Tidal energy extraction modifies tidal asymmetry and transport in a shallow, well-mixed estuary

Abstract

Tidal energy extraction is increasingly being studied as a potential renewable energy resource in estuaries worldwide. Although it is understood that energy extraction via tidal stream turbines can modify currents and transport within estuaries, it is not clear how the underlying nonlinear physical mechanisms dictating tidal hydrodynamics are modulated. This research investigates the influence of a hypothetical tidal stream turbine array on barotropic tidal processes in a shallow, well-mixed system: the Piscataqua River – Great Bay (PRGB) estuary, using a numerical model. The modeled turbine farm includes 180 turbines which would extract an estimated 44.7 GWh of energy, annually. The tidal hydrodynamic model for the existing condition is validated with in-situ observations of currents and water level before analyzing tidal asymmetry and transport with and without tidal turbines. Results indicate that the tidal turbine array will decrease tidal elevation and current magnitudes system-wide, but generally reduce ebb currents and transport more than flood over most of the estuary footprint, thereby diminishing tidal asymmetry. The smaller asymmetric distortion compared to the no-turbine case is attributed to reductions in the storage volume of water over the estuary’s extensive tidal flat regions between low and high waters which decreases the associated nonlinear intertidal storage mechanism up to 25%. This leads to weakened ebb dominance over estuary sections from the mouth to mid-reaches, where depths are deep enough to keep the combined nonlinear shallow water and frictional effects from asserting control over the storage mechanism. Even in upstream shallow regions where depth-dependent friction controls asymmetry in both cases, the frictional mechanism is reduced only by 10% with turbines. Some environmental considerations of this work are discussed, with focus on sediment transport, water quality, and ecology.

Localized Tidal Energy Extraction in Puget Sound Can Adjust Estuary Reflection and Friction, Modifying Barotropic Tides System‐Wide

“…Less habitat for these species could harm the shell fishing industry as well as the broader food web of the region supported by eelgrass. A recent study analyzing the effects of tidal energy extraction in a tide dominated estuary estimated a ∼10% decline in intertidal area was possible with an analogous tidal range decrease of ∼20 cm (Spicer et al., 2023). Similarly, we expect a reduction in the transport of water into and out of South Sound.…”

“…In this work, C e is directly prescribed as a constant 0.5 as we are not modeling any specific turbine design, but rather approximating the effect of generic turbines (e.g., Spicer et al, 2023;Yang et al, 2013). In iterative curvefitting tests between modeled and observed turbine velocity profiles, Li et al (2017) found C e of 0.41 to match observations best, and so a similar value is used here.…”

“…$${F}_{x}^{M}$$ and $${F}_{y}^{M}$$ are the additional momentum sink terms associated with energy extraction which are defined as: $$$\overrightarrow{{F}^{M}}=\frac{1}{2}\frac{{C}_{e}{A}_{b}}{{V}_{c}}\vert \overrightarrow{u}\vert \overrightarrow{u}$$$ where V c is the momentum control volume where the tidal turbine is located (volume of 3D grid cells turbine encompasses), C e is the momentum extraction coefficient (and includes drag forces due to turbine poles, blades, foundation, and thrust force), A b is the flow facing area swept by the turbine, and $$\overrightarrow{u}$$ is the velocity vector ( u or v ). In this work, C e is directly prescribed as a constant 0.5 as we are not modeling any specific turbine design, but rather approximating the effect of generic turbines (e.g., Spicer et al., 2023; Yang et al., 2013). In iterative curve‐fitting tests between modeled and observed turbine velocity profiles, Li et al.…”

“…The simulated turbines were made 10 m in diameter with a center hub height 10 m from the seabed, a typical design employed in other recent energy extraction studies (Spicer et al., 2023; Yang et al., 2014). The modeled turbines were placed in rows bounded by the 20 m depth contour on either side, allowing a minimum 5 m clearance between turbine blades and surface, with considerably more clearance in the channel center where shipping traffic would be expected.…”

“…Specifically, the narrow channels around Johnstone Strait, the San Juan Islands, and Puget Sound feature the strongest currents, and sections such as Tacoma Narrows, which connects the Main Basin to South Sound, has an estimated annual mean kinetic energy flux exceeding 50 MW (Yang et al., 2021), of which some percentage could be extractable as tidal power. It has been well established that tidal energy extraction can modify tidal water levels and currents both local to turbine placement and throughout entire estuarine systems (e.g., Hasegawa et al., 2011; Sánchez et al., 2014; Spicer et al., 2023; Ward et al., 2012; Yang et al., 2013), yet few studies isolate the physical mechanisms which drive these turbine‐induced changes to hydrodynamics within real systems. Although a significant effort has been put forth in characterizing the tidal energy resource within the Salish Sea (e.g., Defne et al., 2012; Haas et al., 2011; Kilcher et al., 2016; Polagye et al., 2009; Sutherland et al., 2007; Yang et al., 2021), there has been disproportionally little work analyzing how tidal turbine farms may modify the important tidal processes there.…”