The waters around New Zealand have some of the highest waves in the world, being exposed to swell coming up from the Southern Ocean, as well as storms in the Tasman Sea and Pacific Ocean.
Causes of wave hazards
Wind blowing over the sea surface produces short waves or ripples. The stronger the wind, and the more time and distance it has to work on the waves, the higher and longer they get.. In a storm the rough sea is a mixture of waves of different heights and lengths, travelling in different directions. Once started, waves can travel long distances (as "swell"), crossing whole oceans.
Potential consequences of wave hazards
Rough seas create dangers to boating, damaging or capsizing vessels at sea, while also damaging wharves, jetties, oil rigs and other structures, and contributing to erosion. They can be particularly dangerous on exposed coastlines, making bars impassable, and creating strong rips at beaches, leading to a drowning hazard.
The combination of storm surge on top of high tides can cause coastal flooding which damages property and impacts on coastal infrastructure (e.g. by blocking roads). When waves are present as well, an additional rise in sea level due to wave setup can exacerbate this inundation, while also allowing the waves to cause coastal erosion and damage to coastal infrastructure (e.g. seawalls) they normally wouldn't reach.
"Wave climate' means the statistical description of how wave conditions vary in time and in different parts of the ocean. This is ideally found by actually measuring waves over many years, e.g. with wave buoys, but relatively few such measurements been made around New Zealand. Consequently, to assess wave climate and derive probabilities of extreme wave conditions, use is made of computer models to hindcast (the opposite of forecast) wave conditions from past wind conditions over a sufficient period of time, usually decades.
Waves tend to be defined by their significant wave height (Hm0), which is an average height over a certain period (around one hour). The highest individual wave in that time could well be twice the significant wave height. On average, the highest (and most energetic) waves (average Hm0 = 3-4 m) are found to the south of New Zealand (Figure 3), which is most exposed to swell from the Southern Ocean. Mean wave heights decrease further north, as exposure to this swell decreases.
In waters south west of New Zealand (Figure 4), significant wave heights of 7-8 m are by no means uncommon.
Recent maritime hazard events in New Zealand waters
Storms of September 2010
A sequence of large energetic storms swept across the ocean south of New Zealand in mid-September 2010, bringing high seas to the Tasman Sea. The image on the left of Figure 1 shows the predicted significant wave height (representing the crest-to-trough height of a "typical" wave) at 1800 on the 16th of September, the start of a 48-hour forecast.
A large low pressure system was centred well south of the bottom of the picture, producing strong south-westerlies over a fetch of more than 2000 km all the way from the Antarctic ice edge to Tasmania, where a wave buoy on the west coast was measuring significant wave heights of 9 m at the time. Over the following days the storm moved eastward, resulting in waves over 8m over almost the full width of the Tasman Sea. These waves were especially high on the North Island west coast as seen in the right-hand image of Figure 1 showing the same forecast's prediction for 0600 on the 18th of September. And that wasn't all – a few days later another large storm passed a little further south, bringing similar conditions again, this time with the largest waves on the South Island west coast.
Wave Hazards research at NIWA
NIWA collects wave data, develops and verifies wave forecasting systems, and is developing and producing wave projections which will help planners and local government to make informed decisions about how climate change will affect coastal regions.
Current NIWA research projects
Reducing the Impacts of weather related hazards
The overall programme aim is to develop a multi-hazards forecasting system for all weather related hazards. This provides accurate and location specific warnings of impending weather, river-flood, flood inundation, land sliding, snow avalanche, sea-state, sea-level including storm surge, and rip current hazards up to 2 days in advance, and more general warnings at longer lead times. In the part of this programme focussed on sea state forecasting, we are:
- collecting wave data from monitoring instruments for operational use and to validate our models
- developing and verifying deterministic and probabilistic forecasting systems for waves, coupled to associated forecast systems for weather, tides and storm surge.
Wave and storm surge projections
The aim of this research programme is to develop and produce regional projections of waves, swell and storm-surge to support local government and engineering or planning consultants in making decisions about adapting to climate change in coastal areas. The programme includes:
- generating a hindcast (30–40 years) and two sets of future projections for waves/swell and storm surge around New Zealand at a consistent national scale and both using the same weather inputs
- developing the delivery system (WASP) to download information on waves/swell and storm surge and the likely climate-change impacts at the regional or sub-regional scale
- undertaking a pilot demonstration in Waitemata Harbour on how the offshore information from WASP can be incorporated into a more detailed assessment of the wave/swell and storm surge hazard at the shoreline (both now and in the future).
Dr Richard Gorman
07 856 1736