Feature

Auckland Erupts!!!

It's just a matter of time before the Auckland Volcanic Field lets loose, and researchers are already assessing the likely impacts.

David Johnston and Michele Daly

Mass evacuations and widespread damage from ash fall are just two of the problems foreseen by disaster preparation teams when considering the likely effects of an eruption in Auckland. A recent study of the Auckland Volcanic Field has provided a comprehensive and realistic basis for disaster response planning. The report, prepared by the Auckland Regional Council and Institute of Geological & Nuclear Sciences outlines five hypothetical eruption scenarios and the effects they would have on Auckland's one million inhabitants.

Auckland is a volcanic field, covering an area of 360 square kilometres, in which activity has occurred from scattered "monogenetic" vents during the past 140,000 years. There are 49 identified centres, although there is evidence that some adjacent centres may have been active in the same episode. By definition, monogenetic volcanoes erupt only once, in a single episode, after which the magma conduit is blocked by solidification of lava. Subsequent eruptions occur from new pathways to the surface, and from different and inherently unpredictable vent locations.

Auckland eruptions have tended to be small in volume -- typically producing 0.1 to 1.0 cubic kilometres of ejecta and lava. All the volcanoes in the field have erupted basalt and are less explosive than most of the cone and caldera volcanoes found in the central North Island. The areas affected by destructive hazards, such as pyroclastic surges, ballistic block fall, lava flows and volcanic gas are quite localised, limited to within a few kilometres of the vent. Ash falls would present a wider hazard but are more disruptive than destructive.

Although none of the Auckland volcanoes has been active since European settlement, further activity is certain at some stage and planning will help reduce the impact on New Zealand's biggest urban area.

Possible Scenarios

The purpose of the scenarios isn't to predict what the next eruption at Auckland will be like, but rather to identify some of the processes and effects that could be expected in a likely eruption. The parameters used in the scenario eruptions are based on evidence from the geological record of the Auckland Volcanic Field, plus observed eruptions at similar volcanoes overseas. However, a future Auckland eruption will not necessarily be similar to any of the scenario events, in sequence, size, duration, or vent location.

The five scenarios are as follows: Scenario 1: An offshore, explosive eruption, involving seawater flashing to steam, centred in the Rangitoto channel, producing a smaller version of Rangitoto volcano. Scenario 2: An explosive eruption centred in the Tamaki Estuary, affecting residential and industrial areas. Scenario 3: A waterfront eruption centred in the railyards, affecting central business district, port, and residential areas. Scenario 4: A fire-fountaining eruption from a vent at the top (south) end of Queen Street, Auckland City, affecting the central business district (CBD). Scenario 5: A fire-fountaining eruption from a vent at the intersection of Mt Albert/Mt Eden roads, affecting residential and commercial areas.

Lava flows, impact of air-borne projectiles, turbulent surges of ash and gas, and lightning strikes from ash clouds present a high risk of damage to people and facilities in the scenario eruptions but the extent of these hazards (in a typical Auckland eruption) is mostly limited to within five kilometres of the vent. Severe ground shaking near vents and accompanying volcanic earthquakes will also damage buildings, possibly also in areas otherwise not greatly damaged by eruption products. Apart from the evacuation of people and removal of plant (if possible) there are few or no mitigation options available to counteract any of these near-vent hazards.

Evacuation, where necessary to save lives in high-risk, near-vent areas, should begin before an eruption commences, and would have to be completed before the eruption peaks. This could involve in excess of 150,000 people. Elsewhere, later evacuation of people may became necessary where loss of services (electricity, water supply, sewerage) makes continuing habitation untenable.

The sole widespread hazard in our model is ash fall from the eruption plume. The scenarios illustrate the vulnerability of urban areas which receive even a few millimetres of ash. Thin tephra fall (2 mm) is sufficient to cause disruption of transportation, electricity, water, sewage and stormwater systems. Most systems, once affected, can be restored within a few days to weeks after an eruption has ended.

Falls of volcanic ash can disrupt electricity supply depending on weather conditions, with power outages occurring if the ash is wet, as it then becomes conductive. Immediate and on-going ash removal is the best mitigation option to prevent widespread outages. Volcanic ash falls can cause severe damage to sewage and stormwater systems with the most effective mitigation measure being simply to reduce the input of ash into the system. Sewage treatment plants can be severely affected both by ash falling directly on the plant and by the receipt of ash-laden sewage. Water supplies are vulnerable to contamination by ash fall into storage lakes preventing their use, and to excessive demand during post-eruption ash clean-up operations.

Urban areas in the region would be forced to undertake expensive and time-consuming clean-up operations as a consequence of any of the scenario eruptions. In each case there is a need to develop coordinated community-wide ash-removal plans, that identify appropriate methods of ash removal, collection and disposal. The public need to be adequately informed about how to deal with volcanic ash.

The eruption scenario/impact exercise has highlighted the value of emergency management planning that will identify in advance likely impacts on community lifelines and strengthen the links between agencies that will have to respond to such events. Planning needs to be done now, hopefully well in advance of any volcanic activity. Emergency managers need to agree on early and proactive responses to warnings. The Auckland Regional Council is currently planning to increase the number of seismometers monitoring the field from four to at least five, providing improved early warning when volcanic activity reoccurs.

Michele Daly works for the Auckland Regional Council.
David Johnston is with the Institute of Geological & Nuclear Sciences Wairakei Research Centre in Taupo.