Natural Hazards Update - No.4 2003
Auckland councils tackle hazards
Auckland volcanic zone (Photograph: Lloyd Homer)
A group of Auckland councils (the Auckland Local Authority Hazard Liaison Group) have joined forces to help them manage the risks they face from a wide range of natural and technological hazards. To make this task simpler and more efficient, the liaison group have developed a series of three guidelines to help these councils with risk management.
- Hazard information management: identifies sources of hazards information and looks at how this information is collected, sorted, recorded, and stored in the Auckland region. It is relevant to the development and maintenance of hazard registers.
- Hazard identification and risk assessment: provides guidance on establishing the context for risk analysis, identifying hazards and risks, and carrying out risk analysis and evaluation.
- Risk treatment and monitoring of hazards: sets out the hazards that affect the Auckland region (at both the local and regional levels) and specifies the range of risk reduction options that can be used to minimise community risk from hazards.
The aims of the guidelines are to achieve better consistency both within and between councils in the Auckland region in the way hazards information is collected, organised, and used to influence decisions about risk management. The guidelines provide a selection of tools and methods that local authorities can use to manage hazard risks in their city, district, or region. While each local authority will manage hazards differently, according to the risks they face as well as community expectations, these guidelines provide a consistent basis for managing and treating those risks.
The guidelines are available from www.arc.govt.nz/auckland-region/hazards, or can be ordered by phoning +64 9 366 2000.
Seismic hazard model for New Zealand
Marlborough-Kaikoura fault systems, both on land and offshore.
In the last 10 years seismic hazard assessment has come of age internationally. Traditionally, the focus has been on where the largest earthquake sources are located relative to a site of interest. Now, this approach is being largely replaced by probabilistic methods, which also consider the important parameter of how often the earthquake sources around a site produce earthquakes. These probabilistic methods have become multidisciplinary, combining seismological and engineering research on historical earthquake occurrence with geological research that can uncover occurrences of prehistoric earthquakes.
The national seismic hazard model for New Zealand being produced by GNS is a relatively new product of these multidisciplinary probabilistic methods. The likely occurrence of future earthquakes and shaking is now based on combining the 160-year historical record of earthquakes with prehistoric earthquake information from over 300 active faults.
GNS is leading a collaborative effort to update, improve, and validate the New Zealand seismic model. Currently, the model is limited to onshore fault sources throughout New Zealand. NIWA’s role is to augment this onshore information with faulting information from offshore coastal and continental shelf areas; in particular, active faulting zones from offshore Canterbury, Hawke Bay, Fiordland, Manawatu, Cook Strait, and Bay of Plenty. With GNS and NIWA joining forces to link the onshore and offshore marine faulting, a much better model of the seismic hazard for New Zealand can be produced. The Earthquake Commission (EQC) and United States agencies are contributing to our development of formal tests of seismic hazard models. We are also working with the Pacific Earthquake Engineering Research Group (PEER) to cross-check seismic hazard codes used to construct models such as the New Zealand model. All these improvements will greatly increase the reliability of the national seismic hazard model and result in a more soundly-based appraisal of seismic risk for different regions of New Zealand.
Natural hazards in autumn 2003
Landslides |
Weather extremes |
Coastal hazards |
Floods and droughts |
Earthquakes and volcanic activity |
Data sources for these maps: GeoNet, NIWA, MetService, Regional Councils
Wahine Disaster
Wahine founders at the entrance to Wellingon Harbour off Seatoun. (The Evening Post, April 1968).
10 April 1968 The interisland ferry Wahine, with 744 people on board, encountered the full fury of ex-tropical cyclone Giselle as it tried to enter Wellington Harbour heads early on 10 April, 35 years ago. Winds from the south rose to 110 km/h, gusting to 150 km/h, whipping waves up to 10 to 13 m high. At 6.40 a.m., Wahine struck Barrett’s Reef in poor visibility. Despite all the rescue efforts, 51 people lost their lives. Insurance payouts for all the impacts of the Wahine storm amounted to $155 million (inflation adjusted). It was one of the two worst regenerating tropical cyclones to hit New Zealand in the last century.
Storms
Satellite image of a deep depression off the South Island on 19 July 2001. (Image: Landcare Research Ltd and NOAA).
Storms produce some of our most frequent natural hazards in the form of floods, gale-force winds, and high waves. Although any storm can be devastating for those experiencing it, meteorologists conveniently divide them into two main groups: tropical cyclones and mid-latitude cyclones. While the ‘label’ reflects their origin, the two types of storms have very different energy sources, structure, formation mechanisms, and challenges in predicting their behaviour.
The ultimate source of energy for weather systems is the sun. If it heated the atmosphere equally everywhere, the Earth’s temperature would be uniform with no air motion and no weather. But it doesn’t, due to the large differences in absorption of the sun’s energy at the equator compared with polar regions, and between the lower and upper atmosphere. The occurrence of tropical and mid-latitude cyclones is evidence that the Earth’s atmosphere is performing it’s essential function of re-distributing this solar heating imbalance, otherwise the equator would cook! To do this, nature organises itself quite efficiently and elegantly at times into giant mixers (see satellite image), but when people and the environments they construct get in the way, the results can be destructive and sometimes deadly.
Tropical cyclones – usually referred to as hurricanes, typhoons, and cyclones in other parts of the world – are intense cyclonic storms of tropical origin. Surface winds can reach over 200 km/h. Luckily for us, there is no evidence of an intact tropical cyclone ever having reached New Zealand. But as a tropical cyclone moves southward towards us it encounters strong vertical shear in the wind, such as high-level jet streams. Coupled with the drop in sea temperature, this increasing background wind shear destroys the mechanism that maintains a tropical cyclone, and so it undergoes a transformation into a mid-latitude or ex-tropical cyclone. It is worth noting, however, that the ‘baggage’ left over from the tropical cyclone, such as the residual clockwise circulation and very moist air it contains, means that these ex-tropical cyclones can become some of the most devastating mid-latitude storms that New Zealand will experience. Examples are tropical cyclone Giselle that re-formed to become the April 1968 Wahine storm and ex-tropical cyclone Bola that washed away huge amounts of topsoil in the Gisborne region in March 1988 and caused $90 million damage.
Mid-latitude cyclones (sometimes referred to as depressions or lows) usually form within the belt of westerly winds encircling the globe between latitudes 30° S and 70° S. They generally move from west to east, bringing a period of unsettled weather, with wind, cloud, and precipitation, most noticeably at fronts. Unlike tropical cyclones, which draw energy from vertical contrasts in temperature, mid-latitude cyclones draw their energy from contrasts in temperature with latitude. As these cyclones develop, the regions of temperature contrasts are organised into cold and warm fronts, giving the mature cyclone a distorted asymmetrical structure, unlike the more symmetrical tropical cyclone. The warm air flows southward and upward ahead of (east of) the low, and the cold air flows northward and downward behind (west of) the cyclone centre. Most of the cloud and precipitation occurs in the rising warm air ahead of the cyclone, in contrast to the symmetrical rainband of the tropical cyclone. The release of latent or ‘stored’ heat, as water vapour condenses to rain droplets in the cloudy region ahead of the cyclone, can help intensify the system. Thus, mid-latitude cyclones that have a surface inflow of moist subtropical air can develop very quickly and on smaller scales, generating storm force winds and torrential rain. The terms explosive cyclogenesis or bomb are used to describe the rapid way these storms evolve and the intense weather that develops. One recent example is the so-called ‘weather bomb’ that hit Thames-Coromandel on 20–21 June last year, producing devastating flash floods.
NIWA’s recent and current research has been to better understand, model, and forecast ex-tropical storms. They are the source of some of the heaviest rain and strongest winds that northern New Zealand gets, so a better forecasting skill will reduce the risk through improved readiness.
Post-earthquake functioning of cities
Ruins of a nurses home, Napier.
New Zealand is a known earthquake-risk country and improved building codes mean that modern (post-1980) buildings are unlikely to collapse from strong earthquakes, which also usually means there are fewer casualties. However, there is still need for improvement. Although the buildings may not collapse, many (as well as essential services) will be so badly damaged in a large earthquake as to be unusable. Such damage to commercial and industrial buildings and their contents could put large numbers of people out of work for days to weeks, and loss of services, especially water, could make large areas uninhabitable for weeks.
GNS has recently been awarded funding from the Foundation for Research, Science & Technology for six years research on minimising post-earthquake trauma and economic impact for people in urban areas, through minimising damage to buildings, contents, and infrastructure. The research will:
- investigate levels of performance and design methods, leading to regulations that will minimise earthquake damage;
- develop novel seismic-isolation technologies to reduce damage;
- develop models showing how damaged infrastructure networks would function and how the networks could be repaired to quickly restore services.
Although the Wellington area is a critical target for the research, the results will have applications for any city.
Cost of the June 2002 ‘weather bomb’
A joint study has just begun to identify the overall economic impact associated with the June 2002 ‘weather bomb’ in the Waikato and Thames-Coromandel regions. Jointly funded by the Ministry for the Environment, Environment Waikato, and the Ministry of Civil Defence and Emergency Management (MCDEM), the project draws on the collective experience of specialists from GNS, NIWA, and the New Zealand Institute of Economic Research (NZIER) in economics, social research, community consultation, and emergency management. The study was driven by:
- the need for local government to adequately prepare for disasters through increasing community resilience and undertaking mitigation works;
- the desire to strive for a consistent methodology to assess the economic impact of disasters across different regions and types of events;
- the fact that climate change is likely to exacerbate some types of extreme weather disasters, so an assessment of the impacts of such events is needed to increase awareness of climate change and flow into mitigation and adaptation strategies.
The economic impact study is due to finish by late September 2003. See also the article on the weather bomb in the June 2003 ‘Wet and Wild’ issue of Tephra, available on the MCDEM website.
Pacific Tsunami Workshop
Planning is well underway for an international workshop on tsunami hazards to be held in the Michael Fowler Centre, Wellington, on 25–27 September. The workshop theme is ‘Tsunamis in the South Pacific – Research towards preparedness and mitigation’. It is organised jointly by the IUGG Tsunami Commission (IUGG/TC), the International Co-ordination Group for the Tsunami Warning System in the Pacific (ICG/ITSU), GNS, and NIWA. The workshop will feature presentations and round-table discussions. So far, more than 50 preliminary registrations have been received from researchers, emergency managers, and decision makers. Details of the workshop and final call for registration and abstracts are now available.
The workshop will be followed by the intergovernmental 19th biennial conference of ICG/ITSU on 29 September to 2 October, hosted this year by the Ministry of Civil Defence and Emergency Management. At this conference, delegates from member Pacific Rim countries will meet in Wellington to review progress and coordinate activities associated with the Pacific Tsunami Warning System.