Metocean and Tsunami modelling of Foveaux Strait to support Green Hydrogen assessment
MetOcean Solutions /MetService has recently completed a detailed wave/hydrodynamic and Tsunami modelling study for the Bluff region in Southland, New Zealand.
This project was commissioned by Meridian Energy Limited as part of their investigation into the potential development of a Southern Green Hydrogen facility in the region.
In this project, a coupled modelling system: SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) and the Wind Wave Model III (WWM-III) was used to simulate hydrodynamic and wave conditions, including storm surges and extreme waves.
The region experiences very strong winds with prevailing westerlies known as the "Roaring Forties", with 10-min sustained wind speed over 20 knots occurring more than approximately 18% of the time and up to 50 knots sustained (gusting up to 60-65 knots) for the 100-year storm. This creates strong wind generated currents
In addition, the tidal currents in the Foveaux Strait and through Bluff Harbour channel are generally strong (up to approx. 2 knots) due to the narrowness of the strait and channel and the interaction between different water masses. When tidal currents and wind generated currents are aligned, the total surface current often reach speeds in the order of 2-3 knots.
The Foveaux Strait is also notorious for its rough seas driven by large, long period ground swell waves from the Southern Ocean and local high windseas.
When the tidal current flows against the wind, the interaction between opposing forces can create confused seas with steep, short-period waves. This effect can significantly increase wave heights. In shallow continental shelves, wave-height modulations of up to 20%–50% have been attributed to tidal currents (Tolman 1990; Wang and Sheng 2018; Lewis et al. 2019). The use of a coupled wave-current model is crucial to accurately replicate these interactions.
One of the focuses of this metocean study was to determine the maximum total water elevation that can be reached during storm events for facility/jetty design. For this, a joint probability approach of extreme waves and water level was adopted. These joint probability contours enable the assessment of how often different combinations of extreme waves and storm tides might occur, rather than looking at these factors independently. Joint probability contours of maximum individual wave height and wave crest versus storm tides were also provided in a similar fashion.
Fitted joint probability ARI contours (for 1, 10, 20, 50, 100, and 1,000-year return periods, red lines) based on fitted Weibull distribution for peak Hs and peak storm tide offshore Tiwai Peninsula.
Tsunami Hazard Assessment near Bluff Harbour/Tiwai, New Zealand
Tsunami hazards pose significant risks to coastal communities. While New Zealand’s Probabilistic Tsunami Hazard Assessment (PTHA) by GNS Science provides a comprehensive evaluation for the entire coastline, it lacks detailed local assessments, requiring additional simulations for specific areas.
This study, in collaboration with GNS, assessed tsunami impacts near Bluff Harbour. GNS simulated tsunami events based on earthquake scenarios from the Puysegur and South American subduction zones, with magnitudes ranging from Mw8.5 to Mw9.68. MetOcean Solutions modelled the nearshore propagation using the SCHISM numerical model, incorporating dynamic tidal signals.
Results show that while static-level simulations (constant water levels) are useful for estimating maximum wave height, they do not accurately predict current velocities. Dynamic simulations, which account for tide-tsunami interactions, provide a more complete picture of current speeds, essential for vessel operations and port safety. Dr Gael Arnaud will be presenting these findings in detail at the upcoming New Zealand Coastal Society Conference in Christchurch later this month https://www.coastalsociety.org.nz/conferences/2024/
In conclusion, static simulations offer a conservative estimate of wave heights, but dynamic tidal modelling is crucial for predicting current velocities and ensuring infrastructure safety.
Animation of the South America M9.5 scenario, with a zoom over Bluff Harbour. Propagation was computed with dynamic tide, but the tide elevation was subsequently removed to display only the residual elevation relative to mean sea level.
Dr Severin Thiebaut – Dr Gael Arnaud – Dr Alexis Berthot
References:
Tolman, H. L., 1990: The influence of unsteady depths and currents of tides on wind-wave propagation in shelf seas. J. Phys. Oceanogr., 20, 1166–1174, https://doi.org/10.1175/1520-0485(1990)020<1166:TIOUDA>2.0.CO;2.
Wang, P., and J. Sheng, 2018: Tidal modulation of surface gravity waves in the Gulf of Maine. J. Phys. Oceanogr., 48, 2305–2323, https://doi.org/10.1175/JPO-D-17-0250.1.
Lewis, M. J., T. Palmer, R. Hashemi, P. Robins, A. Saulter, J. Brown, H. Lewis, and S. Neill, 2019: Wave-tide interaction modulates nearshore wave height. Ocean Dyn., 69, 367–384, https://doi.org/10.1007/s10236-018-01245-z.