An operational high resolution hydrodynamic forecast model for Port Phillip Bay

MetOcean Solutions has recently operationalised a high resolution hydrodynamic forecast model that allows simultaneous simulation of waves, currents and their interaction for Port Phillip Bay, Australia.

This new capability was developed as part of a larger project funded by the Australian Cooperative Research Centres Projects initiative. The project, a partnership between OMC International, Pivot Maritime International, University of Melbourne and MetOcean Solutions will provide an integrated modelling system for predicting under-keel clearance to support port and shipping services in tidal inlets.

“This project was conceived through an industry-research partnership and has leveraged the technical expertise in all partners,” says MetOcean Solutions’ Development Manager Dr Tom Durrant. “By bringing cross-sector experience, it is possible to develop science value-added solutions designed to increase safety and assist informed decisions at sea.”

In this particular project, the circulation model (SCHISM) is coupled with an upgraded wave physics implemented in the Wind Wave Model (WWM-III). The ‘improved WWM-III’ combined with the state-of-the-art unstructured hydrodynamic model SCHISM develops a forecast model capable of simultaneous simulation of waves, currents and their interaction.

“This is of great importance in this part of the world,” says MetOcean Solutions’ physical oceanographer Phellipe Couto.

“Port Phillip Heads, connecting Port Phillip Bay and Bass Strait is a notorious stretch of water that has claimed many ships and lives. Strong tidal currents interacting with waves combine to create significant challenges to ship navigation. Explicit accounting of this interaction, combined with unstructured model grids allowing the complex features of the main channels to be resolved at much higher resolution than previously, offer significant improvements in our ability to accurately forecast both waves and currents in the heads.

“Working closely with the University of Melbourne has provided a great opportunity to rapidly transition cutting edge science into operational systems.”

The operational high resolution hydrodynamic forecast model developed will provide input into the under keel clearance system operated by OMC international, strengthening the offerings available through the Metocean Solutions and OMC partnership (find more information here).

Operational SCHISM is MetOcean Solutions’ powerful new capability in high resolution coastal hydrodynamics, improving forecast by well representing complex nearshore bathymetries. The forecast model was also operationalised for Tasman and Golden Bay, New Zealand. Click here for more information.

The SCHISM model for Port Phillip Bay is freely available at MetOceanView.

For more information visit www.metoceanview.com or contact us at enquiries@metocean.co.nz

An operational hydrodynamic forecast model for Tasman and Golden Bay

MetOcean Solutions has recently operationalised a high resolution hydrodynamic model for Tasman and Golden Bay, New Zealand.

The underlying forecast data is produced by a state-of-the-art unstructured hydrodynamic model (SCHISM), with offshore 3D boundary conditions sourced from a 3-km ROMS implementation of the central NZ region.  This new capability was developed as part of the Sustainable Seas Project together with the  Cawthron Institute and NIWA and will provide valuable information necessary to manage contamination risk in the aquaculture industry and beach water quality forecasts relevant to regional councils and recreational beach users.

General Manager MetOcean Solutions Dr Brett Beamsley says MetOcean Solutions’ science team has many years of experience with the SCHISM model (previously SELFE); applied primarily in high value consultancy services or research projects, with the unstructured domain capability key to representing complex nearshore bathymetries in a computationally efficient manner.

“This particular project has leveraged the strong scientific capabilities in all three research partners (NIWA, Cawthron and MetOcean Solutions) and illustrates what can be achieved when working together collaboratively.”

"SCHISM is a valuable addition to our operational hydrodynamic forecast system,” says MetOcean Solutions’ physical oceanographer Phellipe Couto. “It allows our model applications to account for an even better representation of topographic features (e.g. islands, embayments, navigation channels and tidal inlets) and engineering structures (e.g. ports and breakwaters) that pose critical aspects in the modulation of the hydrodynamic regime surrounding nearshore and coastal waters.

“In practical terms, this enable us to resolve multi-scale geophysical processes such as tides, river plume dispersion and storm surge with an extra degree of accuracy and therefore provide better forecast solutions to the end user.

“The impact of storm surges on coastal areas has become highly topical particularly in the last year and the rapid deployment of this type of operational modelling infrastructure has the potential to more accurately predict coastal nearshore water levels.

SCHISM model grid resolution from approximately 10 m nearshore to 1.5 km offshore.

SCHISM model grid resolution from approximately 10 m nearshore to 1.5 km offshore.

“In this particular project, we developed a model grid with resolution varying from 10 m in the nearshore to approximately 1.5 km offshore, defining estuaries, intertidal areas, channels, streams, major rivers and relevant beaches. The model is a full 3-dimensional implementation with atmospheric and oceanic initial and boundary conditions provided by high resolution in-house models developed for the Central New Zealand oceanic domain encompassing North and South Islands’ coastal areas around the Cook Strait. We also included fluvial discharges from 11 different rivers forecasted by NIWA’s hydrological modelling capability (TOPNET) as an important forcing to our model.”

“SCHISM presents a powerful new capability for Metocean Solutions in high resolution operational coastal hydrodynamics,” says MetOcean Solutions’ Development Manager Dr Tom Durrant. “This is the first of several planned implementations.”

The project ‘Near real-time forecasting using operational oceanographic forecasting of contamination risk to reduce commercial shellfish harvest and beach closures’ is a collaborative effort of experts from the Cawthron Institute, NIWA and MetOcean Solutions. A project to build connected land-river-sea models and provide a timely risk assessment of contamination to beaches and shellfish growing areas. For more information on Sustainable Seas National Science Challenge click here.

The SCHISM model for Tasman and Golden Bay is freely available at MetOceanView.

For more information visit www.metoceanview.com or contact us at enquiries@metocean.co.nz


Research partnership helps ocean industries

MetOcean Solutions and the Coastal and Regional Oceanography Group in the School of Mathematics and Statistics at the University of New South Wales (UNSW) are partnering to ensure that marine industries will benefit from the latest ocean research along the southeast coast of Australia.

“The science team at UNSW produces world-class research. By combining their ocean data and models with MetOcean Solutions’ operational experience we will provide an even better service for clients along the New South Wales coast,” says Prof Moninya Roughan, MetOcean Solutions' Chief Scientist and UNSW`s Coastal Oceanography Group Leader. “This collaboration helps make oceanographic research useable, making valuable knowledge easily accessible to marine industries and the public, and improving safety and efficiency at sea.”

The UNSW team developed a 23-year (1994-2016) hydrodynamic hindcast model using the 3D Regional Ocean Modeling System (ROMS). The model simulates ocean circulation at sufficient resolution (2.5-6 km) to characterise the hydrodynamics in the region. Results have been validated with quality, long-term oceanographic data, and were distributed through the Australian Integrated Marine Observing Program (NSW-IMOS).

The MetOcean Solutions team use model output to generate statistics needed by clients for design purposes. In addition they have characterised the coastal marine environment at a number of locations along the coast of SE Australia, and are conducting research that will improve understanding of dynamical drivers of the coastal circulation. The data has also been used to provide boundary conditions to clients for further downscaling studies.

Model domain and bathymetry showing the East Australian Current and the Tasman Front (Image by  Kerry et al, 2016 )

Model domain and bathymetry showing the East Australian Current and the Tasman Front (Image by Kerry et al, 2016)

“Using the UNSW regional model ensures we are providing the highest quality information for our clients,” says MetOcean Solutions Forecast Operations Manager Dr Rafael Soutelino. “We are very enthusiastic about working with UNSW, and all the improvements we can achieve for industries and governments operating at sea in the region.”

UNSW`s oceanography group has been studying the East Australian Current (EAC) for decades, collecting valuable in-situ data and developing a suite of high-resolution numerical ocean models for the region. The EAC is the Western Boundary Current of the South Pacific subtropical gyre that flows south along the east coast of Australia, dominating the coastal circulation from Brisbane to Tasmania.

“The EAC is characterised by high eddy variability and the correct representation of these eddies into nearshore hydrodynamic models is critical,” says Dr. Colette Kerry, Postdoctoral Research Associate in the School of Mathematics and Statistics at UNSW. “Advancing our understanding of the dynamics of the EAC will help improve model predictions.”

For further information about Australian oceanographic research and consultancy services, please contact Alexis Berthot in Sydney at a.berthot@metocean.co.nz.

Tracking a lost sailing yacht

At the end of October, MetOcean Solutions helped an insurance company find a missing yacht that had been drifting for weeks in the Atlantic Ocean.

Pantaenius, the global yacht insurance company, were trying to help find a yacht abandoned during the Mini Transat 6.50 race. This solo race across the Atlantic Ocean, held every two years, sees sailors brave the roughly 4000 miles from France to the Caribbean only stopping in the Canary Islands on the way.

 
Jolly Rogers adrift in the Atlantic.

Jolly Rogers adrift in the Atlantic.

 

The yacht was Jolly Rogers, a MINI 6.50 (a 6.5 m high-performance sailing vessel), which had been abandoned on 5 October off Spain’s Cape Finisterre when persistent technical issues caused the sailor Luca Sabiu to set off his distress beacon. Luca was airlifted out by helicopter by the Spanish navy, leaving Jolly Rogers adrift.

The yacht was considered lost at sea until a sighting on Friday 20 October provided a position when preparations were made to start a salvage operation. The weather was closing in, however, with forecasts promising strong winds and wave heights in excess of 6 m. Worried that the boat would have drifted too far to be found by the time the weather settled down, Olivier de Roffignac from Pantaenius contacted MetOcean Solutions for advice on where the boat would drift to over two to three days of high winds and heavy seas.

“Searching for a 6 m sailboat in the Atlantic Ocean is a bit like looking for a needle in a haystack,” explains MetOcean Solutions’ oceanographer Simon Weppe, based in France.  “Salvage operations are costly, and Pantaenius wanted guidance on the likely drift track of the boat so that they could narrow down the search radius when the weather was calm enough to initiate the search.”

“To help Pantaenius we modelled the predicted drift of the vessel, continuously updating our predictions as new forecast cycles became available. To account for the uncertainties in the drifting behaviour of the boat, we ran simulations affording different importance to windage (wind-related drifting).

“The global models differ somewhat, and so to get the best results we used several different forecast datasets for winds and currents: the European Centre for Medium-Range Weather Forecasts (ECMWF) winds and MERCATOR currents*, and the US National Oceanic and Atmospheric Administration (NOAA) Global Forecasting System (GFS) winds and Real-Time Ocean Forecast System (RTOFS) currents.

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Jolly Rogers was found only a couple of miles from the predicted location when using ECMWF winds and MERCATOR currents.

Jolly Rogers was found only a couple of miles from the predicted location when using ECMWF winds and MERCATOR currents.

“We found a large variability in the predicted tracks depending on which current data we used. In this location, MERCATOR currents and the ECMWF winds yielded the best predictions. The salvage operation was launched on Monday 23 October, and the yacht was found 2-3 miles from the location we predicted, which is very encouraging given the many uncertainties.

“In this case, getting the current-related component of the drift was important because the yacht had sails and rigging dragging in the water, which would have acted as a sea anchor. In that region of the world, the MERCATOR currents are obviously more accurate than the RTOFS ones”.

The case highlights the importance of using several data sources and drifting model setups for situations where there is limited data on the drifting characteristics of the object lost.

Our team of experienced modellers is able  to quickly simulate the drifting trajectory of anything lost at sea - including vessels, personnel, and oil spills. By using different global datasets we can model a suite of possible trajectories, and compare the outputs, providing the best possible guidance for our clients.

“In the past, we have assisted clients by modelling the trajectory of oil spills, the spread of invasive species, and the dispersal of rubbish in the ocean. It is great that we could assist Pantaenius find the yacht.”

For more information about how we can assist with trajectory modelling, contact us at enquiries@metocean.co.nz.

* All MERCATOR current datasets (and more) can be accessed freely via The Copernicus Marine Environment Monitoring Service. The Copernicus Marine Environment Monitoring Service provides Full, Free and Open Access to Data & Information related to the Global Ocean and the Marine Environment.

Modelling the effects of dredge spoil disposal

As port weather experts, MetOcean Solutions regularly provides modelling support for port dredging operations. 

“The disposal of dredge spoil requires careful consideration,” explains Oceanographer Simon Weppe. “Over time, disposed sediments will be redistributed by currents and waves. When selecting offshore disposal sites, ports have to consider the hydrodynamic processes of the area and evaluate the longer-term changes to the shape of the seabed (morphological changes). Understanding the general morphological behaviour of the region is essential if we are to reduce the potential for sediment recirculation in the channel and port.

“Through our work with numerous ports, we’ve developed a process for modelling the effects of dredge spoil disposal. The changes to the shape of a disposal mound are governed by long-term hydrodynamic patterns slowly transporting sediments, and by extreme events such as storms, which can drastically change the shape of the seabed over short timescales. We use a coupled wave, current, and morphological model system, Delft3D, to understand the processes governing sediment transport in the area, and to identify key transport pathways. This gives us information to help predict the expected long-term morphological changes, as well as those arising from extreme events.”
 

Morphological changes predicted at the end of an accelerated 1-year simulation. A positive magnitude indicates sedimentation. The black polygons show the proposed disposal grounds and channel.

Morphological changes predicted at the end of an accelerated 1-year simulation. A positive magnitude indicates sedimentation. The black polygons show the proposed disposal grounds and channel.

The Delft3D modelling system is placed within broader regional wave and 3D hydrodynamic models of the area. 

For waves, a 10-year historical data set (hindcast) is prepared using the Simulating WAves Nearshore (SWAN) model, carefully downscaled from a New Zealand-wide model to the high-resolution location domain. 

For currents, the Regional Ocean Modelling System (ROMS) is used to create a 10-year hydrodynamic hindcast using a three-step nesting approach, transferring the energy from a New Zealand-scale domain to a high-resolution local domain. 

The Delft3D model is used to simulate coupled wave, current and sediment transport. The modelling system uses boundary conditions from the regional wave (SWAN) and hydrodynamic (ROMS) models.

“Sediment transport depends on the particle size,” continues Simon. “To accurately predict the dispersal of sediments from the disposal site, we need reliable sediment grain size information. The settlement of larger particles, such as sandy sediments, is governed by gravitational forces, whereas for very fine sediments like silts and clays, inter-particle forces caused by ionic charges become significant. 

“When applying process-based models to predict morphological evolution, the main challenge is that the morphology of coastal systems develops over time scales several orders of magnitude larger than the time scale of the hydrodynamic fluctuations responsible for sediment transport, i.e. hours to days versus years to decades, and more. This means that a model system that can predict the time series of instantaneous hydrodynamics and sediment transport will require an unfeasibly long period to compute a multi-year real-time simulation. 

“To overcome this, we use the Morphological Acceleration Factor (MORFAC) method. MORFAC combines the reduction of the input forcing with the use of morphological factors. This means that we reduce the ambient wave and hydrodynamic forcings to a set of representative conditions that reproduce the same morphological evolution as the real-time forcing would. We do this by combining representative wave events with representative tides and current patterns for the site. 

Morphological Acceleration Factors (MORFACs) are used to represent the long-term effects of tides and waves on the seabed morphology within morphological models. Adapted from Ranasinghe et al. (2011).

Morphological Acceleration Factors (MORFACs) are used to represent the long-term effects of tides and waves on the seabed morphology within morphological models. Adapted from Ranasinghe et al. (2011).

“To account for long-term sediment movement as well as that resulting from storms, we generate two types of results: annual morphological simulations and historical storm simulations. The annual simulations provide information on the net sediment dispersion around the disposal mound each year. The real historical storm simulations model the detailed wave, circulation and sediment transport patterns that develop during energetic events, which normally cause the most significant morphological changes.

“Our modelling provides a best estimate of sediment dispersal from the disposal site over time, taking into consideration both day-to-day currents and waves and storm conditions which move large amounts of sediment in one go. The actual sediment transport may differ slightly depending on the timing of storms, but the overall pattern of sediment movement will remain valid. 

“Regulators use the information to determine the potential effects of the dredge disposal on local hydrodynamic processes, and to ensure there is minimal impact on sites of particular ecological or amenity value. Knowing where sediments will end up helps the mitigation, monitoring and management of dredging operations.”

For more information on dredge disposal modelling, contact us at enquiries@metocean.co.nz.
 

Mean wave, current velocity and sediment transport fields for a disposal site for Lyttelton Port off the Canterbury coast in New Zealand. The black polygons show the proposed disposal grounds and channel.

Mean wave, current velocity and sediment transport fields for a disposal site for Lyttelton Port off the Canterbury coast in New Zealand. The black polygons show the proposed disposal grounds and channel.

Modelling support for port modifications

Over recent years, MetOcean Solutions has modelled the effects of dredging for ports all over New Zealand and Australia. 

Using models we can estimate the percentage of time suspended solid concentration thresholds (here 10 mg/L) are exceeded during dredge spoil disposal operations. Inner and outer dashed circles radius: 500 and 1000 m, respectively. 

Using models we can estimate the percentage of time suspended solid concentration thresholds (here 10 mg/L) are exceeded during dredge spoil disposal operations. Inner and outer dashed circles radius: 500 and 1000 m, respectively. 

“Port modifications and maintenance often involve dredging,” explains Project Director Dr Brett Beamsley. “This can include deepening channels or berth pockets, or maintaining existing depths.” 

“Over the years, we have developed modelling processes to support ports and regulators evaluate the impacts from, and manage, port dredging in the best possible manner.” 

Dredging can have complex effects. It causes sediment release and temporarily increased levels of suspended solids. Dredge spoil disposal changes the shape of the seabed at the disposal site, and deepening channels can alter the waves and tidal flows in nearby areas, potentially affecting structures such as wharves and jetties, as well as processes like shoreline erosion. 

Source of a dredge plume for a Trailing Suction Hopper Dredger. 1. Drag head; 2. Overflow; 3. Prop wash. After Becker et al. (2015).  

Source of a dredge plume for a Trailing Suction Hopper Dredger. 1. Drag head; 2. Overflow; 3. Prop wash. After Becker et al. (2015).
 

The dispersal of sediments occurring during dredge disposal can be modelled.

The dispersal of sediments occurring during dredge disposal can be modelled.

Suspended solids can be harmful to marine life. To minimise adverse effects, managers need to know the extent and duration of suspended sediment plumes, and what levels of suspended solids to expect within the plumes. Because the plumes move with the water, hydrodynamic models can be used to predict their extent and duration.

Managers also need to know the effects of discharging sediment at offshore disposal sites. They need to ensure that dredge spoil is disposed at suitable sites, where potential changes to the seabed shape will not cause adverse effects on flow, tidal flushing and local wave climate, and that disposed sediments will not be carried to nearby areas of environmental or amenity importance.

MetOcean Solutions employs a multi-disciplinary team of scientists with a wide range of expertise in port studies and is therefore well placed to provide scientific advice to ports. Two of the company’s founding directors did PhD research on the wave and sediment dynamics at New Zealand ports.

“We use a variety of techniques to get the modelling spot on,” continues Dr Beamsley. “As a starting point, we collate all information we can find for the site, including any wave or current measurements. Additional field data is often collected to ensure that important flow dynamics are captured.” 

“Once we have enough data, we set up a series of models. First, we develop regional and local-scale wave and hydrodynamic models which capture important processes responsible for sediment entrainment and transport. We nest these into larger-scale wave and hydrodynamic models to ensure atmospheric forcing and river discharges are taken into account where necessary. We calibrate and validate the models against all available measured data.

“Once we’re satisfied our models are performing well, we generate a historical data set for the location, detailing wind, waves and currents for multiple years. This historical data set is used to define the wave and hydrodynamic climate of the site and is used as input into all subsequent studies.

“We simulate dredging plumes using models that track sediment particles as they disperse from the site of dredging. Dredging typically releases sediments near the seabed and just below the water surface. Following an initial near-field phase, dredge plumes move with current flows. Over time, the particulates settle out, with larger grain sizes settling faster and finer sediments being transported further, although flocculation needs to be considered. Through modelling the evolution of the plume over time, our oceanographers simulate the particulates as they settle on the seabed, and trace the levels of suspended solids that remain in the water column. 

Example of plume modelling, showing average near-seabed suspended solid concentrations (SSC) for Lyttelton Harbour, New Zealand.

Example of plume modelling, showing average near-seabed suspended solid concentrations (SSC) for Lyttelton Harbour, New Zealand.

“The dispersal of dredge spoil disposal is investigated using high-resolution morphological models, which account for the surficial sediment grain size within the disposal ground. Using these morphological model we simulate the development of the disposal ground over multiple years to decades, modelling how existing flow patterns will affect the disposal mound and how the mound will affect circulation. As part of this, we determine the likely length of time that discharged sediment resides at the offshore disposal ground and highlight sediment transport pathways, determining where the disposed sediments are likely to ultimately end up.”

After five years the disposed sediments at this site have dispersed. The figure shows the cumulative morphological changes after each year over a 5-year morphological simulation of the disposal of 18 million m3 sediment onto a 12.5 km2 disposal ground. Initial bathymetric contours are shown in black. A positive magnitude (yellow and red colours) indicates sedimentation.

After five years the disposed sediments at this site have dispersed. The figure shows the cumulative morphological changes after each year over a 5-year morphological simulation of the disposal of 18 million m3 sediment onto a 12.5 km2 disposal ground. Initial bathymetric contours are shown in black. A positive magnitude (yellow and red colours) indicates sedimentation.

For a discussion about how MetOcean Solutions can help you predict and manage the effects of your dredging operations, contact us at enquiries@metocean.co.nz.

Nearshore renewable wave energy assessment in Mexico

In July 2016, the international wave power company Eco Wave Power commissioned MetOcean Solutions to provide wave statistics for a nearshore site in Cuyutlan, Manzanillo, on the west coast of Mexico.  

Figure 1: Model depths (top) and snapshot of modelled significant wave height (bottom) for a 0.05 degree SWAN domain for 01 January 2006. Extensions of child nests are shown by the black rectangles. Mean wave direction is shown by the arrows..

Figure 1: Model depths (top) and snapshot of modelled significant wave height (bottom) for a 0.05 degree SWAN domain for 01 January 2006. Extensions of child nests are shown by the black rectangles. Mean wave direction is shown by the arrows..

“Our company required wave statistics to assess whether it was feasible to install a wave power plant at the location”, says Eco Wave Power project manager Guillermo Sherwell. “An overview of the wave conditions is essential for the planning of offshore installations - it allows us to assess operability and identify potential hazards. The data also helps document important environmental conditions that may require further attention.”  

Dr Séverin Thiébaut from MetOcean Solutions was in charge of the project, while Dr Rafael Guedes led the wave model implementations. 

“We set up a multi-nest wave hindcast model to replicate the wave climate at the nearshore site,” explains Séverin. “We modelled the 2005-2014 period so that we could reproduce the ambient wave climate and reliably estimate the most extreme wave conditions that can occur in a 30-year period."

In consultation with the client, the team picked three nearshore sites representative of the proposed location of the wave power plant in water depths of 4, 6 and 8 m. Annual, seasonal and monthly wave analyses were carried out for these sites to extract ambient and extreme wave statistics to assess the design, workability and efficiency of the proposed wave power plant. . 

The Simulating WAves Nearshore (SWAN) model was used for the work. A 4-level nesting approach was applied to downscale wave spectra from a global model to the shallow nearshore locations of interest. Wind fields for the model were derived from the Climate Forecast System Reanalysis (CFSR), and tidal constituents from the Oregon State University Tidal Inverse Solution (OTIS). The site is prone to tropical cyclones, and a cyclone mask was used to remove tropical cyclone signatures from the hindcast metocean data.

The model outputs were used to calculate wave power, the rate at which energy is being transmitted. Fatigue analysis was assessed by estimating the total number of individual waves of varying height and period.

Figure 2: Density plot of the total significant wave height (Hs) vs the peak wave period (Tp) at one of the sites. The plot provides a visual representation of the total number of 3-hourly hindcast data (10 years) per Hs-Tp bin, normalised by the bin sizes to obtain a unit of m/s.

Figure 2: Density plot of the total significant wave height (Hs) vs the peak wave period (Tp) at one of the sites. The plot provides a visual representation of the total number of 3-hourly hindcast data (10 years) per Hs-Tp bin, normalised by the bin sizes to obtain a unit of m/s.

“We produced regional summary maps of the conditions,” explains Séverin. “These showed the spatial distribution of variables such as mean significant wave height for the total, swell and sea components, mean peak wave period, mean wave direction and wavelength.” (Illustrated in Figure 1). Joint probability occurrences of variables such as significant wave height and peak period were also included (as illustrated in Figure 2).

Figure 3: Annual wind rose plot for one of the sites. Sectors indicate the direction from which the wind is coming.

Figure 3: Annual wind rose plot for one of the sites. Sectors indicate the direction from which the wind is coming.

Wind statistics were also generated, including monthly and annual wind speed exceedance probabilities, joint probabilities of wind speed and direction and corresponding wind roses (illustrated in Figure 3). 

“Values such as the 99th percentile non-exceedance significant wave height (Hs) level is often used to assess the wave climate and structure design for energetic events. This denotes the significant wave height which is not exceeded for 99% of the time.” states Séverin. “Similarly useful are extreme metocean statistics like the return period values for wind and wave, i.e. the likely maximum that can occur within a specific extended duration.”

“We are very happy with the quality of the work,” states Guillermo Sherwell from Eco Wave Power. 
 

Sea surges as ex-cyclone Debbie moves through

Heavy rains and surging seas plagued New Zealand as the remnants of cyclone Debbie moved  across the country this week.

“Storm surges occurred in different places across New Zealand as the storm moved through,” explains Senior Oceanographer Dr Rafael Soutelino. “We were lucky that the system moved through fast and did not coincide with large tides, otherwise the coastal flooding could have been a lot worse.”

Storm surges occur when the sea rises as a result of wind and atmospheric pressure changes associated with a storm. The surges build up over time and will worsen when a low pressure system lingers. 

“MetOcean Solutions forecast storm surges nationwide using a complex high definition 3D hydrodynamical model,” continues Rafael. “The model computes the atmospheric effects on coastal water levels. It combines this with baseline water levels generated by open ocean eddies and water column expansions and contractions caused by the spatially-variable vertical density distribution.

“Our modelling shows that as the storm progressed, it caused storm surges in different parts of the country. Although mostly mild, the storm surges were still high enough to cause coastal flooding in sensitive areas. 

The storm progresses: wind and rain (right) and storm surges (left) coincide as ex-cyclone Debbie passes over New Zealand.  

The storm progresses: wind and rain (right) and storm surges (left) coincide as ex-cyclone Debbie passes over New Zealand.
 

Storm surges up to 35 cm high occurred in a number of coastal locations around New Zealand.  

Storm surges up to 35 cm high occurred in a number of coastal locations around New Zealand.
 

“We were very lucky that the storm moved through so fast, and that it coincided with neap tides. Had it happened at spring tides, when the high tide levels are higher, flooding in the area could have been much worse. Our models show that at spring tide, the storm surge would have been high enough to almost match the measured river level after all the rain. In that scenario, the river would not have drained into the ocean as fast, which means the flooding would have been worse and lasted longer.”

Coastal flooding could have been much worse had the storm surge coincided with spring high tide at the Whanganui River mouth.

Coastal flooding could have been much worse had the storm surge coincided with spring high tide at the Whanganui River mouth.

Storm surge forecasts provide valuable information for for low-lying coastal locations, and these solutions are readily available to public authorities. 

“MetOcean Solutions’ New Zealand storm surge model has a resolution of 5 km. It’s the only operational hydrodynamical model for the country and we’ve been running it for more than five years now. During that time we have used it for oil spill and search and rescue modelling, and during the Rena disaster is provided essential guidance for the national response activities. We developed a similar model for the south coast of Australia, which has been operational for more than one year. This model is used by our Alliance partners OMC International in their specialist under keel clearance applications. 

“We run these types of models all over the world,” says Rafael - even in his home country of Brazil where the complex flows along the continental shelf are important to the offshore oil field operations.   

For further information about storm surge warnings and the New Zealand 3D hydrodynamical model, please contact us at enquiries@metocean.co.nz