Ocean Dynamics (2013) 63:21–41 DOI 10.1007/s10236-012-0583-z
Contemporary hydrodynamics and morphological change of a microtidal estuary: a numerical modelling study Angela Wenping Jiang & Roshanka Ranasinghe & Peter Cowell
Received: 4 January 2012 / Accepted: 22 November 2012 / Published online: 9 December 2012 # Springer-Verlag Berlin Heidelberg 2012
Abstract Contemporary hydrodynamics and morphological change are examined in a shallow microtidal estuary, located on a wave-dominated coast (Port Stephens, NSW, Australia). Process-based numerical modelling is undertaken by combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. Model results suggest that the complex estuarine bathymetry and geometry give rise to spatial variations in the tidal currents and a marked asymmetry between ebb and flood flows. Sediment transport paths correspond with tidal asymmetry patterns. The SE storms significantly enhance the quantities of sediment transport, while locally generated waves by the westerly strong winds also are capable of causing sediment entrainment and contribute to the delta morphological change. The wave/wind-induced currents are not uniform with flow over shoals driven in the same direction as waves/ winds while a reverse flow occurring in the adjacent channel. The conceptual sediment transport model developed in this study shows flood-directed transport occurs on the flood ramp while ebb-directed net transport occurs in the tidal channels and at the estuary entrance. Accretion of the intertidal sand shoals and deepening of tidal channels, as Responsible Editor: Roger Proctor A. W. Jiang (*) School of Geosciences, University of Sydney, Camperdown, NSW 2006, Australia e-mail:
[email protected] R. Ranasinghe Dept. of Environment, Climate Change, and Water, Sydney, NSW 2001, Australia R. Ranasinghe UNESCO-IHE/Technical Univ. of Delft, 2601 DA Delft, The Netherlands P. Cowell School of Geosciences, University of Sydney, Camperdown, NSW 2006, Australia
revealed by the model, suggest that sediment-infilling becomes advanced, which may lead to an ebb-dominated estuary. It is likely that a switch from flood- to ebbdominance occurs during the estuary evolution, and the present-day estuary acts as a sediment source rather than sediment sink to the coastal system. This is conflictive to the expectation drawn from the estuarine morphology; however, it is consistent with previous research suggesting that, in an infilling estuary, an increase in build-up of intertidal flats/ shoals can eventually shift an estuary towards ebb dominance. Thus, field data are needed to validate the result presented here, and further study is required to investigate a variety of estuaries in the Australian area. Keywords Tidal asymmetry . Numerical model . MIKE 21 . Estuary morphodynamics . Tidal residuals . Sediment transport . Port Stephens . Australia
1 Introduction Estuaries are considered to be valuable natural resources that are of great environmental, social and economical importance (Nordstrom 1992). A widely accepted definition describes an estuary as “a semi-enclosed coastal water body which has a free connection to the open sea and within which sea water is measurably diluted with freshwater derived from land drainage” (Cameron and Pritchard 1963). It has been generally used for water bodies varying enormously from ephemeral (seasonally closed) and small coastal lagoons, to drowned river valleys and barriers and to large infilled and prograding funnel-mouthed coastal plains. The diversity of estuaries has led to numerous studies to address their morphology features, sedimentary environment and general facies distribution as estuaries evolve (e.g. Boyd et al. 1992; Cooper 2001; Dalrymple et al. 1992; Heap et al. 2004; Nicholls 1989; Roy 1984, 1992; Roy et al. 2001).
22
The morphology of estuaries is a result of nonlinear interaction between water and sediment motion and bed topography (Hibma et al. 2004). Sedimentary processes in estuaries are remarkably complicated, consisting of cycles of erosion, transportation and deposition of sediments. These processes strongly depend on the flow dynamics and on the particle properties, their size, shape, density and composition (Nichols and Biggs 1985). Sedimentary processes alter the morphology of estuaries which, in turn, influences the way tide and wave processes are attenuated within estuarine systems (Roy 1984). Understanding and predicting the co-adjustment of processes and morphology are essential in the study of estuarine systems (Wright and Thom 1977; Pethick 1996; Hashimoto and Roy 1997). However, increased research effort is needed to develop a morphodynamic approach to examine the link between geomorphological development and hydrodynamic/sedimentological processes during the evolution of estuaries (Kench 1999). Flood-tide deltas are among the most dynamic features of estuarine systems and often cause instability of the whole system. A marine flood-tide delta forms during estuary infilling, as a function of energy dissipation and attenuation as water moves through the mouth of an estuary (Masselink and Hughes 2003). It has been described as a fan- or horseshoe-shaped sand body deposited inside the inlet (Hayes 1975; Roy et al. 1980), presenting a primary sediment sink. While their presence is well known and studies have developed general models of their formation and evolution (e.g. Boothroyd 1985; Davis et al. 2003; Dyer and Huntley 1999; Hayes 1980), little is known about their contemporary morphodynamics. Large, dynamic and evolving sandy flood-tide deltas formed at the mouths of all drowned river valleys in southeast Australia. Many of these deltas and adjacent shorelines are both highly developed and highly dynamic, leading to substantial management issues, particularly in Port Stephens (Thom et al. 1992), Broken Bay (Cowell and Nelson 1991), Botany Bay (Roy and Crawford 1979) and Port Hacking (Druery and Hurrel 1986). Although globally, there is a good understanding of the processes of sediment transport and deposition within estuarine environments (Dyer 1996), there is a paucity of published research on the contemporary hydrodynamics and morphodynamics of Australian estuaries (Kench 1999). Few studies have attempted, like Moore et al. (2009), Cooper (2001), Brown and Davis (2009) in UK, Bolle et al. (2010) in The Netherlands, Wolanski et al. (1992) and Furukawa and Wolanski (1996) in America, to link contemporary hydrodynamics with estuarine morphological change and to determine an individual estuary as a sediment source or sink to the coastal system. Lessa and Masselink (1995) did attempt to link contemporary hydrodynamics with
Ocean Dynamics (2013) 63:21–41
morphology in a Queensland macrotidal estuary and identify the transition from flood to ebb-dominated estuarine environments. The question remains whether other Australian estuaries have the same tendency. Therefore, it would seem appropriate that more studies are undertaken in Australia to validate global models for a range of estuary types (e.g. microtidal estuaries). Estuaries of New South Wales (NSW, Australia) formed during the Postglacial Marine Transgression, when rising sea levels drowned river valleys and low-lying coastal areas (Roy 1994). They are considered to be sediment sinks (Dalrymple et al. 1992; Roy et al. 1980) and have been infilling with sediments from the land and from the sea since sea level stabilized in the Holocene (Nichols and Biggs 1985; Roy et al. 2001). Rates of infilling of individual estuaries have varied widely depending on the sediment transported into the estuary that is controlled by the interaction of inherited geological nature (e.g. bedrock type, coastal morphology) and contemporary process nature (e.g. tidal currents, river discharge, ocean waves) (Roy 1984). The stage of infilling of estuaries has led to their classification based on the level of maturity (Roy et al. 2001), with ‘mature’ estuaries at a later stage of evolution to those which are ‘immature’. The estuary being investigated in this study is Port Stephens, NSW, Australia, which is a representative of shallow microtidal estuaries located on the wavedominated southeast Australian coast. The estuary has experienced a unique hydrologic regime, being subject to a mixture of tides, waves and wind energy. The morphologic features of the estuary are highly complex comprising a well-developed flood-tide delta, some sand shoals and meandering tidal channels. The estuary provides a natural laboratory to investigate the questions implicit in the aims of the research. The field study of sediment transport in tidal estuaries is difficult, due to the oscillating nature of sediment movement (Cancino and Neves 1999). However, numerical models now provide a valuable means of undertaking experimental investigations due to advances in the last few decades in the understanding of physical processes and mathematical modelling techniques. Process-based models are mathematical representations of physical laws per cross-section for onedimensional models or on a local, discrete grid for two- and three-dimensional models. Typically, these models consist of a number of modules that describe the constituent physical processes (waves, currents, sediment transport), which are coupled via a bottom-evolution module based on sediment conservation. In this study, a process-based numerical modelling approach is carried out, and the main objectives are: (1) to improve the understanding of the mechanisms responsible for contemporary estuary change, i.e. to link hydrodynamic
Ocean Dynamics (2013) 63:21–41
23
processes with estuarine morphological change; and (2) to identify the direction of net sediment transport and erosion/ deposition patterns in the estuary. Particular attention is paid to the flood-tide delta area including a number of tidal channels and sand shoals.
20 m at its deepest points (Fig. 2). The tidal delta sands are predominantly fine to coarse, well-sorted quartz grains (MHL and DPWS 1999). Port Stephens is classified as a tide-dominated estuary located on a wave-dominated open coast. The ecology and processes of the inner bay are predominately estuarine while the outer embayment is dominated by ocean marine processes. Tide is considered to be a dominant process affecting water and sediment transport in the outer bay of the estuary. Tides in the area are semi-diurnal, microtidal (range
