Impact of intertidal area characteristics on estuarine tidal hydrodynamics: A modelling study for the Scheldt Estuary
Introduction
Estuaries and their intertidal areas are of great importance to society, providing for a range of economic activities (e.g. ports and shipping), but also for many ecological functions such as biogeochemical nutrient cycling, biological production and the maintenance of biodiversity (e.g. Barbier et al., 2013, Costanza et al., 1997). Intertidal areas are also valued for their flood and shoreline protection function, as they attenuate wind waves and storm surges (e.g. Möller et al., 2014, Möller et al., 2001, Stark et al., 2016, Stark et al., 2015, Temmerman et al., 2013). However, the ecological value of many estuaries deteriorated during the last decades to centuries, mainly due to land reclamation for agricultural and industrial purposes (e.g. Kirwan and Megonigal, 2013, Moore et al., 2009, Pendleton et al., 2012, Barbier et al., 2013). Moreover, reduced fluvial sediment supply and increased dredging activities may have induced morphological developments that also led to loss of valuable intertidal habitat (e.g. Blott et al., 2006, Meire et al., 2005, Sherwood et al., 1990, Yang et al., 2006). Nowadays, conservation and restoration of intertidal flats and marshes along estuaries is gaining ground as a sustainable strategy to re-establish the coastal defense value as well as the ecological functions provided by these areas (e.g. Cox et al., 2006, French, 2006, Temmerman and Kirwan, 2015). The influence of the above-described intertidal area changes, resulting from embankments, de-embankments, natural or human induced erosion or accretion of tidal flats, on tidal hydrodynamics at the larger scale of adjacent estuaries is the main subject of this study.
Along with intertidal area changes, we may expect hydrodynamic changes in tidal range, tidal prism and tidal asymmetry, which on its turn induce effects on estuarine morphology, ecology and human activities such as shipping (i.e., navigability of estuarine channels). Tidal asymmetry is herein of particular importance to coastal and estuarine morphodynamics as asymmetries in tidal current velocities between flood and ebb lead to a residual sediment transport due to the non-linear relation between current velocity and sediment transport (Van de Kreeke and Robaczewska, 1993, Wang et al., 1999). The asymmetry of the vertical tide is often described by the difference or ratio between the duration of rising tide (flood) and falling tide (ebb). If the flood period is shorter, the asymmetry of the vertical tide is flood-dominant; if the ebb-period is shorter, the asymmetry is ebb-dominant. The reasoning behind this is that a shorter flood duration implies stronger (peak) flood velocities and hence higher transport rates in the flood direction or vice versa (i.e., shorter ebb with higher ebb-velocities and sediment transport rates). This is often the case (e.g. Dronkers, 1998) but not necessarily true in the presence of tidal flats (e.g. Brown and Davies, 2010). Similarly, horizontal tidal asymmetry is flood-dominant if the flood flow is stronger, while horizontal asymmetry is ebb-dominant if the ebb flow is stronger. Furthermore, the difference in duration of the slack water periods after high and low tide is another indicator for horizontal tidal asymmetry as the deposition of fine (suspended) sediment is closely related to the settling time of particles (e.g. Dronkers, 1986). However, this type of tidal asymmetry is not assessed in this study. Finally, tidal asymmetry can also be characterized by the relative contribution of different tidal constituents to the tidal signal, as the amplitude ratios and phase differences between the principal constituents and their overtides or compound tides are indicators for the strength and nature of the tidal asymmetry (e.g. Friedrichs and Aubrey, 1988, Parker, 1984, Wang et al., 1999, Wang et al., 2002).
Variations in tidal asymmetry along an estuary are largely determined by estuarine morphology, including the cross-sectional geometry of estuarine channels and adjacent intertidal flats and marshes, as well as by the asymmetry of the incoming tidal wave itself (e.g. Aubrey and Speer, 1985, Dronkers, 1986, Friedrichs and Aubrey, 1988, Friedrichs and Madsen, 1992, Speer and Aubrey, 1985, Wang et al., 2002). More specifically, asymmetries in the tidal wave originate from mechanisms described by the non-linear terms in the continuity (Eq. (1)) and momentum equations (Eq. (2)) which cause the tidal wave to deform and become asymmetric in shallow waters such as estuaries:in which: t is time, x is the distance from the channel entrance, u(x,t) is the cross-sectional averaged flow velocity, ζ(x,t) is the water level relative to mean sea level, h0 is the water depth relative to mean sea level, btot is the total width of the channel (including tidal flats), bch is the width of the channel solely (without tidal flats), g is the gravitational acceleration, cf represents bed friction and ν is viscosity. The continuity equation is formed by a storage term (A) and a discharge gradient (B). The momentum equation consists of the local inertia term (C), the advective inertia term (D), the water level gradient (E), a bottom friction term (F) and a horizontal diffusion term (G). If the latter is assumed to be small (i.e., assuming a well-mixed situation) so that the horizontal diffusion can be ignored, the advective inertia term (D) and bottom friction term (F) are left as main sources for non-linear interaction (Wang et al., 1999). The effect of the friction term can be further separated in two parts to distinguish between the effect of variations in water depth (i.e., the propagation speed of the high tide exceeds the propagation speed of the low tide due to its dependency on water depth) and the effect of non-linear bottom friction itself, which slows the low tide more than the high tide as the bottom stress is higher during low tide.
The relative tidal amplitude (i.e., ratio between the tidal amplitude and the channel depth) and the relative extent of tidal flats have been identified as main factors for variations in tidal asymmetry in estuarine channels (e.g. Friedrichs and Madsen, 1992, Speer and Aubrey, 1985). In general, shallow channels with little intertidal storage tend to be flood-dominant, whereas estuaries or tidal basins with deep channels and a large intertidal storage volume tend to be ebb-dominant (e.g. Dronkers, 1986, Friedrichs and Aubrey, 1988, Friedrichs and Madsen, 1992, Speer and Aubrey, 1985). More specifically, a flood-dominant-asymmetry arises if frictional and inertial distortion in the channels is dominant via the friction term (F) and advective inertia term (D) in the momentum equation (Eq. (2)) (e.g. Dronkers, 1986, Friedrichs and Aubrey, 1988, Friedrichs and Madsen, 1992, Speer and Aubrey, 1985). Lanzoni and Seminara (2002) even showed one-dimensionally that estuaries without tidal flats are invariably flood-dominant. If tidal flats are present, ebb-dominance is enhanced through the storage term (A) in the continuity equation (Eq. (1)) (Friedrichs and Aubrey, 1988, Friedrichs and Madsen, 1992). Moreover, the bottom friction term (F) also increases during high tide when tidal flats are inundated, while it remains similar during low tide when they fall dry. However, tidal flats that are situated very low in the tidal frame may exert friction during low tide as well, which could even enhance flood-dominance (Fortunato and Oliveira, 2005). The presence of tidal flats then increases bottom friction and reduces the cross-sectional averaged channel depth during most part of the tidal cycle, especially if their position in the estuary is such that they contribute to the flow-carrying cross-section of the estuarine channel system (i.e., allowing the along-estuary propagation of the tidal wave over the intertidal flats). The latter may depend on the physiographic setting of restored intertidal areas, which can be separated into two types (Townend and Pethick, 2002): (1) intertidal areas that only have limited connection with the adjacent estuarine channels and do not contribute to the flow-carrying cross-section, which affect the tidal propagation along the estuary mainly by increasing the intertidal storage volume; and (2) intertidal areas that are situated within the main estuarine channel and flat system, which additionally exert bottom friction for the along-estuary propagating tidal wave. It can be expected that these two types of intertidal storage areas have a different impact on tidal hydrodynamics and tidal asymmetry along the estuarine channels. Therefore, we hypothesize that the position of intertidal areas with respect to the flow-carrying cross-section of the estuarine channel system can be a determining factor for the impact of these areas on estuarine tidal hydrodynamics.
Most existing studies assess the effect of intertidal area characteristics on tidal hydrodynamics in a highly schematized setting or by analytical approximations (e.g. Fortunato and Oliveira, 2005, Friedrichs and Madsen, 1992, Van Rijn, 2011). However, the effect of tidal flat elevation and the physiographic setting and location along the estuary on tidal asymmetry is not yet systematically assessed. Here, we address the influence of intertidal areas on along-estuary tidal hydrodynamics and tidal asymmetry in particular, based on a range of model scenarios simulated with a two-dimensional model of the Scheldt Estuary. This study may provide more generic insights on the effect of intertidal area characteristics on tidal hydrodynamics along the estuary by systematically varying the intertidal flat elevation (i.e., assessing depth-varying friction and storage effects) and the intertidal flat location along the estuary (and hence varying the intertidal storage volume relative to the tidal prism) in different model scenarios.
Section snippets
Study area
The area of interest for this study is the tidally influenced part of the Scheldt Estuary in The Netherlands and Belgium, which has a length of 160 km from Gent to the estuary mouth (Fig. 1). The downstream part of the Scheldt Estuary in The Netherlands is referred to as Western Scheldt, while the tidally influenced part of the Scheldt River in Belgium is referred to as Sea Scheldt. The Scheldt Estuary is tide-dominated by a meso-to macrotidal regime, with a mean tidal range that varies between
Model description
A two-dimensional depth-averaged hydrodynamic model of the Scheldt Estuary was used in this study. This model was set up in TELEMAC-2D (version v7p0) and presented in previous studies (Smolders et al., 2015, Smolders et al., 2012, Stark et al., 2017, Stark et al., 2016). TELEMAC-2D solves the shallow water equations for continuity (Eq. (1)) and momentum (Eq. (2)) in two dimensions (Eqs. (3), (4), (5)):in which U and V (m·s−1)
Scenarios with additional intertidal storage
Adding intertidal storage basins adjacent to the estuarine channels reduces the tidal range along the estuary (Fig. 2a), implying a damping of the tidal wave. The highest tidal range reduction is directly downstream of the additional storage areas, from where the tidal range reduction decreases further downstream. Upstream of the added storage areas, the reduction in tidal range remains more or less constant. In general, tidal range reduction is larger if the added intertidal storage is located
Discussion
The impact of specific intertidal area characteristics on tidal hydrodynamics was assessed with a hydrodynamic model of the Scheldt Estuary. Our results show that intertidal areas can have varying effects on tidal hydrodynamics depending on their elevation and location along the main estuary channels. While previous studies often only considered the storage function of tidal flats (e.g. Friedrichs and Aubrey, 1994, Friedrichs and Aubrey, 1988, Friedrichs and Madsen, 1992), the present results
Acknowledgements
We would like to thank the Port of Antwerp for funding this research and CalcUA for their assistance and usage of their HPC infrastructure. Stefaan Ides, Yves Plancke, Wouter Vandenbruwaene, Bas Borsje, Tom de Mulder and Stefano Lanzoni are gratefully acknowledged for their feedback and suggestions. The water level and discharge data that are used in this paper have been obtained from Rijkswaterstaat (http://live.waterbase.nl) and the Hydrological Information Centre Flanders (//www.waterinfo.be
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2022, Journal of HydrologyCitation Excerpt :Factors influencing morphological characteristics and processes in estuarine areas include tides, river flows, river sediment transport, longshore sediment transport, waves and density-driven flows (Xie et al., 2019). Specifically for interactions between estuarine topographic features, suspended sediment transport and bottom sediment transport are the main morphological processes (Stark et al., 2017; Wang et al., 2020). These processes take place continuously, sometimes reaching an equilibrium condition in which estuarine morphology exhibits a relatively steady state (Du et al., 2016; Nowacki and Grossman, 2020; Zhou et al., 2017).