Impacts of sea-level rise on estuarine circulation: An idealized estuary and San Francisco Bay
Introduction
Rising waters as a result of climate change will likely reshape the world's coastlines and may lead to significant impacts on estuarine areas. Global warming leads to sea-level rise due to the melting of ice caps and thermal expansion of oceans (Gornitz et al., 1982, Root, 1989, Vermeer and Rahmstorf, 2009, Wigley and Raper, 1987). Global warming is also likely to have a major impact on hydrological cycles and consequently lead to a change in freshwater inflows into estuaries (Rapaire and Prieur, 1992, Statham, 2012). Sea-level rise projections by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) range from 0.18 to 0.81 m by 2100 with respect to the base year 1990. A significant acceleration of 0.013 ± 0.006 mm/year2 was reported over the 20th century, and sea levels rose at a rate of 1.7 ± 0.2 mm/year and since 1961 at a rate of 1.9 ± 0.4 mm/year (Church and White, 2006). Recent altimeter observations indicate an increase in the rate of sea-level rise during the past decade to 3.2 mm/year, well above previous estimates of 1.5–2 mm/year (Carton et al., 2005). Tidal gauge and satellite data reveal that sea-level rise is not geographically uniform, and spatial variability in the rates of sea-level rise is expected due to non-uniform changes in temperature, salinity and ocean circulation.
For estuarine areas, sea-level rise leads to an increase in the salinity of surface and ground water through salt water intrusion. Rising sea levels result in a landward shift of the estuarine salinity field, threatening freshwater supplies upstream (Hull et al., 1986, Williams, 1987). Studies have found that sea-level rise results in higher salinities upstream and also affects tidal currents in estuaries (Chua, 2012, Hong and Shen, 2012). Intrusion of salt water and changes in circulation patterns may have serious consequences for marine ecosystems that are unable to tolerate high salinity (Schallenberg et al., 2003, Short and Neckles, 1999). We are particularly interested in the effects of sea-level rise on physical processes such as estuarine circulation, stratification and vertical exchanges, which would play an important role on regulating biological phenomena and aquatic habitats.
Numerical models have been employed to investigate the effects of sea-level rise on salinity and tidal ranges in estuarine environments. Hong and Shen (2012) found the salt content, salinity intrusion length and stratification in Chesapeake Bay will increase subject to seasonal and inter-annual variability as sea level rises. They also quantified the impact of sea-level rise on circulation and transport in the Bay by simulating the transport of passive tracers. With a combination of numerical modeling and statistical techniques, Hilton et al. (2008) demonstrated an increase in bay-averaged salinity of 0.5 psu as sea level rises 0.2 m in Chesapeake Bay. Singha et al. (1997) found a positive sea level trend in the Hooghly Estuary, and that there exists a substantial increase in the amplitude and velocities of the tidal wave due to sea-level rise. Zhong et al. (2008) noted that sea-level rise will change the resonance characteristics of Chesapeake Bay and suggested that the tidal range will increase by 15%–20% with a sea-level rise of 1.0 m.
In this paper, an idealized estuary model is employed to investigate the effects of sea-level rise and freshwater inflows on estuarine circulation. The simulations are performed with an idealized model to remove the influence of irregular coastlines, lateral variation in depth and presence of bathymetric features, so that the effects of sea-level rise and freshwater inflows on estuarine circulation are isolated. Variability of freshwater inflows is difficult to predict due to the huge range of uncertainty in inflows that are expected with climate change. Hence, our simulations are performed for a wide range of inflow conditions. Insights on the physical characteristic of estuaries gleaned through idealized simulations may be incorporated to improve our understanding of estuaries with realistic coastlines and bathymetry.
Supporting three-dimensional simulations from northern San Francisco Bay are presented. In the San Francisco Bay-Delta system which provides freshwater supplies to Southern California and the Central Valley, and for local domestic, industrial and agricultural use, salt water intrusion upstream will result in water intakes that might draw on salty water during dry periods. Coastal aquifers recharged by freshwater upstream are also likely to become saline as salt water is pushed upstream (Bobba, 2002, Meisler et al., 1984, Oude Essink, 1999, Sherif and Singh, 1999, Werner and Simmons, 2009). The availability of freshwater in San Francisco Bay interacts with other factors, including changes to the local hydrology due to climate change (Dettinger et al., 2004, Knowles and Cayan, 2002, Knowles and Cayan, 2004, Miller et al., 2003), and changes in demand due to population growth and urbanization. The combination of these factors is likely to compound water stress in coastal areas. An unstructured-grid SUNTANS model is applied to perform three-dimensional simulations of flow in San Francisco Bay, and a series of simulations is performed to investigate the effects of sea-level rise and freshwater inflows on estuarine circulation.
The remainder of the paper is structured as follows. Section 2 describes the hydrodynamic model. Section 3 presents the results of our idealized simulations. Section 4 describes three-dimensional simulations in San Francisco Bay. Section 5 provides conclusions.
Section snippets
Hydrodynamic Model
The climate change simulations are performed with the SUNTANS hydrodynamic model (Fringer et al., 2006). The governing equations are the three-dimensional, Reynolds-averaged Navier–Stokes equations under the Boussinesq approximation and hydrostatic assumption:subject to incompressibility,where the horizontal gradient operator is , the free-surface height is h, the velocity vector is u
Model Setup
The model domain includes a rectangular channel estuary attached to a shallow coastal ocean (Fig. 1). The estuary has length 500 km and width 5 km, and the ocean boundary extends to 200 km west of the mouth of the estuary. The estuary has a constant depth of 10 m. The depth of the coastal ocean varies from 10 m at the mouth of the estuary to 50 m at the ocean boundary. The dimensions of the idealized estuary have been chosen to be close to that of San Francisco Bay. The finite volume grid is
Application to San Francisco Bay
The idealized simulations performed in the previous section provide a first-order approximation on the effects of sea-level rise and freshwater inflows on estuarine circulation. Further understanding requires that we employ realistic simulations of complex estuaries. In this section, we will describe the scenario simulations performed on a calibrated and validated model of San Francisco Bay.
Conclusion
The effects of sea-level rise and freshwater inflows on estuarine circulation are investigated with an idealized estuary model and supported with three-dimensional simulations of San Francisco Bay. Increased inflows compress the salinity gradient, and lead to a stronger longitudinal salinity gradient, which in turn drives stronger gravitational circulation and larger longitudinal dispersion. Rising sea levels increase the strength of the longitudinal salinity gradient and reduce the impact of
Acknowledgements
The authors acknowledge the support of the National University of Singapore research grant (WBS R-302-000-021-133). MX acknowledge the support of the National University of Singapore PhD research scholarship. We also appreciate the discussions with Dr. Oliver Fringer.
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