COMBINING FLOOD DAMAGE MITIGATION WITH TIDAL ENERGY GENERATION: LOWERING THE EXPENSE OF STORM SURGE BARRIER COSTS

Storm surge barriers across tidal inlets with navigation gates and tidal-flow gates to mitigate interior flood damage (when closed) and minimize ecological change (when open) are expensive. Daily high velocity tidal flows through the tidal-flow gate openings can drive hydraulic turbines to generate electricity. Money earned by tidal energy generation can be used to help pay for the high costs of storm surge barriers. This paper describes grey, green, and blue design functions for barriers at tidal estuaries. The purpose of this paper is to highlight all three functions of a storm surge barrier and their necessary tradeoffs in design when facing the unknown future of rising seas.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/Yjp3b0gU3_U

Turbulence driven by reflected internal tides in a supercritical submarine canyon

The La Jolla Canyon System (LJCS) is a small, steep, shelf-incising canyon offshore of San Diego, California. Observations conducted in the fall of 2016 capture the dynamics of internal tides and turbulence patterns. Semidiurnal (D2) energy flux was oriented up-canyon; 62±20% of the signal was contained in mode 1 at the offshore mooring. The observed mode-1 D2 tide was partly standing based on the ratio of group speed times energy (cgE) and energy flux (F). Enhanced dissipation occurred near the canyon head at mid-depths associated with elevated strain arising from the standing wave pattern. Modes 2-5 were progressive, and energy fluxes associated with these modes were oriented down-canyon, suggesting that incident mode-1 waves were back-reflected and scattered. Flux integrated over all modes across a given canyon cross-section was always onshore and generally decreased moving shoreward (240±15 kW to 5±0.3 kW), with a 50kW increase in flux occurring on a section inshore of the canyon’s major bend, possibly due to reflection of incident waves from the supercritical sidewalls of the bend. Flux convergence from canyon mouth to head was balanced by the volume integrated dissipation observed. By comparing energy budgets from a global compendium of canyons with sufficient observations (6 in total), a similar balance was found. One exception was Juan de Fuca canyon, where such a balance was not found, likely due to its non-tidal flows. These results suggest that internal tides incident at the mouth of a canyon system are dissipated therein rather than leaking over the sidewalls or siphoning energy to other wave frequencies.

Efficiency of tidal dissipation in slowly rotating fully convective stars or planets

ABSTRACT Turbulent convection is thought to act as an effective viscosity in damping equilibrium tidal flows, driving spin and orbital evolution in close convective binary systems. Compared to mixing-length predictions, this viscosity ought to be reduced when the tidal frequency |ωt| exceeds the turnover frequency ωcv of the dominant convective eddies, but the efficiency of this reduction has been disputed. We re-examine this long-standing controversy using direct numerical simulations of an idealized global model. We simulate thermal convection in a full sphere, and externally forced by the equilibrium tidal flow, to measure the effective viscosity νE acting on the tidal flow when |ωt|/ωcv ≳ 1. We demonstrate that the frequency reduction of νE is correlated with the frequency spectrum of the (unperturbed) convection. For intermediate frequencies below those in the turbulent cascade (|ωt|/ωcv ∼ 1−5), the frequency spectrum displays an anomalous 1/ωα power law that is responsible for the frequency reduction νE∝1/|ωt|α, where α < 1 depends on the model parameters. We then get |νE| ∝ 1/|ωt|δ with δ > 1 for higher frequencies, and δ = 2 is obtained for a Kolmogorov turbulent cascade. A generic |νE| ∝ 1/|ωt|2 suppression is next found for higher frequencies within the dissipation range of the convection (but with negative values). Our results indicate that a better knowledge of the frequency spectrum of convection is necessary to accurately predict the efficiency of tidal dissipation in stars and planets resulting from this mechanism.

Convective turbulent viscosity acting on equilibrium tidal flows: new frequency scaling of the effective viscosity

ABSTRACT Turbulent convection is thought to act as an effective viscosity (νE) in damping tidal flows in stars and giant planets. However, the efficiency of this mechanism has long been debated, particularly in the regime of fast tides, when the tidal frequency (ω) exceeds the turnover frequency of the dominant convective eddies (ωc). We present the results of hydrodynamical simulations to study the interaction between tidal flows and convection in a small patch of a convection zone. These simulations build upon our prior work by simulating more turbulent convection in larger horizontal boxes, and here we explore a wider range of parameters. We obtain several new results: (1) νE is frequency dependent, scaling as ω−0.5 when ω/ωc ≲ 1, and appears to attain its maximum constant value only for very small frequencies (ω/ωc ≲ 10−2). This frequency reduction for low-frequency tidal forcing has never been observed previously. (2) The frequency dependence of νE appears to follow the same scaling as the frequency spectrum of the energy (or Reynolds stress) for low and intermediate frequencies. (3) For high frequencies (ω/ωc ≳ 1 − 5), νE ∝ ω−2. 4) The energetically dominant convective modes always appear to contribute the most to νE, rather than the resonant eddies in a Kolmogorov cascade. These results have important implications for tidal dissipation in convection zones of stars and planets, and indicate that the classical tidal theory of the equilibrium tide in stars and giant planets should be revisited. We briefly touch upon the implications for planetary orbital decay around evolving stars.

Tidal current power effects on nearby sandbanks: a case study in the Race of Alderney

A validated numerical model of tidal flows and sediment transport around the Alderney South Banks was used to investigate the potential effects of large (300 MW) tidal turbine arrays at different locations in Alderney territorial waters. Two methods were used, firstly looking at hydrodynamic changes only and secondly modelling sediment transport over a non-erodible bed. The baseline hydrodynamic model was validated relative to ADCP velocity data collected in the immediate vicinity of the sandbank. Real-world sand transport rates were inferred from sand-wave migrations and agree favourably with sediment transport residuals calculated from model outputs. Outputs from the sediment model reproduced realistic morphological behaviours over the bank. Seventeen different locations were considered; most did not result in significant hydrodynamic changes over the South Banks; however, three array locations were singled out as requiring extra caution if development were to occur. The results provide a case for optimizing the array locations for twin objectives of maximizing array power and minimizing impacts on the sandbanks. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Tidal bedforms dynamics, Weser River, Germany

<p>The distribution, morphology and dynamics of tidal bedforms in the Weser estuary, Germany, between the tidal limit (river-km 0 at the tidal weir in Bremen) and the open North Sea (river-km 111 in the Outer Weser) has been analysed for a five-year period based on monthly bathymetric surveys carried out along the main waterway. For the years 2009 to 2014, bedforms were detected from gridded bathymetry data (2x2 m) and their geometric properties described. In particular, the presence and position of a slip face, here defined as the portion of the lee side steeper than 15°, was traced. This was shown to be a practical criterion for the presence of permanent flow separation and turbulent wake in the lee of bedforms. Here it is used as a simplified indicator of bedform roughness: if a bedform does feature a slip face, it is assumed to be an active roughness element. The results were related to river discharge, water levels, and flow velocities.</p><p>Bedforms were present along most of the river channel, apart from a large section between river-km 55 and 75. There, muddy cohesive sediment in the estuarine turbidity maximum zone hindered the formation of bedforms. Along the channel and throughout the years, bedform lengths varied between 20 and 60 m and heights between 0.3 and 1.6 m.</p><p>During times of high fluvial discharge, in winter and spring, ebb velocities were stronger than flood velocities. The bedforms then were small, long and ebb-oriented (i.e. the ebb lee side was shorter than the flood lee side) and many bedforms featured an ebb slip face but no flood slip face. This suggests that throughout the survey area, bedforms were active roughness elements during the ebb phase only.</p><p>In summer and autumn, when the discharge was low, bedforms in the upper reach (ca. river-km 15 to 30) gradually became flood-oriented and many bedforms there developed a flood slip face, implying that these bedforms were active roughness elements during the flood. Between km 30 and 55, bedforms were predominantly ebb-oriented, and many bedforms had an ebb slip face but only few had a flood slip face, so most bedforms were only active during the ebb phase.</p><p>The annual variations of bedform dimensions and shapes reveal an intricate feedback between river and tidal flows, channel morphology, sediment dynamics and bedforms. The results have implications for bedform research, river management and numerical modelling.</p>