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Sea Level Rise and Tidal Power Plants in the Gulf of Maine Holly E

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Sea Level Rise and Tidal Power Plants in the Gulf of Maine Holly E JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 2863–2873, doi:10.1002/jgrc.20221, 2013 Sea level rise and tidal power plants in the Gulf of Maine Holly E. Pelling1 and J. A. Mattias Green1 Received 4 July 2012; revised 11 March 2013; accepted 25 April 2013; published 11 June 2013. [1] The response of the Bay of Fundy and Gulf of Maine to large-scale tidal power plants and future sea-level rise is investigated using an established numerical tidal model. Free stream tidal turbines were simulated within the Bay of Fundy by implementing an additional bed friction term, Kt. The present-day maximum tidal power output was determined to be 7.1 GW, and required Kt ¼ 0.03. Extraction at this level would lead to large changes in the tidal amplitudes across the Gulf of Maine. With future SLR implemented, the energy available for extraction increases with 0.5–1 GW per m SLR. SLR simulations without tidal power extraction revealed that the response of the semidiurnal tides to SLR is highly dependent on how changes in sea level are implemented in the model. When extensive flood defenses are assumed at the present-day coast line, the response to SLR is far larger than when land is allowed to (permanently) flood. For example, within the Bay of Fundy itself, the M2 amplitude increases with nearly 0.12 m per m SLR without flooding, but it changes with only 0.03 m per m SLR with flooding. We suggest that this is due to the flooding of land cells changing the resonant properties of the basin. Citation: Pelling, H. E. and J. A. M. Green (2013), Sea level rise and tidal power plants in the Gulf of Maine, J. Geophys. Res. Oceans, 118, 2863–2873, doi:10.1002/jgrc.20221. 1. Introduction Fundy in combination with large levels of future sea-level rise (SLR)? [2] The tides of the Gulf of Maine (Figure 1) are some of [3] The large tidal range of the Gulf of Maine has the largest in the world with an observed tidal range of attracted the attention of renewable energy industries, keen some 16 m in the Bay of Fundy due to resonance of the on exploiting this natural resource. However, the tides in semidiurnal tides [e.g., Garrett, 1972]. This very large tidal the Gulf are extremely sensitive to the implementation of range makes the Bay of Fundy a desirable location for TPP due to feedbacks induced by the increased dissipation large-scale tidal power plants (henceforth ‘‘TPP’’; see e.g., of tidal energy [Garrett and Cummins, 2005; Sutherland Garrett [1972] and Greenberg [1979] for details about the et al., 2007]. For example, Karsten et al. [2008] showed bay], but at the same time the near resonant state of the bay that the placement of a free stream TPP in Minas Passage makes the system sensitive to perturbations of the basin could affect the tides as far away as the Bay of Boston (Fig- [Ray, 2006; Karsten et al., 2008]. Furthermore, the near- ure 1). Also, because TPP are designed to have a working resonant properties of the Bay of Fundy in combination lifespan of some 120 years [Woollcombe-Adams et al., with its location—close to where the tidal wave enters the 2009], they will operate in an environment which may ex- Gulf of Maine from the North Atlantic—ensures that any perience SLR, and the combined response of the tides to changes to the tides within the bay will have back effects SLR and TPP is unknown. Here, we therefore ask how will far into the Gulf of Maine [Muller€ , 2011]. The sensitivity TPP and SLR change the tides in the Gulf of Maine if they of the tidal system in the Gulf of Maine therefore opens for are implemented individually and in combination? a fundamental question: how will the tide change in the [4] The current estimate of present global SLR is 1 Gulf of Maine if tidal power is extracted within the Bay of approximately 1:9mm yr À and this is expected to acceler- ate over the present century [Church and White, 2006, 2011; Woodworth et al., 2011]. Surprisingly, even the The copyright line for this article was changed on 2 September 2014 moderate SLR (a fraction of a meter) the ocean has experi- after original online publication. enced over the last few decades has already had an impact 1School of Ocean Science, College of Natural Science, Bangor University, on regional tidal systems [e.g., Woodworth et al., 2009; Menai Bridge, LL59 5AB, UK. Muller€ et al., 2011]. The SLR of 0.2–2 m estimated for the Corresponding author: H. E. Pelling, School of Ocean Science, College year 2100 [Denman et al., 2007; Rahmstorf, 2007] may of Natural Science, Bangor University, Menai Bridge LL59 5AB, UK therefore have a large impact on regional tides [e.g., Muller€ ([email protected]) et al., 2011; Pickering et al., 2012; Ward et al., 2012]. Tide gauge data show that sea level in the Gulf of Maine This is an open access article under the terms of the Creative Commons has already risen some 30 cm between 1880 and 1980, but Attribution License, which permits use, distribution and reproduction in that the rate had dropped to 20 cm per century over the past any medium, provided the original work is properly cited. 30 years [Greenberg et al., 2012]. We may thus expect a © 2013. The Authors. Journal of Geophysical Research: Oceans published significant change in sea level within the area over the life by Wiley on behalf of the American Geophysical Union. span of a potential TPP in the Bay of Fundy. Greenberg 2169-9275/13/10.1002/jgrc.20221 et al. [2012] also show that tidal high water is likely to 2863 PELLING AND GREEN: SLR AND TPP IN THE GULF OF MAINE significantly increase over the next century, due to a combi- nation of ice melt, thermal expansion, isostatic adjustment, and associated changes in the tidal dynamics. They use a very sophisticated tidal model with a high resolution, unstructured grid, and included wetting and drying of inter- tidal areas. Consequently, they produced high resolution estimates of probable future scenarios. [5] Reconstructions of paleotidal systems have shown that the tides in the Gulf of Maine have undergone signifi- cant change over the last 7000 years [Scott and Green- berg, 1983; Gehrels et al., 1995]. Using a paleotidal model, Gehrels et al. [1995] found that mean high water 7000 years ago was just over half of present-day values and 2.4–2.8 m of this increase is due to amplification of the M2 tide caused by resonance. Tide gauge data from the last century also show significant increases in the M2 amplitude [Godin, 1992; Ray, 2006; Muller€ et al., 2011]. The forcing behind these changes has been somewhat elu- sive, although Greenberg et al. [2012] pointed out that Figure 1. The bathymetry of the model domain. The SLR is likely to be a very important contributory factor. black points mark the locations used to discuss results in [6] Pelling et al. [2013] show, using a numerical tidal the paper. model without intertidal wetting and drying, that the way SLR is implemented in a tidal model is important for how 2. Methods the tides responds the change in sea level. If permanent flooding of land is allowed on the European Shelf, the 2.1. Oregon State University Tidal Inversion Software response of the tides to SLR is dramatically different com- [9] Model simulations were conducted using the Oregon pared to if vertical walls are introduced at the present-day State University Tidal Inversion Software (OTIS, see coastline (referred to as ‘‘no flooding’’ by Pelling et al. Egbert et al. [2004] for details), which was run with a hori- [2013]). This is because the newly flooded areas will have zontal resolution of 1 arc min in both latitude and longitude high tidal velocities due to their shallow water depth, and (see Figure 1 for the domain). The model solves the shal- consequently dissipate significant amounts of tidal energy low water equations with rotation and bed friction: due to bed friction. This effect shifts the amphidromic @U points toward the locations with enhanced dissipation [Tay- þ Ur_ u þ f  U ¼ghrðÞþ À kr2U þ F þ  ð1Þ lor, 1922] and consequently alters the entire tidal regime. @t SAL The introduction of high vertical walls at the present-day @ coastline on the other hand, which of course prevents the ¼rÁU ð2Þ flooding of land, solely introduces changes in the propaga- @t tion speed of the tidal wave, but may also impact on the resonant properties of the certain basins [e.g., Arbic and [10] Here, U ¼ uH is the depth-integrated volume trans- Garrett, 2010; Green, 2010; Pelling et al., 2013]. port given by the velocity u times the water depth H, t is time, f is the Coriolis vector, and are the tidal elevation [7] It is important to make a clear distinction here e between what we call flooding/no flooding, and wetting and equilibrium tide, respectively, F is the dissipative stress  and drying. The latter is an intertidal flooding process from bed friction, is the astronomic tide-generating force, and k 100m2sÀ1 is the horizontal turbulent eddy which consequently acts during a tidal cycle, whereas our ¼ viscosity. definition of flooding/no flooding is a feature in which SLR [11] The bed friction is parameterized using the standard is enough to permanently flood land cells and thus not quadratic law: allowing them to dry out during the tidal cycles.
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