Douglas, 1991), increasing to 3.4 ± 0.4 mm yr−1 over the interval 1993–2014 (Nerem et al., 2010; Chen et al., 2017). Although local SLR rates vary slightly in different studies (Dangendorf et al., 2017; Frederikse et al., 2020), it is generally accepted that the rate of SLR is globally accelerating and will continue to accelerate in the future (IPCC, 2014; Chen et al., 2017). It has become a worldwide concern that tidal flat accretion in estuaries and coasts may not be able to keep pace with an accelerated rate of SLR in the coming century. This results in submergence and loss of tidal flats and salt-marshes and associated important habitats and ecosystems (Craft et al., 2009; Kirwan and Megonigal, 2013; Valiela et al., 2018), such as in the Wadden Sea (van Wijnen and Bakker, 2001; Wang et al., 2018; Lodder et al., 2019), San Francisco Bay (Takekawa et al., 2013), and the Mississippi River delta (Blum and Roberts, 2009). Global estimates suggest that 40–90% of coastal wetlands may be lost by the end of the 21st century even when considering marsh accretion and expansion (Ganju et al., 2017; Valiela et al., 2018). The decline in river-borne sediment supply and land subsidence may further accelerate the coastal wetland loss (Syvitski et al., 2009).
There is an ongoing debate about the likely impact of an accelerating rate of SLR on estuaries and deltas in the forthcoming century, which is the period of most relevance for present coastal management and planning. In river-dominated deltas, SLR causes delta submergence, shoreline recession and changes in habitat depending on the availability of fluvial sediment and the rate of SLR (van de Lageweg and Slangen, 2017). Differing from open coasts and river deltas, the impact of SLR on tidal basins and estuaries tends to be more complicated because of the non-linear behavior of tidal wave propagation, the interactions between basin geometry and tidal flats, and large-scale estuarine morphodynamic adjustment and feedback mechanism in response to SLR (Du et al., 2018; Lodder et al., 2019). Furthermore, whilst marine transgression on the open coast is invariably normal to the shoreline, the changes in an estuary are more 3-dimensional. For clarity, we consider changes along the axis (thalweg) of the estuary to take place landward, for example, if the tidal limit extends further inland. In contrast, lateral changes are those that are normal to the axis or cross-shore, such as erosion of the shoreline which causes a lateral expansion of the estuary.
Many previous studies have documented changes in tidal wave propagation and hydrodynamics when imposing a higher mean sea level on a fixed morphology (Friedrichs et al., 1990; Wolanski and Chappell, 1996; Du et al., 2018; Talke and Jay, 2020). These studies have stressed the importance of tidal basin planform variations under different water levels and consequent impacts on tidal wave propagation and sediment transport. Others have examined the large-scale response of flats and channels using aggregated models (van Goor et al., 2013; Townend et al., 2016). Examining the likely response, whilst taking account of the redistribution of sediments and the potential changes in morphology, has received far less attention. Schuerch et al. (2018) estimated that 0–30% of the global coastal wetland might be lost until 2100 provided that sediment supply remains at present levels and that there were no constraints on shoreline migration. This estimated loss is smaller than previous predictions, because of the assumed possible inland expansion, where new wetlands are created. Ladd et al. (2019) and Mariotti and Carr (2014) also stressed that sediment from the lateral erosion of tidal flats might provide sources for vertical accretion. These studies emphasize how tidal systems are able to adjust their own morphology as part of the dynamic response to SLR. They imply that the fate of a tidal system to be drowned or not, depends on its ability to accrete vertically at rates equal to or larger than SLR, and/or to migrate inland at rates faster than shoreline erosion. However, the mechanisms and modes of morphological adjustment that would enable tidal basins to adapt to SLR at the decade to century time scales, when considering both vertical accretion and horizontal migration, are not yet clear. The main evidence for possible mechanisms relies on studies of shoreline retreat and system transgression from sedimentary stratigraphic studies over historic and geological time scales (Allen, 1990; Townend and Pethick, 2002; Dalrymple et al., 2006).
Large-scale morphodynamic modeling is a powerful tool in exploring the impact of SLR on estuarine and coastal morphodynamics at the decade to century time scales. Modeling approaches range from highly schematized box-models (e.g., Rossington et al., 2007) to case studies including complex process interactions (e.g., van der Wegen, 2013). Based on an aggregated approach considering morphological equilibrium concepts (van Goor et al., 2003; Wang et al., 2018; Lodder et al., 2019), it was suggested that a tidal inlet-basin system, like those in the Dutch Wadden Sea, can survive SLR up to a rate of 15 mm yr−1 owing to the sediment import from ebb-tidal deltas. In contrast, process-based models take complex process descriptions as a starting point. This type of model has a high spatial and temporal resolution but is computationally more expensive than an aggregated approach. The morphodynamic modeling approach has been applied to schematized tidal lagoons and estuaries (Dissanayake et al., 2012; van Maanen et al., 2013; van der Wegen, 2013) and also to actual estuaries and tidal basins, such as the sub-embayments of San Francisco Bay (Ganju and Schoellhamer, 2010; Elmilady et al., 2019; Zhang et al., 2020) and the Western Scheldt Estuary (Dam et al., 2013). Most of the past studies documented that intertidal flats in tidal lagoons and estuaries are prone to drown under an accelerating rate of SLR (van der Wegen, 2013; van der Wegen et al., 2016; van de Lageweg and Slangen, 2017; Elmilady et al., 2019).
The above-mentioned modeling studies highlight the morphodynamic sensitivity to SLR rates and the high probability of drowning of tidal basins under enhanced rates of SLR scenarios. An important yet under-explored aspect of estuarine adaptation to SLR is the presence of lateral migration of the estuary shoreline, leading to an expansion of plan area under rising sea levels and its subsequent impact on tidal dynamics and morphodynamic evolution. Many tidal basins and estuaries worldwide have a convergent planform and are fringed by large areas of low-lying lands in the lower reaches, which are currently just above high water (see Fig. 1 and S1 in the Supporting Information; Dalrymple and Choi, 2007; Bamunawala et al., 2020). Moreover, the low relief of coastal plain tidal basins and estuaries implies that a relatively small increase in mean sea level can lead to a large increase in intertidal area (Kirby, 2000; Friedrichs, 2011). This will have an impact on tidal propagation, subtidal flow and salinity distribution, sediment transport and associated morphodynamic adaptations. Lateral expansion under SLR potentially allows the survival of intertidal flats and marsh systems. Sustainable coastal management strategies, e.g., by introducing more flexible flood protection schemes, could be better developed if there is more knowledge on the benefit of preserving low-lying lands. Therefore, it would be of substantial value for coastal management to understand the degree to which large-scale estuarine morphodynamics adapt to SLR of different rates.
The objective of this work is to explore the morphodynamic impact of SLR on a long tidal basin at the centennial time scale when considering the possibility of lateral expansion by using a process-based numerical modeling approach. We first outline the modeling method and settings before presenting the model results in terms of morphological evolution, tidal dynamics and net sediment transport. We then assess the impact of SLR and the implications for estuary management.
Section snippets
Method
We construct a 2D model of a schematized tidal basin based on the Delft3D software (Lesser et al., 2004) which is a process-based model widely used in modeling estuarine and coastal morphodynamics (see e.g., van der Wegen, 2013; Guo et al., 2015). The model domain is 250 km long and 100 km wide. Longitudinally, the first 150 km is prescribed as a long tidal basin which meets an open coastal ocean extending 100 km offshore (Fig. 2a). The tidal basin width at the mean sea level increases from
Morphodynamic evolution
The initial 500-year morphodynamic simulation leads to the development of meandering channels and shoal systems inside the tidal basin and the formation of an ebb-tidal delta with bifurcating channels seaward of the basin mouth. Given no external sediment sources, erosion of the channel bed and channel banks inside the tidal basin and spatial redistribution of the sediment initiates the morphodynamic changes. The ebb-tidal delta builds up rapidly by sediment export from the basin that leads to
Importance of lateral expansion
To indicate the importance of the low-lying land under SLR, we run extra simulations by removing the low-lying floodplains with elevation above the high water at the beginning of the SLR scenarios, to represent a constrained tidal basin. The shorelines within the tidal basin are then fixed and do not migrate laterally under SLR (see Fig. S12). Compared with an unconstrained tidal basin (with low-lying lands), the tidal prism still increases with rising sea levels in the constrained basin, but
Conclusions
Understanding the impact of SLR on tidal basins and estuaries in the coming 100 years is of practical interest for coastal management and human development. In this work we deployed a numerical model to explore centennial morphodynamic evolution of a schematized tidal basin with broad tidal flats in response to SLR up to a rate of 20 mm yr−1. We find that sediment export at the basin mouth increased with SLR, owing to increased hydraulic storage on the tidal flats, which favors ebb dominance.
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
This work is supported by the Ministry of Science and Technology, P.R. China (MOST) (No. 2017YFE0107400; 2016YFE0133700), Natural Science Foundation of China (Nos. 51739005; U2040216; 41876091), and the Science and Technology Commission of Shanghai Municipality (Nos. 19QA1402900; 20DZ1204700).
References (85)
The Severn Estuary of southwest Britain: its retreat under marine transgression, and fine sediment regime
Sediment. Geol.
(1990)- et al.
Do salt marshes survive sea level rise? Modeling wave action, morphodynamics and vegetation dynamics
Environ. Model. Software
(2018) - et al.
Identifying knowledge gaps hampering application of intertidal habitats in coastal protection: opportunities & steps to take
Coast. Eng.
(2014) - et al.
Morphologic and facies trends through the fluvial-marine transition in tide-dominated depositional systems: a systematic framework for environmental and sequence-stratigraphic interpretation
Earth Sci. Rev.
(2007) Tidal asymmetry and estuarine morphology
Neth. J. Sea Res.
(1986)Tidal flat morphodynamics: a synthesis
- et al.
Long-term, process-based morphodynamic modeling of a fluvio-deltaic system, Part I: the role of river discharge
Continent. Shelf Res.
(2015) Practical implications of tidal flat shape
Continent. Shelf Res.
(2000)- et al.
Morphological response of tidal basins to human interventions
Coast. Eng.
(2004) - et al.
Characterization of intertidal flat hydrodynamics
Continent. Shelf Res.
(2000)
Development and validation of a three-dimensional morphological model
Coast. Eng.
Increasing flood risk and wetland losses due to global sea-level-rise: regional and global analyses
Global Environ. Change
Modelling tides and sea-level rise: to flood or not to flood
Ocean Model.
A preliminary net sediment budget for the Humber Estuary
Sci. Total Environ.
Transient coastal landscapes: rising sea level threatens salt marshes
Sci. Total Environ.
Impact of sea-level rise on the morphological equilibrium state of tidal inlets
Mar. Geol.
Long-term surface elevation change in salt marshes: a prediction of marsh response to future sea-level rise. Estuarine
Coastal and Shelf Science
Influence of the nodal tide on the morphological response of estuaries
Mar. Geol.
The response of tropical Australian estuaries to a sea level rise
J. Mar. Syst.
An analysis on half century morphological changes in the Changjiang Estuary: spatial variability under natural processes and human intervention
J. Mar. Syst.
Is 'morphodynamic equilibrium' and oxymoron?
Earth Sci. Rev.
An approach to the sediment transport problem from general physics
US Geological Survey Prof
A holistic modeling approach to project the evolution of inlet-interrupted coastlines over the 21st century
Frontiers in Marine Science
Critical dependence of morphodynamic models of fluvial and tidal systems on empirical downslope sediment transport
Nat. Commun.
Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise
Nature
The increasing rate of global mean sea-level rise during 1993-2014
Nat. Clim. Change
sea level change
Impact of Holocene and modern sea-level changes on estuarine mangroves from northeastern Brazil
Earth Surf. Process. Landforms
Forecasting the effects of accelerated sea level rise on tidal marsh ecosystem services
Front. Ecol. Environ.
Incised Valleys in Time and Space
Long-term process-based morphological model of the western Scheldt estuary
Long-term performance of process-based models in estuaries
Proceedings of the Coastal Dynamics Conference
Reassessment of 20th century global mean sea level rise
Proc. Natl. Acad. Sci. Unit. States Am.
A morphogenic approach to world shorelines
Zeitschrift fur Geomorphologie
Morphodynamics of tidal inlet systems
Annu. Rev. Fluid Mech.
User Manual Delft3D-Flow: Simulation of Multi-Dimensional Hydrodynamic Flows and Transport Phenomena, Including Sediments
The morphological response of large tidal inlet/basin systems to relative sea level rise
Climate Change
Global sea level rise
J. Geophys. Res.
Tidal responses to sea-level rise in different types of estuaries: the importance of length, bathymetry and geometry
Geophys. Res. Lett.
Intertidal area disappears under sea level rise: 250 years of morphodynamic modeling in San Pablo Bay, California
J. Geophys. Res.: Earth Surface
A Monograph on Sediment Transport in Alluvial Streams
Barrier to and opportunities for landward migration of coastal wetlands with sea-level rise
Front. Ecol. Environ.
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