Volcanic hazards in New Zealand

• Will act as an irritant to lungs and eyes.
• Airports will close due to the potential damage to aircraft.
• Possible minor damage to vehicles, houses and equipment caused by fine abrasive ash.
• Possible contamination of water supplies, particularly roof-fed tank supplies.
• Dust (or mud) affects road visibility and traction for an extended period.

Effects that occur with < 1 mm of ash will be amplified, plus:

• Possible crop damage.
• Some livestock may be affected. Most will not be unduly stressed but may suffer from lack of feed, wear on teeth, and possible contamination of water supplies.
• Minor damage to houses will occur if fine ash enters buildings, soiling interiors, blocking air-conditioning filters, etc.
• Electricity may be cut; ash shorting occurs at substations if the ash is wet and therefore conductive. Low voltage systems more vulnerable than high.
• Water supplies may be cut or limited due to failure of electricity to pumps.
• Contamination of water supplies by chemical leachates may occur.
• High water-usage will result from ash clean-up operations.
• Roads may need to be cleared to reduce the dust nuisance and prevent storm-water systems from becoming blocked.
• Sewage systems may be blocked by ash, or disrupted by loss of electrical supplies.
• Damage to electrical equipment and machinery may occur.

Effects that occur with < 5 mm of ash will be amplified, plus:

• Burial of pasture and low plants. Foliage may be stripped off some trees but most trees will survive.
• Most pastures will be killed by over 50 mm of ash.
• Major ash removal operations in urban areas.
• Most buildings will support the ash load but weaker roof structures may collapse at 100 mm ash thickness, particularly if the ash is wet.
• Road transport may be halted due to the build up of ash on roads. Cars still working may soon stop due to clogging of air-filters.
• Rail transport may be forced to stop due to signal failure bought on by short circuiting if ash becomes wet.

Effects that occur with < 100 mm of ash will be amplified, plus:

• Buildings that are not cleared of ash will run the risk of roof collapse, especially large flat roofed structures and if ash becomes wet.
• Severe damage to trees, stripping of foliage and breaking of branches.
• Loss of electrical reticulation due to falling tree branches and shorting of power lines.

Effects that occur with < 300 mm will be amplified, plus:

• Heavy kill of vegetation.
• Complete burial of soil horizon.
• Livestock and other animals killed or heavily distressed.
• Kill of aquatic life in lakes and rivers.
• Major collapse of roofs due to ash loading.
• Loading and possible breakage of power and telephone lines.
• Roads unusable until cleared.

Electrical Discharges

Ash clouds can generate powerful electrical fields producing intense and frequent lightning discharges, which can interfere with radio communications and damage electrical installations, or start fires in buildings and installations.

PYROCLASTIC FLOWS AND SURGES

If a large volume of volcanic debris is erupted quickly from a volcano the eruption column can collapse. Like pointing a garden hose directly up in the sky. As the eruption column collapses it can transform into an outwardly expanding flood of hot solid ejecta in a fluidizing gas cloud. This is known as a pyroclastic flow, or surge if the flow is dilute and turbulent.

The flow direction may be topographically controlled. Flows and surges often travel at speeds up to 200 km/h, and cause total destruction of the areas they cover. Flows and surges maybe very hot (several hundred ^oC) and can start fires. Some pyroclastic surges are cooler (usually less than 300^oC) and often deposit sticky wet mud. Pyroclastic flows and surges represent the most destructive manifestations of volcanic activity.

People caught in the direct path of a flow or surge are most unlikely to survive. Those who are lucky enough to survive will receive severe injuries. Buildings offer some protection at marginal areas of flows and surges but they are often destroyed. The best protection is to evacuate prior to the event.

LAVA FLOWS

Lava flows are streams of molten rock that will travel down valleys on the slopes of volcanoes. The distance they travel depends on the viscosity of the lava, output rates, volume erupted, steepness of the slope, topography and obstructions in the flow path. Basalt flows have low viscosity (flow easily) and have been recorded to travel more than 50 km from a volcano but usually only flow 5-10 kilometres. Andesite flows are more viscous and rarely travel more than 5 kilometres. Dacite and rhyolite lavas have high viscosity and typically form short, thick flows or domes.

Lava flows will seldom threaten human life because of their slow rate of movement. However, the steep fronts of flows may became unstable and can collapse, causing small pyroclastic flows. Lava flows will causes total destruction of buildings and other infrastructure in their path.

LAHARS
Lahars are "mudflows", mixtures of volcanic ash, blocks and water, formed on volcanoes. The source of a lahar maybe a crater lake, a dam collapse or heavy rainfall washing ash from the slope of a volcano.

People caught in the path of a lahar have a high risk of death from severe crush injuries, drowning or asphyxiation. Lahars are often highly erosive to river banks and eyewitnesses should remain at a safe distance. Lahar events will cause destruction of buildings, installations and vegetation caught in their path.

VOLCANIC GASES

Volcanic gases predominately consist of steam, followed by carbon dioxide and lesser amounts of sulphur and chlorine compounds. Hazardous concentrations of gases are present only very close to the crater (within 1 -2 km). Away from the vent gases pose no more than an irritant or nuisance.

LANDSLIDES (DEBRIS AVALANCHES)

A debris avalanche is the sudden catastrophic collapse (landslide) from an unstable side of a volcano. Many volcanic cones are steep sided and unstable due to rapid growth of the cone. Rising magma, earthquakes, weakening due to hydrothermal alteration and heavy rain can trigger a debris avalanche of this unstable material. Avalanched material follows valleys as it moves down the side of the volcano under the force of gravity. Debris avalanches can be wet, dry or both, and if wet, an avalanche may evolve and continue to flow further down slope as a lahar.

TSUNAMI AND SEICHES

Tsunamis are seismic sea waves of long period caused by disturbances on the sea floor. They can be produced by a submarine earthquakes, by debris avalanches and by underwater volcanic eruptions. A number of waves may be produced and they may travel long distances to far-off shores. The height of a tsunami varies and may be affected by the sea floor bathymetry, resonance effects and other factors. Volcanic tsunami may also be produced by landslides or debris avalanches flowing into the sea around an island volcano.

Large earthquakes before or during a volcanic eruption from a vent in a lake, may generate seiches (waves) on the lake. The mass entry of volcanic debris into a lake from an eruption itself may also create seiches. Low-lying land on the edge of a lake would be flooded by a seiche. Seiches may also travel down any rivers that flow from the lake.

Earthquake or volcanic activity can trigger a decrease in the fluid pressure of a geothermal system, inducing instability and boiling in the shallow portions. Thus causing steam (hydrothermal)explosions to occur. Damage from hydrothermal eruptions are generally limited in extent (a few hundred metres from the vent), and cause damage only to the immediate area around the active vent. They do not produce ash clouds, but can deposit muds.

Mt Ruapehu's 1995-1996 eruptions cost New Zealand at least $130 million. Mt Ruapehu last erupted in 1945, and although the the physical effects then were similar, the social and economic impact of the recent eruptions was much greater. The significant increase in risk over the past 50 years is due to an increase in the population and more tourism.

Tourism suffered the biggest financial impact -- estimated at $100 million - largely because the 1996 ski season was ruined. Electricity generation also suffered heavy losses estimated at $22 million. About half of that was for repairing the Rangipo power station, which had two turbines damaged by the ash-laden Tongariro River.

Losses to the aviation industry, with many airports and flights disrupted by ash, is harder to estimate. However, the value of cancelled flights alone was $2.4 million. Emergency service costs were $6.5 million. Agriculture suffered relatively light losses, estimated at $400,000. About 2000 ewes and lambs died from eating ash-covered grass, and ash destroyed the annual cauliflower crop in Gisborne.

The 1995-96 Ruapehu eruptions lasted a total of four months and threw out more ash, cinder blocks, magma jets and more lahars (mudflows) than in the history of Mt Ruapehu since European settlement.

At least 16 organisations, including the Institute, had major roles during and after the eruptions. The eruptions demonstrated the need for clear lines of communication between these organisations, and particularly among key decision-makers. The Institute's involvement at crisis management meetings was a key factor in the success of managing the response.