MetOcean Solutions joins CSIRO for Port Phillip Bay Coastal Hazard Assessment

MetOcean Solutions is partnering with CSIRO to undertake a coastal hazard assessment for Port Phillip Bay, performing data analysis and numerical modelling of climate change scenarios to understand the potential coastal hazards impact on the area.

This project is funded by the Victorian Government and aims at assessing the environmental effects of climate change along the Port Phillip Bay coastline, to help land managers understand the hazards they may face in the future (click here for more information).

“MetOcean Solutions has a strong focus in providing the latest developments in oceanography to real applications and therefore collaboration with government research and university research groups are fundamental for us,” says Dr Alexis Berthot, MetOcean Solutions’ Marine Project Consultancy Manager.

MetOcean has developed a coupled hydrodynamic and wave model for the region using the Semi-implicit cross-scale hydroscience integrated system model (SCHISM). SCHISM is a powerful capability for MetOcean Solutions in high resolution coastal hydrodynamics. The model will be used to perform multi-year simulations under various sea level rise scenarios and historical and future climate conditions.

 
SCHISM model mesh for Port Phillip Bay.

SCHISM model mesh for Port Phillip Bay.

 

“This state-of-the-art unstructured mesh model is key to providing a detailed representation of complex nearshore bathymetric features and engineering structures, such as breakwaters and ports, that enable us to better understand the hydrodynamic regime at nearshore and coastal waters,” says MetOcean Solutions’ physical oceanographer Phellipe Couto. “This achievement is critical to support planning around potential coastal hazards in the currently changing climate scenario.” 

The Coastal Hazard Assessment, led by CSIRO with support of MetOcean Solutions, Federation University and others, investigates the extension of inundation, groundwater change, and erosion. The outcome will assist the community in future planning considering climate change scenarios.

 
Photo: State Government of Victoria, available  here .

Photo: State Government of Victoria, available here.

 

Learn more about Port Phillip Bay Coastal Hazard Assessment at the Coastal Programs website of the Department of Environment, Land, Water and Planning, Victoria (Australia).

For more information about Operational SCHISM, 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


Senegal-Mauritania wave and hydrodynamic hindcast models now available

MetOcean Solutions recently completed the development of high-resolution wave and hydrodynamic hindcast models offshore Senegal and Mauritania, West Africa.

“Senegal and Mauritania coastal areas are influenced by oceanographic processes, including tides, coastal upwelling/downwelling, eddies, internal waves, and highly-energetic wave conditions,” says Senior Oceanographer Dr Séverin Thiébaut. “Our challenge was to ensure the models would adequately replicate those multi-scale processes and both ambient and extreme metocean conditions.”

The Simulating WAves Nearshore (SWAN) model was used to resolve the wave climate and the Regional Ocean Modeling System (ROMS) was applied to simulate the hydrodynamic circulation. The technique implemented is known as ‘dynamical downscaling’, using information from large scale global models to drive regional/nearshore models at much higher resolution. All models were carefully calibrated with measured data from several current meters and wave buoys that were made available.

The SWAN model simulates the growth, refraction and decay of each frequency-direction component of the complete sea state, providing a realistic description of the wave field as it changes in time and space.

“In order to reliably replicate the regional and nearshore wave climate, the SWAN nests were defined with increasing resolutions of 5 km, 1 km and 100 m,” explains Séverin. “This approach allows the model to resolve fine-scale features near the coast while still accounting for remote influences to the area from far-field generated swell.”

Full spectral boundaries were prescribe from the MetOcean Solutions’ global wave model to the 5-km SWAN domain. The latter was used to force the boundaries of the 1-km SWAN domain, which in turn was applied to the boundaries of the high-resolution 100-m SWAN domain (Figure 1). All model nests were simulated in series over 39 years (1979 to 2017).

Figure 1: Snapshot of modelled significant wave height from the 5-km resolution SWAN parent nest off the Senegal/Mauritania coasts, delimited by the outer rectangle on (a). Extents of the 1-km resolution child nest are represented by the outer and inner rectangles on (a) and (b), respectively. Extents of the 100-m resolution child nest are represented by the inner rectangle on (b).

Figure 1: Snapshot of modelled significant wave height from the 5-km resolution SWAN parent nest off the Senegal/Mauritania coasts, delimited by the outer rectangle on (a). Extents of the 1-km resolution child nest are represented by the outer and inner rectangles on (a) and (b), respectively. Extents of the 100-m resolution child nest are represented by the inner rectangle on (b).

This ROMS model is an open source state of the art ocean model which has been used widely in the scientific community and industry for a range of ocean basin, regional and coastal scales. ROMS has a curvilinear horizontal coordinate system and solves the hydrostatic, primitive equations subject to a free-surface condition. Its terrain-following vertical coordinate system results in accurate modelling of areas of variable bathymetry, allowing the vertical resolution to be inversely proportional to the local depth. Two ROMS nests were defined with horizontal resolutions of approximately 6 km and 2 km for the regional and local model grid domains, respectively, as shown in Figure 2.

Figure 2: ROMS (a) regional (6 km) and (b) local (2km) computational model grids. The red lines illustrate the transect corresponding to the vertical sigma grid structures provided in the following figure. Note the bathymetry is represented with distinct colorbars in (a) and (b).

Figure 2: ROMS (a) regional (6 km) and (b) local (2km) computational model grids. The red lines illustrate the transect corresponding to the vertical sigma grid structures provided in the following figure. Note the bathymetry is represented with distinct colorbars in (a) and (b).

The terrain-following grid configuration consisted of 30 and 23 vertical levels with increased resolution at surface and near-bottom to better represent the boundary layers (Figure 3). The model was produced over 25 years (1993 to 2017), delivering 3-dimensional current, water temperature and salinity and sea surface elevation data.

Figure 3: Representation of the 30 vertical sigma levels of the regional grid domain over a cross-shelf transect along the latitude of 16.064०N.

Figure 3: Representation of the 30 vertical sigma levels of the regional grid domain over a cross-shelf transect along the latitude of 16.064०N.

Hindcast datasets offer key baseline information for project scoping, offshore and coastal design, project planning and environmental impact assessments.

For further information about MetOcean Solutions hindcast datasets, please contact hindcast@metocean.co.nz.

Hauraki Gulf current hindcast now available

MetOcean Solutions recently completed a 26-year hindcast detailing currents and water elevation of the Hauraki Gulf. The hindcast was created using a 2-dimensional Regional Ocean Modeling System (ROMS) model run at 250 m resolution, delivering detailed depth-averaged currents and water elevation data from 1989 to 2016.

Oceanographer Phellipe Couto carried out the modelling. He explains: “The model resolves barotropic tides - the depth-averaged velocity component of the tide - as well as water levels. The Hauraki Gulf is subject to storm surges, which are driven by wind setup and atmospheric pressure. Low-pressure systems sweeping across New Zealand result in surge, which can act to amplify the tides, causing very low or high water levels."

Snapshots of modelled atmospheric and oceanic fields during a meteorological event. Upper panels show mean sea level pressure contours and non-tidal sea surface elevation provided by the New Zealand-wide grid. The animation at the bottom illustrates the depth-averaged current flow field inside the Hauraki Gulf reproduced in the high-resolution grid.

“To accurately resolve the tides and water levels inside the Gulf, we had to get the forcings right. We used high-resolution tidal constituents at the boundaries combined to fine resolution bathymetry constructed from available data, including data from nautical charts and independent surveys. We also ran a New Zealand-scale ROMS model at 5 km resolution to provide boundary conditions, allowing us to properly downscale energy and associated sea surface perturbations from the larger scales into the 250 m grid of the Hauraki Gulf model. For atmospheric forcing, we used the Hau-Moana data set - the 12 km resolution atmospheric hindcast for the New Zealand-wide domain, and the 4 km resolution Hauraki Gulf data for our higher resolution grid.”

The hindcast data was validated against water elevation data from a tide gauge at Tiritiri Matangi Island.  

“The validation shows good agreement between modelled and measured water elevation levels,” continues Phellipe. “The hindcast provides a robust baseline tide and water elevation data set for the area, outlining the prevailing conditions for use by people who operate within or manage the Gulf. The data can also be used as boundary conditions for higher resolution modelling of local areas within the Gulf, e.g. for the hydrodynamic modelling of estuaries or embayments. This study forms the foundation for developing a full 3-dimensional hydrodynamic model which will help with the management of water quality, and gain insight into transport (e.g. of larvae or sediments), dispersion, and flushing timescales in the Hauraki Gulf.”

For further information about the Hauraki Gulf ROMS hindcast, please contact enquiries@metocean.co.nz.     

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
 

Kaikoura wave model data now available

Hindcast wave model data are now available for the Kaikoura region. The data, which includes two high-resolution nested domains around the Kaikoura Peninsula and Clarence (regions where active coastal construction is taking place) were generated by dynamically downscaling waves from MetOcean Solutions’ global wave model using a series of Simulating WAves Nearshore (SWAN) model nests. 

Snapshot of significant wave height (Hs) and peak wave directions for the SWAN domains defined for Kaikoura. Inserts show the Clarence (right) and Kaikoura Peninsula (bottom) nests.   

Snapshot of significant wave height (Hs) and peak wave directions for the SWAN domains defined for Kaikoura. Inserts show the Clarence (right) and Kaikoura Peninsula (bottom) nests. 
 

“We prioritised the model runs to ensure that suitable time-series and boundary data are available for the Kaikoura rebuild effort,” explains Project Manager Dr Brett Beamsley. “The model domains are representative of the pre-earthquake bathymetry, but we don’t expect wave characteristics at depths exceeding ~30m to be significantly different between before the earthquake and now, because depth changes in deeper water are not expected to significantly influence wave propagation.” 

“Much of the rail and SH1 roading network north of of Kaikoura historically went very close to the sea and as a result were often closed due to waves washing over the narrow foreshore. Design tolerances for reconstruction of these networks will require an understanding of the likely impacts of large waves, including storm return periods and maximum expected wave heights. In the absence of measured data, this understanding can only be achieved through long period hindcasts.

“Additionally, these hindcast datasets can be used for boundary conditions for specific high-resolution wave models employed to understand implications of the new harbour (which is expected to be completed by mid year) or breakwaters, including wave energy penetration, overtopping and infragravity waves.”

For further information on the data available, please contact b.beamsley@metocean.co.nz

An operational oceanographic forecast / hindcast model for Shanghai, China

MetOcean Solutions recently completed the development of operational high-resolution wave and hydrodynamic models for the Yangtze River mouth and coastal areas off Shanghai. 

The work combined cutting-edge science within our agile operating system to set up wave and current models for Hangzhou Bay, a region within the East China Sea which is partially enclosed by the Ryukyu chain of islands. 

“It is a very tricky area to model,” notes Senior Oceanographer Dr Rafael Guedes. “The region is characterised by a wide, shallow and highly irregular shelf with many small islands and underwater reefs. Accurate bathymetry for the area is limited. The site is strongly influenced by the phenomenal seasonal discharge from the Yangtze River, which is one of the largest rivers in the world, and is also subject to strong tidal currents.” 

 Bathymetry of the East China Sea. Red dots show the locations of measured data used to validate the models.

 Bathymetry of the East China Sea. Red dots show the locations of measured data used to validate the models.

 
Progressive downscaling of outputs from MetOcean Solutions’  global wave model WAVEWATCH III  using two SWAN nests. 

Progressive downscaling of outputs from MetOcean Solutions’ global wave model WAVEWATCH III using two SWAN nests. 

Snapshot of surface salinity from the ROMS model. Blue denotes low salinities; red high.

Snapshot of surface salinity from the ROMS model. Blue denotes low salinities; red high.

“In order to model the location well, we had to capture the meteorological events occurring within the East China Sea as well as the swell generated beyond the Ryukyu Islands which propagates into the bay. Frequent typhoons ravage the area, and these are always hard to resolve well. All in all, the area displays a challenging combination of highly variable bathymetry, strong temperature and salinity differences and complex mixing processes.”

The SWAN (Simulating WAves Nearshore) model was used to resolve the wave climate and the Regional Ocean Modeling System (ROMS) was applied to simulate the circulation. 

“To model the area we used a technique known as ‘dynamical downscaling’,” explains Rafael. “This process uses information from large scale global models to drive regional models at much higher resolution. The technique allows us to resolve fine-scale features near the coast while still accounting for remote influences to the area from long-generated swell or meso-scale currents.”

Quantile-quantile plot comparing measured and modelled significant wave height (Hs) for wave hindcast using (black) existing  CFSR  wind fields and (red) adjusted wind fields to correct for observed wind bias.

Quantile-quantile plot comparing measured and modelled significant wave height (Hs) for wave hindcast using (black) existing CFSR wind fields and (red) adjusted wind fields to correct for observed wind bias.

“High-quality input data sources are critical to running wave and hydrodynamic models in such complex settings,” continues Rafael. “We found persistent wind speed bias near the bay in the global reanalysis data source that we used to calibrate the high resolution models. Correcting this bias before running the wave model significantly improved model results just offshore of the bay as shown in the comparison of measured and modelled significant wave height.

The area has heavy shipping traffic, and the operational system outputs, including 7-day forecasts of site-specific waves, winds and currents, are now available to marine users. Please contact us and we will connect you with our partner agency in China.

Modelling the impacts of seasonal variability in freshwater input into the Waikouaiti Estuary

Understanding the dynamics of estuaries is important when setting minimum flow levels for the rivers flowing into them.

Model domain showing depth relative to mean sea level in metres.

Model domain showing depth relative to mean sea level in metres.

When Otago Regional Council consulted with local communities about possible minimum flows for the Waikouaiti River, interest groups voiced concern about what potential changes in freshwater inflow would do to the health of the estuary. So Rachel Ozanne, the Council water quality scientist, contracted MetOcean Solutions to investigate the natural variations in freshwater input to help council explore the potential impacts of flow reduction on estuary processes and ecology.

“Regional councils are tasked with setting minimum flow-levels and water allocation limits for rivers,” explains Rachel. “In order to determine whether the proposed minimum flows would cause adverse effects, we need to understand the estuary hydrodynamics.”

The council was particularly interested in assessing the difference that the minimum flow levels would make to the summer flows, when the natural riverine input into the estuary is at its lowest. 

Hydrodynamic model setup

The first step in setting up a hydrodynamic model is to get accurate bathymetry. A survey was carried out by Hunter Hydrographics and the data were combined with council LiDAR and chart data from LINZ. 

A finite-element (triangular mesh) numerical model domain was set up, with 5 m resolution inside the estuary, reducing to 200 m offshore. The SCHISM model was established and validated with measurements of water properties, water levels and currents made during a targeted field campaign undertaken by the Cawthron Institute. 

The effects of different low summer flows on estuary hydrodynamics

“We ran the model for a range of river flow rates representing summer conditions,” explains Dr Brett Beamsley, who was the science leader for the project and is an expert user of the SCHISM code. “The modelling showed that the range of summer flows has negligible effect on the overall hydrodynamics of the estuary. The tidal flows are so strong that they, rather than the river input, dominate the estuary hydrodynamics in the summer.”

Rachel feels that the modelling confirmed council suspicion that the natural variability in the area exceeds potential changes caused by varying the freshwater input.

“In the past, big storms have dramatically changed the estuary. The study showed that the small changes in summer flow investigated (changes in the order of up to 200 l per second) would make little difference to the dynamics of the estuary.” 

Tracing the dilution of pollutants

A eulerian tracer method was used to examine the dilution of fresh water within the estuary, and these simulations clearly showed that the rate of flushing varies for specific areas. 

“Such results are very useful when considering potential pollution events,” adds Rachel. “In the upper reaches of the estuary, it takes more than 10 tidal cycles to flush out the fresh water. This means that if a pollutant enters the estuary through the rivers, it  can take more than a week to dilute to negligible levels.”

Which areas are prone to siltation?

The model was also used to examine sediment transport potential within the estuary.
“When current flows exceed a threshold value, the sediments are mobilised and transported along in suspension,” adds Brett. “The sediments settle out when the current velocity drops. The modelling showed that even the smaller channels within Waikouaiti Estuary have flows that can entrain silts and sands for considerable periods of time on each tide. However some of the adjacent shallow intertidal flats do not, so these areas are susceptible to siltation.

“To help council manage the estuary, we produced a series of maps showing where sediments of different sizes are likely to entrained.” 

Mapping intertidal areas

The number of hours the estuary bed is wetted over the tidal cycle.

The number of hours the estuary bed is wetted over the tidal cycle.

Another aspect that the council was interested in was assessing how large the intertidal area is at different stages of the tide, and at the spring and neap tides. 
“Exposure to air is a critical component determining which species live where in the estuary, and which species prey on them” states Rachel. “Areas that are rarely inundated are susceptible to sedimentation, and maps like these help us understand the ecology and potential future changes that might affect the biological communities.   

Model and data are freely available

“SCHISM is an open-source science model which is freely available,” explains Brett. “This means that all the code is fully transparent, so that other researchers can replicate and modify previous modelling efforts. Council also made available all the measured data, model setup, boundary conditions and configuration files for use by the local community, including university students.”

Making the data and model freely available was important to Otago Regional Council. 
“We expect improvements to the model will be made over time as other users become involved in the model development and more data becomes available for validation,” explains Rachel. “We hope that Otago University will be able to use it for student projects, and are keen to promote it as an active, living model which can be used by community interest groups or organisations. The ultimate goal is for the model to become a useful tool for the community to further understand how the estuarine hydrodynamics affect ecosystem processes.”   

For further information on Otago Regional Council, see their website.
For more information on MetOcean Solutions’ coastal service, click here or view a pdf here.

Kaikoura tsunami waves measured in Wellington Harbour

The location of the wave meters on the eastern side of the entrance to Wellington Harbour.

The location of the wave meters on the eastern side of the entrance to Wellington Harbour.

The MetOcean Solutions science team made some unique wave measurements of the 14 November Kaikoura New Zealand tsunami event.

Oceanographer Florian Monetti has been studying the complex swell wave transformations into Wellington Harbour since 2015 as part of the environmental assessments for the Wellington Harbour Deepening Project. CentrePort, who operates Wellington Harbour, recently commissioned additional wave measurements at the popular surf breaks on the eastern side of the harbour entrance, with the intention to use these data to more precisely validate the numerical wave models of the harbour. MetOcean Solutions supplied three wave meters (RBRsolo) on seabed frames that were placed very close to the surf breaks. These meters record water levels continuously at twice per second, and as luck would have it they were deployed just a few days before the 7.8 magnitude earthquake. 

“The measurements are quite remarkable,” says Florian. “Because the meters were spaced along 3 km of the entrance coastline we can clearly see the progression of the tsunami waves as they enter the harbour.” 

The first noticeable change to the sea surface occurred only 2 minutes after the 12:02 am earthquake, with waves around 0.5 m occurring - likely radiating from the adjacent shoreline.  Then, after 24 minutes a mild disturbance was observed for about 10 minutes, which is probably the waves generated by the quake within the Wellington Harbour. Next, some 48 minutes after the quake, the water level first receded by about 0.8 m and then 12 minutes later it rose by 1.6 m. The tsunami waves moved relatively slowly through the harbour entrance, their speed limited to around 35 km/h by the shallow depth. The time lag between the first wave arriving at the Pipes and then Lion Rock surf breaks was 5 minutes. The two first waves were the largest but another 13 waves between 0.4 m and 1.0 m in size occurred over the following 6 hours. Notably, all these waves had a well-defined periodicity of (i.e. they recurred every) 27 minutes, and this distinctive tsunami signal could still be detected 19 hours after the arrival of the first wave. 

The tsunami waves started soon after the earthquake and continued until 19 hours after the quake. The graphs show the water level as measured (top) and with the tide and normal wave patterns removed (bottom).

The tsunami waves started soon after the earthquake and continued until 19 hours after the quake. The graphs show the water level as measured (top) and with the tide and normal wave patterns removed (bottom).

At Wellington, the size of the largest tsunami wave was quite close to the tidal range (which is 1.4 m), and the largest waves occurred when the tide was still low. This means the sea level changes were similar to what happens most days with the typical rise and fall of the tide. In fact, the highest water level occurred during the 9th tsunami wave, which coincided with the high tide. Some coastal flooding from the tsunami waves would certainly have occurred if the largest waves had arrived during the high tide.      

CentrePort has kindly made the data freely available for international tsunami researchers, and interested people should contact Florian for access (f.monetti@metocean.co.nz).    

Tracing gun cartridges using MetOceanTrack

Cartridges released from the Waitara River potentially spread far.

Cartridges released from the Waitara River potentially spread far.

An unusual query from Taranaki Regional Council resulted in an interesting unravelling of the fate of plastic waste washing into the sea.
 
In August 2016, Dr Emily Roberts, Taranaki Regional Council’s Marine Ecologist, approached MetOcean Solutions to ask for help with some marine detective work. Emily is involved with Project Hotspot, a Taranaki based pilot project that is using citizen science to support the conservation of threatened and iconic species. As part of the project, Council worked with Oakura School and Highlands Intermediate to clean up Taranaki beaches. The children were puzzled to find dozens of plastic wads from shotgun cartridges washing up on beaches around Taranaki, including within the environmentally sensitive Tapuae Marine Reserve. They wanted help determining likely sources of the cartridges, and who better to answer this than New Plymouth’s resident oceanographers.

Emily discussed the case with Allen Stancliff, the Taranaki Fish & Game Council Field Officer. He suspected that the plastic wads came from the Manganui River (which flows into the Waitara River), where an annual club clay bird shoot event is held. The shoot has grown in popularity over the years, attracting about 200 shooters in recent years. As some of the traps are located on the riverbank, the plastic wads could easily have ended up in the river. Newer ammunition uses fibre wads, but up until 2014 the ammunition used had plastic wads. If Allen was right, thousands of plastic wads could have been washed down the Waitara River over the years. Allen also thought that some plastic wads may originate from gamebird hunters shooting ducks along streams and rivers throughout Taranaki. These hunters are required to use steel shot (rather than lead, which is toxic) and at present the only suitable wads are plastic, but he doubted that this was a significant source.

Some of the plastic wads found by Oakura School and Highlands Intermediate.

Some of the plastic wads found by Oakura School and Highlands Intermediate.

Emily asked MetOcean Solutions to help trace the likely source of the plastic wads.

“There’s a number of reasons why stuff washes ashore in certain places,” says Mariana Horigome, the oceanographer working on the project.  “The wind and currents are the drivers, but the coastal aspect and shoreline profile also has to be right for objects such as the plastic wads to beach and not get refloated on the next tide." 

Mariana met up with Emily who pinpointed the locations where the higher concentrations of cartridges were noticed, and with Allen who explained the potential sources. To investigate whether the Waitara River could be the source of the shotgun cartridges, Mariana ran the MetOceanTrack modelling tool, a marine particle tracing software developed by MetOcean Solutions. When the model was set to release particles from the Waitara River they spread widely to locations both north and south of New Plymouth, nicely replicating where the wads had been found.

Emily was also keen to determine the source of plastic parking tickets that had been found at the beach cleanup. A MetOceanTrack run showed that the location of the tickets recovered was consistent with them entering the water in New Plymouth City, thus making local beach-goers the likely culprits. The model runs indicated that over time both the cartridges and the parking tickets could potentially spread very far from the source, with some particles ending up north of Mokau.  

"It was great to get some help determining where the plastic came from," says Emily. "The modelling confirmed our suspicions and we can now take action to minimise the waste entering rivers and the marine environment."
 
Mariana enjoys being able to help the local community. “Software like MetOceanTrack has got wide applications, providing useful information for anyone wanting more information about where objects go once they have entered the ocean. In addition to tracing rubbish, MetOceanTrack can be used to trace the spread of invasive species, the fate of oil spills and even people lost at sea.”

For more information about the findings of project Hotspot, click here.

Predicting estuarine coastal inundation

At the upcoming New Zealand Coastal Society Conference in Dunedin Dr Brett Beamsley will present recent work from a project with Tasman District Council. 

The project was designed to help the council assess coastal hazard risks due to inundation and coastal flooding over the coming 100 years. Normally, coastal flooding risks are calculated using a combination of predicted storm tide and sea level rise. However, this technique produces overly conservative estimates in estuarine environments where the non-linear nature of tidal propagation affects inundation levels. 

The study quantified the likely inundation within the Moturere Estuary, a complex inlet which includes a barrier spit. A numerical model was set up for the site and a combination of ‘worst-case’ water levels were run over several tidal cycles to examine the non-linear propagation of the open coast water levels into the inlet, their effect on inundation levels, and the effect of sequential high-stands on water levels. The findings suggest that because of the tidal wave lag within estuaries the actual inundation extents will likely be significantly different than those predicted if simplistically assuming that land lower than or equal to the storm surge level will become inundated. If the storm surge event spans multiple tidal cycles, or multiple storm surge events are predicted to occur in quick succession, inundation extents are likely to increase significantly. 

Inundation levels for a 3.5 m combined Sea Level Rise + Storm Tide + Mean High Water Springs for the first high-tide stand (left), a high-tide stand 6-days later (centre) and assuming all land sub 3.5 m becomes inundated (right). 

Inundation levels for a 3.5 m combined Sea Level Rise + Storm Tide + Mean High Water Springs for the first high-tide stand (left), a high-tide stand 6-days later (centre) and assuming all land sub 3.5 m becomes inundated (right). 

The conference is held in Dunedin 16-18 November and has as its theme 'He waka eke noa - Linking science, engineering, management and community'.  'He waka eke noa' is a Maori proverb which means 'We are all in this boat together'.

For further information about the conference, see http://www.coastalsociety.org.nz/NZCS_Conference_2016/

Water quality maps for Sustainable Seas

MetOcean Solutions, the Cawthron Institute and NIWA join forces in a project to map and forecast water quality. The project, which is funded as part of the Sustainable Seas National Science Challenge, aims to use oceanographic forecasting to produce forecast charts of expected water quality levels in Tasman and Golden bays, New Zealand. 
 
"The idea is to create forecast charts and site specific forecasts that councils, boaties and marine farmers can check, similar to a weather map," explains MetOcean Solutions Project Director Dr Brett Beamsley. "It will show the expected distribution patterns of river plumes formed after heavy rainfall, highlighting their paths once they enter the coastal marine area and become influenced by coastal circulation caused by wind and tides."
 
Rivers often carry contaminants such as sediments and faecal coliforms and the tool will include information about the predicted risk of bacterial contamination in the coastal receiving environment. "The maps will provide helpful information for stakeholders interested in coastal water quality, such as councils tasked with managing beach closures and the shellfish aquaculture industry who cannot harvest until the water quality is sufficiently good," says Dr Beamsley. 
 
Currently, water quality in the bays is estimated through the monitoring of river flow rates only. It is hoped that the research will provide higher resolution data on the exact flow patterns of river plumes in the coastal environment, thus allowing more accurate and responsive management of activities sensitive to water quality.  
 
The project is one of the eight scientific research projects that have recently received funding as part of the Sustainable Seas National Science Challenge. The challenge aims to enhance the use of New Zealand's vast marine resources, while ensuring the environment is used wisely for future sustainability. 
 
For further information on the water quality mapping project, see http://www.cawthron.org.nz/coastal-freshwater/news/2016/weather-maps-water/

The water quality maps will help shellfish farmers establish when water quality is good enough to harvest stock.

The water quality maps will help shellfish farmers establish when water quality is good enough to harvest stock.

Taranaki Surf Reserve proposed

Taranaki Regional Council has proposed a national surfing reserve for the Taranaki coastline in their draft coastal plan. The proposal covers the coastline from Cape Egmont to Okato and aims to protect the region's renowned surf breaks from anything that might adversely impact the waves.
 
The proposed reserve, the first of its kind in New Zealand, was proposed to the council in May by Dr Peter McComb during the presentation of a regional surf assessment report prepared by MetOcean Solutions.  "It's great to see our council showing leadership. Surfing is important for the local communities and economy, and establishing a reserve will afford this unique density of quality surf breaks the protection they deserve," he says.  
 
The proposed coastal plan will be going through consultation for the next year. Taranaki Region already has four nationally significant surf breaks (Waiwhakaiho, Stent Road, Backdoor Stent and Farmhouse Stent), and the proposed reserve will ensure that the wider coastline is considered as important as those breaks.

For further information about the proposed reserve, see a recent article on stuff.co.nz or view the draft Taranaki Regional Coastal Plan.

The proposed reserve will protect the Taranaki Region's surf breaks.

The proposed reserve will protect the Taranaki Region's surf breaks.