Prepared by:Adam MunroDavid Parkin

For:Environment WaikatoPO Box 4010HAMILTON EAST

26 May 1999

ISSN: 1174-7234

Environment Waikato Policy Series 1999/10

Volcanic Risk Mitigation Plan

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Table of ContentsTable of Contents i

Executive Summary iii

Background and Explanation v

1 Introduction 1

2 Pre-eruption: Mitigation Techniques for Non-crisis Periods 32.1 Geological Studies 32.2 Planning 32.3 Scientific Alert Levels and Science Alert Bulletins 62.4 Monitoring 62.5 Satellite Remote Sensing 7

3 During an Eruption: A Description of Different Volcanic Hazards and MitigationMeasures for those Hazards 9

3.1 Tephra Falls 93.1.1 People 103.1.2 Agriculture and Horticulture 103.1.3 Building Structures 133.1.4 Electricity 133.1.5 Water Supply 143.1.6 Wastewater Networks (Stormwater Drainage and Sanitary Sewers) 143.1.7 Sewage Treatment Plants 143.1.8 Gas 143.1.9 Transportation 143.1.10Communications 153.1.11Mechanical, Electrical and Electronic Equipment 15

3.2 Mitigation Measures for Tephra Fallout 153.2.1 People 153.2.2 Agriculture and Horticulture 153.2.3 Building Structures 163.2.4 Electricity 163.2.5 Water Supply 173.2.6 Wastewater Networks (Stormwater Drainage and Sanitary Sewers) 173.2.7 Sewage Treatment Plants 183.2.8 Transportation 183.2.9 Mechanical, electrical and electronic equipment 193.2.10Ash Disposal 193.2.11Detailed Mitigation Measures 19

3.3 Ballistic Fallout 203.3.1 Mitigation Measures for Ballistic Fallout 20

3.4 Lahars 203.4.1 Mitigation Measures for Lahars 20

3.5 Pyroclastic Flows 223.5.1 Mitigation Measures for Pyroclastic Flows 22

3.6 Pyroclastic Surges 223.6.1 Mitigation Measures for Pyroclastic Surges 22

3.7 Directed Volcanic Blasts 233.7.1 Mitigation Measures for Volcanic Blasts 23

3.8 Lava Flows 233.8.1 Mitigation Measures for Lava Flows 23

3.9 Debris Avalanches 243.9.1 Mitigation Measures for Debris Avalanches 24

3.10 Volcanic Gases 243.10.1Mitigation Measures for Volcanic Gases 25

3.11 Tsunamis and Seiches 253.11.1Mitigation Measures for Tsunamis and Seiches 26

3.12 Flooding 26

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3.13 Hydrothermal Eruptions 263.13.1Mitigation Measures for Hydrothermal Eruptions 26

3.14 Volcanic Earthquakes 273.14.1Mitigation Measures for Volcanic Earthquakes 27

3.15 Electrical Discharges 273.15.1Mitigation Measures for Electrical Discharges 27

3.16 Other Hazards 27

References 29

Appendix I: Scientific Alert Levels (Johnston, 1997a) 37

Appendix II: Detailed Mitigation Measures 38

Appendix III: 1995-1996 Ruapehu Eruptions Survey 52

Table of FiguresFigure 1: Summary of volcanic hazards from a composite cone volcano

(after Myers et al., 1997) 1Figure 2: Volcanic hazard management during non-crisis (pre-eruption) and

crisis (during an eruption) periods (after Johnston and Houghton, 1995). 2Figure 3: Summary of the applications of remote sensing for volcanology (after

Oppenheimer, 1997). 8Figure 4: Ash from Mount Ruapehu carried by southeasterly winds over Lake Taupo

during the 1995-1996 Ruapehu eruptions. 9Figure 5: The interaction of volcanic gases during an eruption (after Johnston, 1997a). 25Figure 6: An eruption from Mount Ruapehu on 8 July 1996. 53Figure 7: Question 2 -What category do you class your business in? 54Figure 8: Turnover of businesses that answered the Ruapehu survey (logarithmic scale). 56Figure 9: The Arrangement of Zones around Mount Ruapehu 57Figure 10: Question 6 Please indicate where your home or business is located. 58Figure 11: Series of graphs looking at the relationship between peoples location and

how likely they think they would be affected by a future eruption. 59Figure 12: Respondents knowledge of what to do during an eruption. 61Figure 13: How did you first learn that there was an eruption occurring from

Mount Ruapehu? 63Figure 14: Location of the respondent versus whether they were affected by the 1995-1996

Ruapehu eruptions. 64Figure 15: Question 19 Did you suffer any economic loss related to the Ruapehu eruptions?69Figure 16: Losses (NZ$) suffered by different types of business (Logarithmic Scale). 69Figure 17: Types of stress suffered as a result of the Ruapehu eruptions. 70

TablesTable 1: Educating the public about volcanic hazards (after Gregory 1995; Peterson, 1996;

Voight, 1996). 5Table 2: Impacts on plants and soil from increasing ash thickness (after Folsom, 1986, and

Blong, 1984; in Neild et al., in prep). 11Table 3: Periods of high crop risk from ash (after MAF, 1995; Neild et al.,1998). 12Table 4: Mitigation measures for volcanic ash and the water supply

(after Johnston, 1997a, 1997b). 17Table 5: Number of survey participants involved in each type of business. 55Table 6 : How respondents solved or fixed problems caused by the Ruapehu eruptions. 66Table 7: Common hints suggested by survey respondents. 67Table 8: Lifestyle adaptations made in response to the Ruapehu eruptions. 70

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Table 9: Range of benefits from the Mount Ruapehu eruptions. 72Table 10: Organisations that respondents turned to for advice or general

information during the 1995-1996 Ruapehu eruptions. 73Table 11: Impact of a volcanic eruption from Mount Ruapehu in different seasons. 74

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Executive SummaryThe Volcanic Risk Mitigation Plan has been written:

a) To achieve the natural hazards objectives of the Waikato Regional PolicyStatement. These are to define the management functions of Environment Waikatoand the district councils and to minimise the adverse effects associated withnatural hazards.

b) In response to Environment Waikato s volcanic risk management responsibilitiesunder the general provisions of the Resource Management Act 1991 and the CivilDefence National Plan.

c) To achieve Environment Waikatos responsibilities under the Civil Defence Act1983. The three aims of civil defence are to prevent loss of life, to help the injured,and to relieve personal suffering and distress.

d) To meet the International Decade for natural Disaster Reduction requirements.These include the provision of mitigation plans involving long term prevention,preparedness and community awareness.

e) To integrate Environment Waikatos activities with other organisations, and assistthem to achieve their organisation and professional responsibilities.

The plan confirms the principles accepted by Environment Waikato as the basis of theVolcanic Risk Mitigation Plan. There is an emphasis on working in partnership withdistrict councils and communities to find acceptable solutions to volcanic issues.

The first section of the plan outlines the roles and responsibilities of district councils inimplementing volcanic risk mitigation measures, the second section outlines pre-eventtechniques and the third section outlines techniques that could be used during aneruption.

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Background and ExplanationThe Waikato Region has more volcanic hazards than any other region in New Zealand,because a large part of the Taupo Volcanic Zone (TVZ) lies within or adjacent to itsboundaries. There are three presently or potentially active volcanic centres locatedwithin the Waikato Region, these being Tongariro, Taupo, and Maroa. Due to thelocation of many urban areas (e.g. Turangi, Taupo, and Tokoroa) within the TaupoVolcanic Zone, mitigation and contingency planning is an essential ingredient inproviding maximum protection to people living in these areas. Many of the NorthIslands vital Lifelines are also located in the Taupo Volcanic Zone - all of which couldbe affected during a major eruption.

Five active volcanic centres are located outside the Regional boundary that pose justas much risk to the residents of the Waikato Region than those volcanic centreslocated within the Waikato Regional boundary. This contingency plan does notspecifically address these other areas. Information about these volcanic centres canbe found in the respective regions volcanic mitigation and/or contingency plans.

Environment Waikato is responsible under statute to manage volcanic risk.Environment Waikato and District Councils have responsibilities to avoid and lessennatural hazards under the Resource Management Act (RMA) 1991. The emphasis forEnvironment Waikato is on regional risk management. The emphasis for DistrictCouncils is on the controlling the effects associated with the use of land. BothEnvironment Waikato and District Councils have responsibilities for pre-event planning,response, and recovery under the Civil Defence Act 1983.

The purpose of this plan is to outline suitable mitigation options that will minimise theadverse effects of future volcanic activity on the Regional community and economy.

Principles

A number of principles have been used to develop Environment Waikatos riskmitigation plans. The principles used in previous plans are also applicable to thevolcanic risk mitigation plan. The general principles are:

a) Recognise the Primacy of the Resource Management Act 1991

The Resource Management Act 1991 repealed and amended much of theprevious legislation relating to watercourses. The legislation that remains issubject to the RMA. For example, Part 1 section 10A of the Soil Conservationand Rivers Control Act 1941 states nothing in this Act shall derogate from theResource Management Act 1991.

Environment Waikato considers it vital in the development of mitigation policy torecognise the primacy of the RMA, while promoting the integration of the variousActs in policy development:

i) Resource Management Act 1991.ii) Soil Conservation and Rivers Control Act 1941.iii) Land Drainage Act 1908.iv) Local Government Act 1974.v) Building Act 1991. The advantages of integration are clarity, with responsibilities dealt with by themost appropriate and legally bound agency, issues will not be passed from oneagency to another and administration will become more efficient. Integration willbe given overall authority through the Regional Policy Statement (RPS).

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Mitigation plans produced by Environment Waikato therefore reflect theprovisions of the Acts and is the approved policy of Environment Waikato(Waikato Regional Council).

b) Promote a Strategy of Avoidance then Mitigation

The RMA and the RPS give a strong lead for avoidance and mitigation throughthe control of land use. Land known to be subject to natural hazards should besubject to clear land use controls. New development, in particular, should beanalysed for potential exposure or risk from natural hazards. VolcanicContingency Plans, which complement mitigation plans, are written under therequirements of the Civil Defence National Plan.

Mitigation is a tool which can be used to deal with situations where a combinationof the natural hazard and the vulnerability of the community create a risk.Mitigation can be used for existing or proposed development to ensure anacceptable level of risk is maintained.

c) Information Readily Available High quality information on hazards and potential risks and the widespreaddissemination of this information are vital for effective risk management and riskreduction.

f) Partnership with District Councils

Environment Waikato and district councils both have responsibilities for naturalhazard management. This Plan clearly defines the respective responsibilitiesand promotes a partnership approach to management. The natural hazardsSection (3.8) of the Waikato Regional Policy Statement addresses the uncertaintyover the allocation of responsibilities of the regional and district councils. Section3.8.3 Management of Natural Hazards, Policy One, acknowledges the role districtcouncils have historically undertaken for the control of the use of land.

h) Community Safety

Environment Waikato recognises the value of community input in decisionmaking. Environment Waikato and district councils have a responsibility toenable communities to provide for their health and safety under the RMA.Environment Waikato has responsibilities for community safety under the CivilDefence Act 1983. The Waikato Region Civil Defence Plan 1996 states the threeaims of civil defence. These are:

vi) to prevent loss of lifevii) to help the injuredviii) to relieve personal suffering and distress. Therefore when Environment Waikato assesses volcanic risk managementoptions, community safety has overriding importance.

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Related Documents:

This Mitigation Plan represents the final milestone in completing a comprehensivestudy of the volcanic hazard and risk in the Waikato Region. Other relatedreports/sources of information that this plan complements include:

Regional Civil Defence Plan (and Standard Operating Procedures) National Civil Defence Plan National Volcanic Contingency Plan Civil Defence Act 1983 Waikato Region Volcanic Hazard Assessment Volcanic Risk Mitigation in the Waikato Region (University Thesis) Crater Lake Instability Study Lahar Hazard Assessment of the Tongariro River Taupo District Council Volcanic Hazard Analysis District Councils Civil Defence Plans District Councils Volcanic Contingency Plans IGNS Internet Site: www.gns.cri.nz Ministry of Civil Defence Internet Site: www.mocd.govt.nz

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1 IntroductionAs defined in the introduction, a volcanic hazard describes the physical characteristicsof an eruption (Blong, 1996). While a volcano is in eruption it will produce a variety ofhazards. Near-vent volcanic hazards tend to be very destructive, while distal hazardsmay cause damage to structures or disrupt everyday life. Even when a volcano is notin eruption, volcanic hazards such as debris avalanches or remobilised secondarylahars can still occur. Figure 1 summarises some of the hazards that may be expectedfrom a typical composite cone volcano.

Mitigation of volcanic hazards can be undertaken during periods of crisis, while avolcano is in eruption. Studies of recent eruptions have led to the identification ofmitigation measures that were used successfully while an eruption was in progress.Extensive measures have been identified for the mitigation of problems caused by ashfall. However, there are still a number of hazards that have few mitigation optionsavailable. For example, pyroclastic flows and surges are so destructive that the onlyreally viable option is to evacuate the population at risk prior to the event.

The management and mitigation of volcanic hazards should not only occur during crisisperiods. It is also important that management of volcanic hazards is initiated andundertaken in periods of non-crisis, prior to an eruption occurring. Pre-planning willensure that the mitigation measures employed in response to a crisis are successful.Figure 2 illustrates the different aspects of volcanic hazard management under crisisand non-crisis (pre-event) conditions.

Figure 1: Summary of volcanic hazards from a composite cone volcano(after Myers et al., 1997)

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Figure 2: Volcanic hazard management during non-crisis (pre-eruption)and crisis (during an eruption) periods (after Johnston andHoughton, 1995).

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2 Pre-eruption: Mitigation Techniques forNon-crisis Periods

2.1 Geological StudiesIt is essential to carry out extensive geological investigations of potentially activevolcanoes during periods when those volcanoes are in repose. Studying a volcanoseruptive record enables scientists to reconstruct how each volcano has erupted in thepast. From this information, it is possible to ascertain the types and magnitude ofhazards posed by the volcano, and determine how frequently active the volcano is.This information is fundamental, and is always the starting point when planning andpreparing for a future eruption.

2.2 PlanningDuring periods when volcanoes are not active, planning and preparation should beundertaken to ensure the effects of a volcanic eruption are minimised. In the Mount St.Helens eruption, the value of planning was one of the strongest lessons learnt by thoseinvolved. Planning is important at national, regional, local and even individual levels(Saarinen and Sell, 1985).

The following aspects should be considered when planning for a volcanic eruption.

Land use development and regulation to prevent development in zones that are ofhigh risk to volcanic hazards (Johnston and Houghton, 1995).

Where thick ash fall is likely to occur, building codes that require roofs to havesteeper pitches could be implemented (Spence et al., 1996; Johnston, 1997a).This is especially important for critical buildings such as hospitals, fire stations,police stations, public buildings and schools (Johnston, 1997a).

Plans must be established regarding procedures during a volcanic eruption. Plansmay need to detail procedures for notifying the public about the eruption,procedures for shutting down operations and maintenance and clean upprocedures (Federal Emergency, Management Agency, 1984; Johnston, 1997a;1997b). Recovery planning should also be considered within the contingency plan(Johnston, 1997a).

Plans and procedures need to be flexible enough to adapt to what may be rapidlychanging conditions during a volcanic eruption (Peterson, 1996; Johnston 1997a).

Sample emergency ordinances should be prepared in advance (FEMA, 1984).

Johnston (1997a) suggests making a list of facilities that must be kept operative,versus those that can be shut down during and after ash fall.

It is advisable to consider the need for stress counselling both for the general publicand emergency workers (Finnimore et al., 1995).

Pre-test the plan so that people know what roles they must fulfil (FEMA, 1984).

The 1996 Mount Ruapehu eruptions confirmed that the preparedness of a district isbased on past experiences. As a result of the 1995 eruption experience,organisations were able to respond quickly and more effectively. It is important topass on information about lessons learnt from past eruption experiences to new

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staff in the organisations, so that they too can use that information effectively (Neildet al., 1998).

EvacuationEvacuation may be necessary in the event of a volcanic eruption. Near to the sourceof the eruption it may be advisable to evacuate the area prior to activity in order to savelives. It is also important to note that heavy tephra falls may cut off transport routesafter the eruption, thus hindering any effort to evacuate people (Johnston and Nairn,1993).

There is a need to plan for the transportation, sheltering, feeding, clothing and medicaland hygiene needs of any evacuees or those that are stranded by an eruption. In theevent of a volcanic eruption there may be a large number of displaced people that needto be cared for, and pre-planning will mean that those people have places where theycan stay (FEMA, 1984; Johnston and Nairn, 1993; Finnimore et al., 1995).

Before an eruption, it is necessary to identify resources that can be used to assist inthe evacuation of large numbers of residents. For example this may include towingfirms, mechanical repair firms, emergency fuel supplies and bus companies. Otherissues that should be considered include the control of traffic, and animal transport andwelfare. The early identification of needs during a volcanic eruption will allow readyarrangement of outside assistance when an eruption occurs (Environment BOP, inprep).

Spare PartsSpare parts or critical equipment that may be needed during a volcanic eruption shouldbe stockpiled. This may include air filters, cleaning equipment, protective clothing, facemasks and extra fire hoses (Novak et al., 1981; FEMA, 1984; Johnston, 1997a). Extravehicles for emergency use by police and other personnel may also be required(FEMA, 1984).

EducationEducation of the public about volcanic hazards and how to mitigate against the effectsof a volcanic eruption is important. Education will lessen the physiological and physicalimpacts of an eruption on the public. Warnings can be better understood if the publicunderstands the nature of the hazard. Also, since communications may be disruptedduring and after an eruption, it is necessary to distribute information before an event sopeople know what to expect and what to do (Johnston and Nairn, 1993).

The public can be educated through newspaper articles, television, radio, the Internet,exhibits at museums, brochures, talks by scientists to clubs and organisations andschool classes. Education about volcanic hazards aimed at school children has theadded benefit that parents become informed too, through their children (Peterson,1996).

Table 1 summaries what the public needs to know about volcanic hazards and outlinessome techniques in disseminating hazard information.

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Table 1: Educating the public about volcanic hazards (after Gregory 1995;Peterson, 1996; Voight, 1996).

What the publicneeds to know

- Basic information about volcanoes in general, and the volcanoes intheir area. (This may include their shape, size, places from whereeruptions have issued and common types of eruption in the past. Alsoit may be useful to provide some facts about particular eruptive eventssuch as size violence, volume of ejecta, areas affected and how ofteneruptions have occurred).

- Successive eruptions at the same volcano can have great variations instyle, size and violence.

- Time intervals between eruptions can vary widely.- Neighbouring volcanoes may differ greatly from one another in their

eruptive habits and characteristics.- Science has capabilities, but it also has limits too.- Some volcanic unrest ends without an eruption.- People need to know the time frames that apply to different statements

about possible future activity.- Practical measures for personal protection and mitigation of volcanic

hazards.

The Volcanic Hazards Information Series published by IGNS is an exampleof an education campaign that incorporates most of the features mentioned

above.

Techniques forrelaying themessage

- Provide concrete personalised information (personalise the risk) sothat the public understand that volcanic risk applies to them (forexample, mention specific locations).

- Information must come from, and be relayed by a credible source.- Accurate and clear information should be provided.- Messages should be presented with confidence and conviction.- Repeat the message to provide consistent reinforcement (confirmation)

and to reach a broader population.- Use a range of communication channels including printed media,

electronic media and personally delivered messages.

The MediaMost people rely on the media for receiving information. Surveys by Johnston et al.,(1997) show that public knowledge and awareness of events during the Ruapehueruptions were derived almost entirely from the media.

Effective management of the media is required so that accurate information can beconveyed to the public during a volcanic eruption. In a recent survey of organisationsby Paton et al. (1998), 43 percent of respondents reported that they had sufferedmedia problems during the 1995 Ruapehu eruption. These results highlight the needfor organisations to develop an effective media response, and to provide training formedia spokespersons. Paton et al. (1998) suggest addressing this problem byincluding a media management component in training programs.

The increased public demand for information during a volcanic eruption may besupplemented by distributing printed information (Johnston, 1997b).

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NetworksThe FEMA (1984) recommend that prior to a volcanic eruption, roles andresponsibilities of the different organisations should be defined, and a network ofauthority under which individuals would work in an emergency should be established.

In the survey of organisations by Paton et al. (1998), it was found that manyrespondents believed there was a lack of clear responsibility for co-ordination overthe duration of the 1995 Ruapehu eruptions. There is therefore a need to establishinter-organisational networks among those organisations that may be involved indealing with a future volcanic eruption. Paton et al (1998) suggest that moresimulations and exercises would help identify and resolve co-ordination problems.Another recommendation was for groups to work together in the planning stage todevelop their capability to work as an integrated team (Paton et al., 1998).A volcanic eruption may cover more than one local authority, and a shift in winddirection may even change the entire area of impact. Because volcanic eruptionscover wide areas, a nationally co-ordinated effort could reduce duplication. Neild et al.(in prep) suggest that this is particularly true for providing information to the public andmedia. However, concerns have been expressed over how Emergency ManagementGroups would function without local knowledge if co-ordination were controlled from anoutside centre (Neild et al., in prep).

2.3 Scientific Alert Levels and Science Alert BulletinsThe New Zealand Scientific Alert Levels (Appendix 1) are used to determine differentlevels of volcanic unrest. The system is numbers based and ranges from zero(volcano in a dormant state) to five (large hazardous eruption in progress). There aretwo scales one for frequently active volcanic cones, and one for re-awakening ofdormant volcanoes.

A dual system is necessary, as the different types of activity require differentresponses. The Scientific Alert Levels are useful for organisations as they can pre-planfor different responses depending on the level that a volcano has been assessed at(Johnston, 1997a).

Science Alert Bulletins are issued by IGNS during volcanic eruptions, and provideinformation on the status of a volcano. The Science Alert Bulletins are useful becausethey contain information regarding the scale of the activity, highlight developing orexpected problems, and may contain predictions about activity. The informationcontained in a Science Alert Bulletin may allow organisations to put response plans intoeffect before being overwhelmed by a volcanic eruption (Johnston, 1997a).

2.4 MonitoringWithin New Zealand, monitoring of all our active volcanoes takes place. Volcanosurveillance enables scientists to note any changes to a volcano, and if possibleprovide warning of an impending eruption. At this stage, appropriate steps can thenbe taken by organisations to reduce the risk to lives and property (Scott et al., 1995).

Three main types of monitoring are undertaken in New Zealand. The first technique ismonitoring of volcanic earthquakes. There are five volcano seismic networks inoperation around New Zealand. There are networks situated at Tongariro, Taupo andin the Bay of Plenty and these are monitored by IGNS. The other two networks arelocated in Auckland and Taranaki and are monitored by the relevant regional council(Scott et al., 1995).

Measurement of ground deformation is a second monitoring method used on NewZealand volcanoes. Measurement of ground deformation can be done in a number ofways:-

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- Measuring distances with electric distance measuring equipment (EDM).- Ground tilting measurements are made by precise levelling and using some of the

volcanic lakes as large scale, natural spirit levels (Scott et al., 1995; Scarpa andGasparini, 1996).

- Horizontal control surveys using triangulation and trilateration techniques (Scott etal., 1995).

- Global Positioning Surveys (GPS) can be used to measure horizontal and verticalearth shifts (Scott et al., 1995; Scarpa and Gasparini, 1996). Two GPS receivershave recently been installed at Mount Ruapehu to try and detect grounddeformation. Previously, plotting land deformation on Mt Ruapehu has beendifficult, but if the GPS receivers prove to be successful more will be installedfurther up the mountain, closer to the crater (Hurst, 1998).

Changes in gas chemistry, the rate of gas emission from craters and the chemistry ofcrater lake and thermal spring waters can also be used to detect changes in a volcano.Other evidence of unrest can also be detected from changes in groundwater, lakelevels, rate of stream flow and water temperature (Scott et al., 1995; Giggenbach,1996).

2.5 Satellite Remote SensingSatellite remote sensing can be used successfully to detect changes in activevolcanoes before they erupt. Infrared detectors can detect changes in the temperatureof the volcano. As magma moves to the surface a volcano will get hotter. Crater lakeswill also heat up before an eruption. Both of these types of changes can be detectedusing infrared remote sensing (Oppenheimer, 1993; Oppenheimer, 1997). Theamount of swelling or deflation of a volcano can also be determined by looking atchanges in topography. This is done using a technique called SAR interfermometry(Oppenheimer, 1997).

Satellite imagery can be used for mapping geology, and it can also be used to detectmorphological features in a fashion similar to air photography. Both of theseapplications are useful in hazard assessment (Oppenheimer, 1997).

After a volcanic eruption has occurred, satellite data can be used to monitor the ashcloud that is produced (Francis et al., 1996; Oppenheimer, 1997). Satellite remotesensing was used to detect and track clouds from Mount Pinatubo in 1991 (Casadevallet al., 1996). The eruption cloud can be tracked on successive satellite images, andthe horizontal spreading velocity of the plume can be established. From thisinformation it is possible to predict whether the ash cloud will reach populated areas,and if so, when. The height of the eruption cloud can also be determined by usingsatellite imagery. Determination of cloud height is important for aviation hazardmitigation (Oppenheimer, 1997).

Mount Ruapehu eruption clouds were tracked using an Advanced Very High ResolutionRadiometer (AVHRR) during the 1995-1996 eruptions (White and Hockey, 1996).Unfortunately several problems were encountered that limited the usefulness of theimages:-

- The eruptions were short-lived and generally did not coincide with satelliteoverpasses;

- Many of the eruptions that occurred were obscured by cloud cover; and- AVHRR could not be used to measure the water temperature of Crater Lake as the

lake is too small.

As a result of these restrictions, the monitoring of Mount Ruapehu was mostlydependent on field data and aircraft observations (White and Hockey, 1996).

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Ground based radar can also be used to track drifting volcanic clouds. Radar candetermine the height of the eruption cloud and the structure of the cloud. Portableradar are useful as they can be moved around depending on where an eruption hasoccurred. Doppler radar systems can sense particle size distribution in a volcaniccloud (Rose and Kostinski, 1994).

Massive releases of sulphur dioxide from eruptions can be determined using the TotalOzone Mapping Spectrometer (TOMS). Measurement of volcanic aerosols can also beundertaken using remote sensing techniques (Francis et al., 1996; Oppenheimer,1997). It is important to measure these as they are relevant to circulation, radiativeenergy balance and chemical processes in the atmosphere. Volcanic aerosols canhave a climatic impact, and this was demonstrated after the Mount Pinatubo eruption in1991 (Oppenheimer, 1997).

A summary of the applications of remote sensing to volcanology can be seen inFigure 3.

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Figure 3: Summary of the applications of remote sensing for volcanology(after Oppenheimer, 1997).

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3 During an Eruption: A Description ofDifferent Volcanic Hazards andMitigation Measures for those Hazards

3.1 Tephra FallsAsh has erupted volcanic material that is less than 2 mm in size. Ash particles aresmall enough to be carried by the wind, and therefore the areas where ash is depositedare determined by how high the ash is carried into the atmosphere, and the directionand strength of the wind at the time (Figure 4). During a major eruption, very largeareas can be affected by tephra fallout (Blong, 1984). Pumice lapilli is erupted pumicematerial that ranges from 2 to 64 mm. Pumice lapilli may also be spread over asignificant area during a pyroclastic eruption.

Figure 4: Ash from Mount Ruapehu carried by southeasterly winds overLake Taupo during the 1995-1996 Ruapehu eruptions.

Ash rarely causes direct damage, but instead accumulates and causes structures (forexample, buildings, tree branches, electricity lines and telephone lines) to collapse(Blong, 1984; Houghton et al., 1988). Ash particles may carry a film of corrosive acidand this causes corrosion on metallic surfaces (Houghton et al., 1988). Ash isabrasive, and can be conductive especially when wet (Labadie, 1983).

Danger exists where large pumice lapilli have not reached thermal equilibrium beforeimpact. The impact temperatures of the lapilli may be high enough to ignite a variety ofmaterials at considerable distances (Blong, 1984).

The hazardousness of ash fall may be influenced by a number of other factorsincluding whether the ash is wet or not, and the grain size of the ash (Blong, 1984;Houghton et al., 1988). When ash is wet, it is very heavy and causes buildings andother structures to collapse sooner than those covered in dry ash. A finer grain size ofash may represent a greater hazard than coarser grain sizes (Blong, 1984). Finer grainsizes will penetrate machinery and other human structures more readily. Also, finegrained ash becomes cohesive when wet, resulting in crusting of ash layers which

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causes increased rain run-off (Blong, 1984). Toxic fluorine compounds are alsoconcentrated on fine grained particles (Thorarinsson, 1979; Blong, 1984).

3.1.1 PeopleRespiratory problems, eye irritations, skin irritations (ash rash or acid rash) andstress reactions will be experienced by people in the event of ash fallout (Blong, 1984).As well as experiencing minor respiratory problems, it is possible to encounter seriousrespiratory problems as a result of breathing in falling ash. Chronic bronchitis,pneumovolconiosis or silicosis can be contracted from breathing in ash. Silicosis is alung disease resulting from the inhalation of fine particles of free crystalline silica whichhave toxic effects on the lungs causing fibrotic changes. For silicosis to develop thevictim must be exposed to crystalline quartz (quartz, cristobalite or tridymite) of arespirable range (that is, less than 10 microns) (Blong, 1984).

During the Mount St. Helens eruptions of 1980 it was noted that while many peopledeveloped medical problems that were directly related to the ash fallout, there was alsoa rise in ash-related accidents. For example, motor vehicle accidents and falls fromrooftops increased during the period that ash was present (Blong, 1984).

3.1.2 Agriculture and HorticultureA large area of New Zealand is utilised for agriculture and horticulture and is especiallyvulnerable to the effects of volcanic activity. While heavy ash falls would be disastrous,even light ash falls of less than 5mm would cause problems for livestock (Neild et al,1998).

Ash that falls on pasture or in drinking troughs can affect the health of grazing animals(Gregory and Neall, 1996). Food will be scarce where ash fall has been heavy, andanimals may require supplementary feed.

Volcanic ash poses a pneumoconiosis risk for animals, as it does for humans. In rarecases, tephra can also cause asphyxiation during heavy ash falls with the formation ofan obstructing plug of ash and mucus in the upper respiratory tract (Gregory and Neall,1996).

Where tephra falls have been light, ash is easily ingested by grazing animals. Asfluorine coats fine ash particles readily, animals at a distance from the erupting volcanomay ingest ash that is coated with fluorine and as a consequence contract fluorosis(Thorarinsson, 1979). Signs of poisoning include lesions in the nose and mouth, andhair falling out around the mouth (Thorarinsson, 1979; Gregory and Neall, 1996). Inextreme cases, especially where animals are at risk (for example, pregnant orlactating), deaths may occur. If ash falls were thick and widespread, significant stocklosses could be expected.

Fluorine may also find its way into water courses. If fluorine covered tephra is rainedon, or if the tephra lands on wet ground, then the fluorine may be leached out (Gregoryand Neall, 1996).

Animals could also suffer complications, such as polioencephalomalacia in sheep orthe development of copper deficiency, from eating excessive amounts of sulphur(Gregory and Neall, 1996).

If vegetation is destroyed by a volcanic eruption, fish may be killed as a result of theincreased water temperature in the river. Fish may also be killed by suspendedsediments, higher acidity and higher concentrations of fluorine in water bodies after aneruption. Aquatic floral and faunal populations are also susceptible to ash suspendedin rivers or lakes (Neild et al., 1998).

If there are widespread ash falls, birds may die from a lack of food. Gases may also killbirds near the vent area. Insects are particularly susceptible to ash, as the epicuticular

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wax layer is abraded by ash particles which causes rapid desiccation and death (Cooket al., 1981; Neild et al., 1998).

Tephra falls from an eruption would have both physical and chemical effects onhorticulture (Neild et al. 1998). Table 2 shows the impacts on plants and soil fromincreasing thicknesses of ash.

Table 2: Impacts on plants and soil from increasing ash thickness (afterFolsom, 1986, and Blong, 1984; in Neild et al., in prep).

Ash Thickness Impact on Plants and SoilThin Burial(< 5mm tephra)

- No plant burial or breakage.- Ash is mechanically incorporated into the soil within one year.- Vegetation canopies recover within weeks.

Moderate Burial(5-25 mm tephra)

- Buried microphytes may survive and recover.- Larger grasses are damaged but not killed.- Tephra layer remains somewhat intact on the soil surface after one

year.- Soil underneath remains viable and is not so deprived of oxygen or

water that it ceases to act as a topsoil.- Vegetation canopies recover within next growing season.

Thick burial(25-150 mm tephra)

- Complete buries and eliminates the microphytes.- Small mosses and annual plants will only be present again in the local

ecosystem after recolonisation.- Generalised breakage and burial of grasses and other non-woody

plants.- Some macrophytes of plant cover do not recover from trauma.- Large proportion of plant cover is eliminated for more than one year.- Buried soil is revitalised when plants extend roots and decaying

organic matter from the surface of the tephra layer down to the top ofthe buried topsoil and affect an integration of the tephra and buried Ahorizon. Generally accomplished in four to five years.

- Vegetation canopy recovery takes several decades.Very thick burial(>150 mm tephra)

- all non-woody plants are buried.- Burial will sterilise soil profile by isolation from oxygen.- Soil burial is complete and there is no communication from the buried

soil to the new tephra surface.- Soil formation must begin from a new time zero.- Several hundred (to a few thousand years) may pass before new

equilibrium soil is established.

The time of year, or stage of plant growth, will also affect the impact of ash fall onvegetation. For example, a thin layer of tephra deposited in the growth season may domore harm than a thicker layer deposited in other seasons (Thorarinsson, 1979).Volcanic dust may also affect pollination time. The dust may impede the transfer ofpollen to the receptive parts of the flower, resulting in fewer fruit set and smallerdeformed fruit (Neild et al., 1998). The following table (Table 3) shows the stage thateach crop is most at risk.

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Table 3: Periods of high crop risk from ash (after MAF, 1995; Neild etal.,1998).

Crop Period at RiskPea From emergence until the end of flowering.Squash During the initial stages of growth and flowering.Tomatoes During seed emergence and flowering stages.Sweetcorn During the early stages of growth.Pipfruit Has three danger periods:-

- blossom where severely acidic ash (pH less than 3) could burn plant tissueand result in poor pollination;

- 6 to 8 weeks after blossoming, when the skin of the- fruit is particularly sensitive; and- later stages of development when fruit is prone to cosmetic blemishing.

Stonefruit Stone fruit is also susceptible at the same times as pipfruit, except that theearly fruit development period is four to six weeks after blossoming, whensensitive fruit skins could be damaged, and show russet or deformation insevere cases.

Kiwifruit Kiwifruit is also at risk at, and six to eight weeks after, blossom. There wouldalso be a problem at harvest time. As kiwifruit cannot be washed prior topacking, the hairy nature of the fruit would make ash removal very difficult.

Grapes Grapes have three main periods when damage could occur:-- flowering, when acidic ash could burn plant tissues, reduce pollination and

reduce bunch fill;- fruit development, where ash deposits would block sunlight and reduce

quality; and- harvest, where ash deposits would be a contaminant with the extra acidity

of the ash possibly having a significant impact on wine quality. Ash wouldhave to be removed prior to harvesting by washing and allowing bunchesto dry.

Grains Ash showers near maturity will make harvesting difficult and reduce the qualityof the grain.

Evergreenperennial crops

(For example, avocado and citrus) Susceptibility is more uniform throughoutthe year due to their persistent foliage cover.

Maize The critical period for maize yields is three weeks before tasselling to twoweeks after pollination. Even light falls over this period could result in barrenstalk and crop failure.

During the Mount St Helens eruption, it was found that ash that had fallen on appleleaves reduced photosynthesis by up to 90 percent. Peaches and raspberries couldnot be cleaned of volcanic ash easily and as a result a significant percentage of thecrop could not be sold. Near mature blueberries were damaged by the salts in wetash. Ash covered the leaves of strawberry plants and compressed the fruit to the soilsurface, where conditions for infection and decay were ideal. In many cases, while theash that fell did not cause disease, it contributed to creating an environment thatdisease could thrive in (Cook et al., 1981).

A layer of ash on the ground surface will lower permeability to air, water and watervapour. Ash may also abrade horticultural or agricultural machinery (Cook et al.,1981). Plant survival may be influenced by the weight of ash on the leaves. Forexample, plants such as lucerne or peas have delicate leaves and stems, and thesemay be easily damaged by the weight of the fallen ash. As well as damaging leavesand stems, volcanic ash cancause significant physical damage to fruit (Neild et al.,1998).

Pest species are not as prone to volcanic dust as their predators, and therefore after avolcanic eruption there may be an increase in pests (Cook et al., 1981; Neild et al.,1998).

Ash suspended in the atmosphere may cause a reduction in temperatures, and affecthorticulture. Horticultural crops may be stunted or may fail completely. As a result ofvolcanic dust in the atmosphere from the eruption of Mount Pinatubo in 1991, New

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Zealand temperatures were reduced by 1-2C. This temperature reduction retardedplant growth and productivity declined (Neild et al., 1998).

If an eruption were to occur near Taupo or Rotorua, then major forest fires in the pineplantations could occur due to ignition by pyroclastic flows or hot falling tephra (HawkesBay Civil Defence Organisation, 1994; Neild et al., 1998). Breakage of tree branches,and burial of trees could also occur if eruption deposits were thick enough. Coping withproblems such as forest fires would be made even more difficult if the infrastructurewas also damaged (Neild et al., 1998).

3.1.3 Building StructuresWhen ash accumulates on the roofs of buildings, building collapse is dependent on theslope of the roof (the lower the angle of the roof, the more likely roof collapse willoccur), the amount of ash accumulated on top and whether the ash is wet or dry (Blong1981, Johnston, 1997a). Roof collapse will occur when there has been anaccumulation of between 100 and 300 mm of dry ash. Only a small number of roofswill collapse with an ash thickness of 100 mm, but as the thickness of ash increases,the incidence of building collapse also increases. It has also been noted from pasteruptions that wide-span roofed buildings have a tendency to collapse more quicklythan short span domestic scale construction (Johnston, 1997a).

Ash may infiltrate structures and cause damage to materials and equipment insidebuildings. Exterior surfaces on buildings, particularly those parts that remain unwashedby normal rainfall, will also suffer damage from the effects of volcanic ash and acidrain. Metallic surfaces are especially vulnerable and may corrode due to the acidity ofthe ash and rain that falls (Johnston, 1997a).

3.1.4 ElectricityThe most common effects of volcanic ash on electricity distribution systems includeinsulator flashover, electricity outages, and line breakage (Stemler and Batiste, 1981;Johnston, 1997a).

Insulator flashover can be caused by volcanic ash and will result in outages to theelectricity supply. Ash that is dry causes no immediate flashover problems. However,ash particles that have a soluble coating and have also been moistened are highlyconductive and can cause insulator flashover. Ash is moistened either by falling rain,or from water present in the eruption plume (Federal Emergency Management Agency(FEMA), 1984; Johnston, 1997a).

As well as the state of the ash, the size and dimension of an insulator may also affectflashover. For example, lower voltage insulators with smaller weather sheds are morevulnerable to flashovers due to the fact that they are more prone to exposure from ashand water (Sarkinen and Wiitala, 1981).

After ash fall has occurred, controlled electricity outages are necessary to clean ashfrom affected parts of the electrical system. Another problem is line breakage, and thisoccurs when the weight of ash collected on power lines becomes too great (Johnston,1997a).

In addition, Johnston (1997a) also notes a number of other problems associated withvolcanic ash and the electricity supply. These are:-Ash contamination on insulators and conductors increases corona activity which in turncauses increase in audible noise (around 10-15 dB) and radio interference.

Volcanic ash will abrade and clog mechanically moving parts used in the electricitysystem.

Saturated volcanic ash on ground surfaces has the potential to be hazardous dueto its conductivity.

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Tree limbs that are laden with wet ash may fall on electricity distribution lines.

3.1.5 Water SupplyAsh may affect the water supply by causing an increase in the turbidity and acidity ofthe water (Johnston and Houghton, 1995; Johnston, 1997a). Small bodies of water,such as roof water tanks and drinking troughs, may also be contaminated by ashleachates, rendering them undrinkable (Johnston, 1997a). At water treatment plants,ash may cause wear and tear on equipment, and may also short circuit electricalequipment (FEMA, 1984). Another problem regarding ash and the water supply, is thatan increased demand for water resources may occur as water is used to clean up afterthe volcanic eruption (FEMA, 1984, Johnston, 1997a).

3.1.6 Wastewater Networks (Stormwater Drainage and SanitarySewers)Ash, especially of a fine grain size, is easily washed into storm water systems byrainfall or via the clean-up process. Because ash has a high density, it is not held insuspension in the wastewater but instead accumulates easily causing pipe blockagesand local flooding. Very fine ash or pumice (which is low density) may be transportedto sewage treatment plants, and this will result in damage to the plant (Johnston,1997a).

3.1.7 Sewage Treatment PlantsJohnston (1997a) cites a number of problems that may be experienced at sewagetreatment plants. These include:- Ash may cause damage to milliscreens, mechanical grit and sludge removal

systems, comminutors and other equipment. Ash falling into sedimentation tanks will add to the volume of material to be

removed. Ash entering oxidation ponds or biofilters will tend to halt the oxidation process until

the ash settles out or is removed. Ash may affect the acidity or toxicity level of effluent to such an extent that

beneficial bacterial growth may be damaged or lost.

3.1.8 GasGas supplies are not significantly affected by ash falls as most pipes are located belowground and are protected from the ash. Gas facilities above ground such as aboveground-pumping stations, pressure reduction facilities, pipeline bridge crossings andgas meters, may suffer ash-related damage (Johnston, 1997a).

3.1.9 Transportation Motor Vehicles and Road TransportAsh reduces visibility for road vehicles. Ash clouds are stirred up by moving traffic,making it difficult for drivers to see (FEMA, 1984; Johnston, 1997b). When ash is wetit causes problems to moving vehicles, as the surface becomes slippery to drive on(FEMA, 1984; Johnston, 1997a). Ash can clog vehicle air filters, and can cause wearto moving parts due to its abrasiveness (Hawkes Bay Civil Defence Organisation,1994; Johnston and Houghton, 1995; Johnston, 1997a). Ash that fills roadside ditchesand culverts may prevent proper drainage and cause erosion on the shoulder of theroad (FEMA, 1984).

Rail TransportDecreased visibility due to stirred up ash is a problem for the rail system. Rail crewsmay also suffer from breathing problems due to the suspended ash. Ash may causewear to moving parts of the train, and if the ash is wet it may lead to short-circuiting ofsignal equipment (Johnston, 1997a).

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In the Mount St. Helens eruption it was found that rail transport fared better thanautomobile or air transport. Only minor train slow-downs were required and some railequipment suffered ash-related problems (Schuster, 1981).

Aircraft and Airline TravelAircraft and the airline industry are also prone to falling ash. On the ground, aircraftand aerospace equipment may be contaminated by falling ash (Labadie, 1983). In theair, temperatures of jet engines are hot enough to melt ash and thus effect the engine,causing it to lose power. Ash cannot be detected by aircraft radar, so aircraft exclusionfrom particular areas of potentially hazardous airspace is common. Even where ashfall is minor, or there is simply the potential of ash fall, it may result in the closure ofairports (Labadie, 1994).

3.1.10 CommunicationsAsh fall may cause direct damage to communication systems, or have indirect effectson them. For example, a particular communication system may not be operablewithout electricity (Johnston, 1997a).

Interference to radio waves may occur due to large quantities of electrically chargedash in the atmosphere. Telephone systems may also be affected by ash falls. Ashentering telephone exchanges can cause abrasion, corrosion and conductivity damageto electrical and mechanical systems. The switching gear at telephone exchangesneeds to be kept below critical temperatures, so exchanges with external air-conditioning systems are vulnerable to overheating if these units fail due to ashingestion, or need to be switched off. Another problem that telephone systems mayexperience is overloading, due to the increased demand by the public and emergencyservices in response to an eruption (Johnston, 1997a).

3.1.11 Mechanical, Electrical and Electronic EquipmentDue to volcanic ash being abrasive, corrosive and conductive, it can cause problemsfor mechanical, electrical and electronic equipment. Air-conditioning units can becomeblocked and damaged by volcanic ash. Short-circuiting and fires can occur in electricalequipment, and ash can cause wear on motors (FEMA, 1984). Computers are alsovulnerable to volcanic ash.

3.2 Mitigation Measures for Tephra Fallout3.2.1 People

If possible, the best measure to guard against inhalation of ash particles is to stayindoors. If it is necessary to leave the shelter of a building then the best protectivemeasure against suspended ash is to wear a face mask when out of doors (Blong,1984, FEMA, 1984). If a face mask is not available then a wet cloth held over the facewill act as a makeshift mask. Those people that are likely to be heavily exposed to ash(for example, outdoor workers) should ensure that they have adequate breathingprotection (Blong, 1984). To avoid eye irritations caused by ash particles, contactlenses should not be worn (FEMA, 1984).

3.2.2 Agriculture and HorticultureIn the 1995-1996 eruptions from Ruapehu volcano there were some stock deaths fromfluorosis poisoning due to low levels of fluorine in ash deposits (Cronin et al., 1998).Another cause of stock deaths was from stock ingesting ash when the ash coveredfeed. Cronin et al., (1998) suggest mitigation measures to combat these problems. Toreduce stock losses from fluorosis poisoning, it was advised to supply supplementaryfeed to at-risk pregnant or lactating animals following tephra fall. It was also suggestedto move stock or supply supplementary feed to stock to avoid the animals ingestingash.

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To avoid animals drinking contaminated water, water troughs should be emptied andrefilled with uncontaminated water.

In regards to horticulture, Neild et al. (1998) suggest that a prompt determination ofthe physical (e.g. particle size) and chemical (composition and reactivity) properties ofash from an actual event will help to predict its effects and guide mitigation orrehabilitation strategies. Other mitigation suggestions include:

- Relocate beehives to assist hive survival.- For cauliflower, break a leaf over on each plant to shelter the white curd.- Horticultural machinery will require maintenance.- Stock up on fungicide sprays and/or pest control sprays.- Remove ash from leaves.- Where ash falls have been around 5-25 mm, clear ash from the base of trees to

prevent plant disease or death from crown rots fostered by the contact of damp ashwith the trunk.

- It may be possible to mix thinner layers of ash into the topsoil.- Under evergreen trees there may be less ash underneath the trees than in

surrounding areas. It may be worthwhile to thin out the ash by spreading someunderneath the trees.

- Where plants have been partially defoliated, fruit should be thinned to better alignleaf resource to fruit numbers. Applying fertiliser will also promote the growth ofnew leaves (Neild et al., 1998).

Nairn (1991) suggests that ventilation systems for crops grown indoors will need tohave filtering systems installed to reduce the amount of ash entering the structures.

3.2.3 Building StructuresRoofs of buildings should be cleared of ash immediately so that ventilation systems canbe reactivated, and streets can be cleaned without any risk of being re-contaminatedby ash reworked from roofs. Roof collapse is also a possibility if ash is left on the roof.The best way to remove ash from a roof is to lightly dampen the ash with water andthen sweep the ash off (FEMA, 1984).

It is advisable not to sweep the ash off in a dry state as this will cause it to billow upinto a cloud. Wetting the ash completely is also not advisable, as this may induce roofcollapse due to additional weight.

When removing ash from roofs care should be taken not to wash the ash into drains,downspouts and soak holes as it will clog pipes and seal up wells (FEMA, 1984).

Johnston (1997a) suggests that where communities are exposed to 100 mm or more ofash fall, they would be evacuated before the climactic phase of the eruption. Thosepeople that are unable to be evacuated should take shelter in buildings with steeppitched roofs, or at least avoid wide-span roof structures (Johnston, 1997a).

3.2.4 ElectricityTo prevent widespread power outages it is necessary that all surfaces in the electricalsystem be cleaned immediately after ash fall. Dry ash should be cleaned by airblastingor brushing the affected surface (Stemler and Batiste, 1981; Labadie, 1983; Johnston,1997a). Wet ash is more difficult to remove. It should be cleaned off either with waterat high pressure or by hand (Stemler and Batiste, 1981; FEMA, 1984). To decreasethe chance of insulator flashover insulators should be washed from the bottom upwardsto remove as much ash as possible (FEMA, 1984; Johnston, 1997a). Cleaning andprotection of the electricity system should be continuous until the threat of windblownash is over (FEMA, 1984).

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3.2.5 Water SupplyJohnston (1997a,b) suggests a number of mitigation measures regarding ash andwater supplies (Table 4).

Table 4: Mitigation measures for volcanic ash and the water supply (afterJohnston, 1997a, 1997b).

Mitigation measures forthe general water

supply

- - Water supply intakes should be closed before turbidity and acidity

levels become excessive.

- Vulnerable plant equipment and pumps should be covered whenash fall is impending.

- High turbidity levels may be able to be managed if watertreatment filters are cleaned regularly. It is necessary, however tobe aware that they may become blocked. People should beadvised to boil water when turbidity levels are high, as suspendedash may decrease the effectiveness of any disinfection orflocculation process.

- As fine ash can remain in suspension for long periods (days toweeks) a coagulation-flocculating agent may need to be added.Alum is found to be the best agent.

- Regular monitoring is necessary to determine when the normalwater supply can be resumed.

- There is the need for a water management plan to handleexcessive demands for water after an eruption. Reservoirs mayrequire filling and public information messages regarding waterconservation may need to be broadcast.

Mitigation measureswhere the water comesfrom a tank on a roof

- - Disconnect the downpipes leading to the tank.

- Cover open tanks.

- Where downpipes have not been disconnected, do not use thewater until tests have been done to ensure that it is not toxic.

- Where the roof supply is found to be non-toxic, but the turbidity ishigh, boil water before drinking.

If no tests can be done, the water tank should be drained, flushed andrefilled with uncontaminated water.

3.2.6 Wastewater Networks (Stormwater Drainage and SanitarySewers)It is important to reduce the input of ash into stormwater systems and sanitary sewers(Johnston and Daly, 1997). When washing ash off streets avoid washing it down thedrains and manholes of stormwater drains and sanitary sewers. Use protectivemeasures such as sandbags around manholes and drains, or weirs in the manholes totrap the ash (Markesino, 1981; FEMA, 1984). When cleaning areas served by freedischarging or dry well storm drainage systems, use dry methods of removing ash (forexample, hand sweep ash out from the gutters) prior to flush cleaning. To avoid ashentering wastewater networks, it is important to educate residents on the acceptablemethods of disposing of ash (FEMA, 1984).

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3.2.7 Sewage Treatment PlantsThe key to preventing ash from entering a sewage treatment plant is to limit ashentering the stormwater and sanitary sewer system.

Based on the eruption of Mount St Helens in 1980, FEMA (1984) made the followingrecommendations regarding wastewater treatment systems:-

Temporarily cover all mechanical equipment that might be exposed to ash fall. Where possible, place sandbags or other devices at the entrance channel to the

plant to trap ash. (This procedure requires frequent attention due to normalsettleable solids present in sanitary waste).

Consider removing or bypassing the comminutor during the initial heavy flows of ashinto the plant.

Frequently check the primary clarifier to prevent (a) damage to the sludge collectionmechanism and/or the digester sludge pumps and (b) the transference of ash to thedigester. Depending on the type of mixing employed in the digester, further damagemay occur in the sludge transfer pumps.

To clear ash from individual sections of the treatment facility, bypass individual units,or in extreme instances, make a complete plant bypass to a holding pond or lagoon.

The effects of ash on the pH value of influent or effluent are not clearly understood.Toxicity may occur in the plant effluent to the extent that the bacterial growth isdamaged or lost. At the first signs of distress on a biofilter, check and adjust the pHlevel of the influent to the biofilter (FEMA, 1984).

3.2.8 Transportation Road NetworkSpeed restrictions or road closure may be necessary to combat visibility problems andslippery road conditions caused by ash falls (FEMA, 1984). After the 1980 Mount StHelens eruption a number of dust retardants were used successfully to control the ashbefore it was removed. Coherex (an emulsion of petroleum resins), lignin sulphateand rock salt were among those used to stabilise the dust (FEMA, 1984; Labadie,1983; Johnston, 1997a). However, these dust control methods did not control heavyash deposits for a long period of time, and they were also expensive (FEMA, 1984).

The best method of removing ash from roads is to sprinkle the ash with water andblade it to the side or middle of the road. The ash can then be picked up by belt orfront-end loaders. A power broom can be used or water flushed over the road toremove the remainder of the ash (Labadie, 1983; Johnston, 1997a). Conventionalsnow removal methods should not be used to remove ash off roads. Snow removalmethods only stir the ash up and cause it to resettle on the roadway (FEMA, 1984).

Where roads are made of gravel, try to avoid removing too much of the gravel off thesurface during the clean-up process. Additional gravel may be required to replace anythat is lost, and may also assist in stabilising any dust that cannot be collected. FEMA(1984) recommended adding graded material of 5/8 inch to 0 in size and crushed tostandard specification, for dust control (FEMA, 1984).

Ash deposits should be removed from any catch basins as soon as possible or ash willform a crust making it difficult to remove later on (FEMA, 1984).

Motor VehiclesRegular checks and maintenance of car parts are essential to preventing damage tomotor vehicles from volcanic ash. Checks and maintenance should be carried out onvehicles after every two and a half to three hours of exposure to volcanic ash. If ashenters the air filter and electrical equipment, it should be cleaned off using compressedair of 30 psi or less. The outside of the car, engine and radiator should also be cleaneddaily, if necessary using water to flush the ash. After volcanic ash has ceased to fall,

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then a thorough inspection should be undertaken and repairs carried out (FEMA, 1984;Labadie, 1983).

Rail TransportLike motor vehicles, regular cleaning and maintenance of trains and rail equipment isnecessary during periods of ash fall (Labadie, 1983).

Aircraft and Airline TravelIt is necessary to apply flight restrictions when ash is falling, or when there is apossibility of it being in the atmosphere (FEMA, 1984). Aircraft that are groundedshould, if possible, be kept free of volcanic dust. If aircraft have been exposed to ash,then careful cleaning procedures are required to avoid damage to the aircraft(Labadie, 1983; Labadie, 1994).

Runways should be kept clean as volcanic ash is easily re-entrained by the wind,aircraft take-off and ground vehicle movement. The ash should be wetted down,bladed to the sides of the runway and then picked up by belt or front-end loaders. Theremaining residue of ash should then be flushed away with water, or swept away.Landing aids, air traffic control systems and ground equipment will also require periodiccleaning, maintenance and monitoring (Labadie, 1983; Labadie, 1994).

Volcanic Ash Advisory Centres (VAACs) have been established to keep track ofvolcanic activity in different parts of the world. A VAAC is situated in Wellington, NewZealand and covers the South West Pacific region. VAACs collate information aboutvolcanic clouds and provide this information to aircraft that are in flight or to those whoare planning flights (Metservice, 1997; Mayberry and Rose, 1998).

3.2.9 Mechanical, electrical and electronic equipmentTo clean volcanic ash from electrical equipment, the equipment should first be switchedoff and then blown clean with an air compressor at 30 psi or less (FEMA, 1984).

The best way to protect electronic equipment when there is ash falling is to discontinueuse of the equipment and protect it by covering it up or sealing it off. If the equipmentstill needs to be used, then the next best option is to restrict access to the equipmentand ensure those that use it clean their clothing before entering the room in which it iscontained (FEMA, 1984). Continual protection and cleaning of computer systemsshould ensure that they can continue to be used (Labadie, 1983).

Prior to, and while ash is falling, air-conditioning systems should be shut down. Whenthe ash has stopped falling, the air-conditioning intake and filters should be cleanedbefore the system is reactivated (FEMA, 1984).

3.2.10 Ash DisposalAn important consideration after ash removal is the disposal of the unwanted ash. Ashshould be disposed of in areas where it does not constitute a hazard, and where it isacceptable to the adjoining property owners. It is necessary that disposed ash isstabilised so that it does not blow away from the disposal site. Ash can be stabilised bycovering dump sites with a blanket of heavy material such as gravel, growingvegetation on the area or using straw and other mulching materials on the site (FEMA,1984). Ash should not be disposed of in the garbage as it will cause damage to therunners on the inside of garbage trucks (Johnston, 1997b).

3.2.11 Detailed Mitigation MeasuresMore detailed information on mitigation measures for ash fall is contained in AppendixII.

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3.3 Ballistic FalloutBallistic fallout occurs when projectiles of rock (volcanic blocks, bombs and lapilli) arethrown on parabolic trajectories from a volcanic vent in an eruption. Generally ballisticfallout is confined to less than three kilometres radius from the vent, as the projectilesare too big to be ejected any further (Houghton et al., 1988).

Ballistic projectiles are a significant proximal hazard. Damage to structures can begreat (Johnston, 1997a) and loss of life severe (Baxter, 1990) due to the size of theblocks and bombs (> 64 mm) that are ejected. Ballistic fallout can be hot, causingburns to humans if they are situated in the path of the fallout. The heat of the projectilesmay also start fires amongst vegetation or structures (Blong, 1984; Houghton et al.,1988).

3.3.1 Mitigation Measures for Ballistic FalloutWhen ballistic material is falling in a volcanic eruption, the best mitigation measure is torelocate people or restrict them from entering the affected area (Blong, 1984).

Where it is not possible to leave the area then staying under cover in a sturdy buildingis recommended. If it is necessary to go outside, then padding to protect the bodyshould be worn with special attention paid to covering the head. If outside, it is a goodidea to stay alert and watch for any incoming ballistic fallout so as to avoid being hit(Blong, 1984).

3.4 LaharsA lahar is defined by Houghton et al (1988) as a rapidly flowing mixture of water andvolcanic rock fragments of all sizes, particularly with fine ash which may combine withthe water to form a slurry capable of transporting larger rock fragments.

Lahars can be produced in many ways:-- Snow or ice may be melted by erupted ash or lava.- An eruption through a crater lake or emptying of the crater lake to cause water and

mud to flow down the side of a volcano.- Heavy rain falling on to unconsolidated ash.- Movement of a pyroclastic flow or debris avalanche into a river or lake (Houghton

et al., 1988; Gregory and Neall; 1996).

Lahars follow valleys, travel great distances and travel at high speeds (Blong, 1984;Houghton et al., 1988). Lahars may continue to occur for months to years after avolcanic eruption with the subsequent mobilisation of secondary lahars (Blong, 1984;Rodolfo et al., 1996).

Lahars are capable of destroying everything in their path including buildings, bridges,other structures and vegetation. People and animals are at risk from crush injuries,drowning or asphyxiation (Baxter, 1990; Johnston and Houghton, 1995).

3.4.1 Mitigation Measures for LaharsBecause of their speed there is a potential for great loss of life from a lahar. However,detection systems for lahars can be put in place to provide early warning of anapproaching lahar. Sensors can also be used to detect a sudden drop in the level of acrater lake (Pierson, 1989).

Currently there are already a number of lahar early warning gauges in places onmountains and rivers in New Zealand. Cronin et al. (1997a, 1997b) suggests that thereshould be more of these gauges on New Zealand rivers. A new Eruption DetectionSystem has been installed at Ruapehu in light of the failure of the Lahar WarningSystem (LWS) during the 1995 lahars (Bryan, 1997).

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Another mitigation measure for lahars is hazard mapping. Because most lahars flowdown valleys, areas likely to be at risk can be predicted fairly readily and mapped(Blong, 1984).

If one is caught in or near the path of an approaching lahar, then moving to higherground away from any valley should afford protection from the lahar (Blong, 1984). If itis not possible to move to higher ground in time then it may be possible to avoid thelahar by climbing on to the roof of a building. If the lahar is not travelling at too great avelocity then the building may remain intact while the lahar passes around it (Johnston,1997a).

In the 1995-1996 Ruapehu eruptions, the Rangipo Dam and Power Station wasaffected by lahars travelling down the Tongariro River. Lahars have continued to affectthe power station as volcanic deposits in the catchment of Rangipo continue to beremobilised (Malcolm et al., 1997).

Volcanic material suspended in the lahars caused excessive wear on the turbines ofRangipo power station and the replacement of parts was necessary. To mitigateagainst the effect of the abrasive ash, improvements were made to the replacementparts so that it would take three times longer for them to be damaged by volcanic ash(Malcolm et al., 1997).

Other mitigation options considered by Rangipo power station to avoid damage toturbines included:-- Shutting down power generation when the amount of suspended volcanic ash in

Tongariro River causes an excessive wear rate in the turbine.- Retaining the ash near source using sediment traps or retention dams.- Diversion of the water sources carrying suspended volcanic material.- Separation of the ash at Rangipo Dam.- A sediment trap in the Waihaha pipe bridge (Malcolm et al., 1997).

The eruptions of Mount Ruapehu in 1995-1996 resulted in a deposit of ash blocking theoutlet of Ruapehu Crater Lake. When the lake refills, it is anticipated that this tephradam may collapse and produce a dangerous lahar down the Whangaehu River. Astudy by the Department of Conservation (1998) has been carried out to look atmitigation options for this hazard. Some of the more feasible options for mitigationinclude:-

Option 1: The development of a warning/response system and revised hazardplanning in lahar run-out zones. For example, an acoustic based real-time warning system should be designed and installed.

Option 2: Engineering works in lahar run-out zones. Construction of dams andbunds at strategic locations along the Whangaehu River seems themost practical method of achieving this.

Option 4: Excavation of a trench through, or partly down into the 1995-96 tephradeposits at the former outlet of Crater Lake. Engineering works usingmachinery (for example, a bulldozer) would be most effective, althoughmanual digging of a shallow trench is also possible.

Much consultation is still required before a decision can be made on which mitigationmeasures will provide adequate protection and account for cultural, philosophical andother values (Department of Conservation, 1998).

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3.5 Pyroclastic FlowsPyroclastic flows occur from the collapse of an eruption column or the generation oflaterally directed blasts. The pyroclastic flow, which is made up of gas and volcanicparticles, sweeps away from the volcanic vent at a very rapid speed (up to 100 to200km/hr) (Nairn, 1991; Froggatt, 1997). Small pyroclastic flows are stronglycontrolled by the topography, and will affect an area close to the source of the flow.Large pyroclastic flows travel radially outwards, traversing valleys and climbingobstacles (Blong, 1984). Pyroclastic flows are hot and move at such high velocitiesthat they envelop and destroy everything in their paths. The resulting ignimbritedeposit varies in thickness laterally and even where little evidence of the flow is left,human casualties and destruction are likely (Blong, 1984).

The heat of a pyroclastic flow may cause the ignition of fires in vegetation andstructures (Blong, 1984). Pyroclastic flows may also trigger major secondary hazardssuch as lahars and flooding (Houghton et al, 1988).

3.5.1 Mitigation Measures for Pyroclastic FlowsAs pyroclastic flows are so destructive, the best protection for human lives is toevacuate hazardous areas prior to any event (Johnston, 1997a). If a small, valleyhugging, pyroclastic flow were likely to occur, then some protection may be afforded bymoving to a higher elevation. However, if there was the possibility of a largepyroclastic flow occurring, then the only mitigation option would be to distance thepopulation from the hazardous area, as the flow would be capable of travelling overtopographic highs (Blong, 1984).

Most deaths that occur from pyroclastic flows can be attributed to asphyxia, burns tothe body and blows from hurling rocks. At the edges of the pyroclastic flow, somemeasures may prove useful in mitigating against the heat produced by the flow. Facemasks may slow down the onset of asphyxiation by a few minutes. Structures such aslarge diameter concrete pipes walled in at the end may offer some protection from lowvolume pyroclastic flows. Also, protective clothing may decrease the likelihood ofburns and scalds (Blong, 1984).

3.6 Pyroclastic SurgesLike pyroclastic flows, pyroclastic surges are very fast moving and contain a mixture ofgas and particles. However, unlike pyroclastic flows, surges contain more gas and lessparticles, travel at high velocities and are more turbulent (Houghton et al., 1988;Houghton et al., 1994). While pyroclastic surges travel faster than pyroclastic flows,they cover less distance making them a more proximal hazard. Pyroclastic surges donot follow valleys, but deposit thin layers of volcanic material in both depressions andon topographic highs (Houghton et al., 1994). Because of this ability to surmount thetopography, pyroclastic surges constitute a high volcanic risk (Johnston, 1997a).

Pyroclastic surges can be made up of either hot and dry volcanic particles or they canbe made up of wet volcanic particles. The hot and dry surges are very destructive andare deadly close to source. Humans and animals are killed by the heat of the surgeand by asphyxiation. Wet (phreatomagmatic or phreatic) surges may also be verydestructive, although they tend to decelerate more quickly than dry surges (Houghtonet al., 1988).

3.6.1 Mitigation Measures for Pyroclastic SurgesThe best mitigation measure for pyroclastic surges is prior evacuation of the area atrisk, as surges can surmount topography.

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3.7 Directed Volcanic BlastsWhere an explosive blast is directed across the land surface rather than verticallyupwards, a gas and particle cloud of high velocity and high temperature is produced. Ablast will override ridges and hills near the source, but will begin to become moreinfluenced by topography as it slows down and moves away (Houghton et al., 1988).

Commonly, large blasts are associated with composite volcanoes of andesite or dacitecomposition. They can also occur from lava domes, but this is less common. Basalticphreatomagmatic eruptions may produce smaller destructive blasts (Houghton et al.,1988).

3.7.1 Mitigation Measures for Volcanic BlastsAs there are few immediate precursors to an event of this type, it is necessary to belocated outside the zone of the blast in order to survive (Houghton et al., 1988). Thelateral blast from Mount St. Helens in 1980 created total destruction within a 13 kmradius of the crater (Schuster, 1981). There may be a chance of surviving a blast onlyif the person is located at the edge of the blast and in an airtight building (Johnston,1997a).

3.8 Lava FlowsThe behaviour of a lava flow is dependent on the viscosity of the magma, output rates,volume erupted, steepness of the slope, topography, and obstructions in the flow path.Viscosity is the most important control on lava flow behaviour and depends mostly onthe composition of the lava. Basaltic lavas are the least viscous and thus are likely toflow for longer distances. At the other end of the scale, rhyolite lavas have a very highviscosity which means that they are more likely to form short thick flows or domes (Casand Wright, 1987).

Lava flows move relatively slowly and cover limited areas, so the risk to life from lava isnot high (Blong, 1984; Johnston and Houghton, 1995). Lava flows may reach speedsof up to 50km/hr, but in general they flow at speeds of less than 10km/hr (Johnston,1997a). Lava flows will cover and destroy structures that cannot be moved away fromthe path of the flow, and can start destructive urban and forest fires (Thorarinsson,1979). Avalanches of lava blocks from steep, high, hot flow fronts represent anadditional hazard (Blong, 1984).The risk from the growth of an actual lava dome is not high, although destructive blockand ash flows can occur if the dome becomes unstable (Houghton et al., 1988).

3.8.1 Mitigation Measures for Lava FlowsOnly a low number of deaths have been attributed to lava flows, and many of thesedeaths were considered avoidable. Most deaths resulted from people approachinglava flows too close (for example, burns from the lava after falling through a lava crust)rather than the danger afforded by the hazard itself (Blong, 1984). To mitigate againstinjuries inflicted by curiosity, it is advisable to restrict onlookers from the site of the lavaflow.

Attempts have been made to divert lava flows away from areas with varying success.The successful diversion of a lava flow away from a harbour was carried out inHeimaey, Iceland in 1973. This was achieved by chilling the lava with large volumes ofwater (Thorarinsson, 1979). Successful cooling has also been carried out in Hawaii. In1992 at Mt Etna, Italy, several unsuccessful attempts were made to divert a lava flowbefore it was finally diverted using channels and explosives (Barberi et al., 1993).Earth banks can also be used to divert lava flows. (Thorarinsson, 1979). It is possibleto use already standing structures such as houses to divert lava, although the hightemperatures of the flow would increase the danger of ignition of these objects (Blong,1984).

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3.9 Debris AvalanchesA debris avalanche is the sudden catastrophic collapse from an unstable side of avolcano. Many volcanic cones are steep sided and unstable due to rapid growth of thecone (Houghton et al., 1988). Rising magma, earthquakes, weakening due tohydrothermal alteration and heavy rain can trigger a debris avalanche of this unstablematerial (Houghton et al., 1988; Johnston, 1997a). Avalanched material follows valleysas it moves down the side of the volcano under the force of gravity (Houghton et al.,1988). Debris avalanches can be wet, dry or both, and if wet, an avalanche mayevolve and continue to flow further down slope as a lahar (Johnston, 1997a).

3.9.1 Mitigation Measures for Debris AvalanchesAs debris avalanches are very destructive, travel a considerable distance at greatspeed, and occur with little or no warning, evacuation of areas that could potentially beaffected by debris avalanches is the best mitigation measure (Johnston and Houghton,1995; Johnston, 1997a).

3.10 Volcanic GasesThe main gases present during an eruption include water vapour, and carbon dioxidewith smaller amounts of other gases such as sulphur gases (SO2 and H2S), andchlorine and fluorine (Houghton et al., 1994).

During an eruption volcanic gases spread in three main ways from the volcanic vent:-- as acid aerosols;- as compounds absorbed on tephra particles; and- as salt particles (Thorarinsson, 1979).

Close to the volcano (within a few kilometres) volcanic gases can be sufficientlyconcentrated so that they are harmful, but further away the concentrations of gasesdilute, posing little risk to communities (Johnston, 1997a). Direct contact with volcanicgases can cause eye and breathing irritations, and where heavier-than-air gases (forexample, carbon dioxide) collect in depressions around the volcano, suffocation canoccur (Thorarinsson, 1979; Houghton et al., 1988; Johnston, 1997a).

Some of the gases (for example, sulphur dioxide and hydrogen fluoride) mix with waterdroplets in the eruption plume and atmosphere to form acid rain (Figure 5) (Froggatt,1997). As the dispersion of volcanic gas decreases with distance from the vent, thenthe acid rain hazard also decreases downwind (Blong, 1984). When acid rain falls, itcauses damage to metal surfaces, skin, clothing and vegetation, and contaminateswater supplies (Houghton et al., 1988; Froggatt, 1997).The release of gas with only a minor amount of ash during an eruption can producevog or volcanic smog. Vog was seen in late October 1995 and late July 1996 duringthe Ruapehu eruptions. Vog results from chemical reactions between SO2 and oxygen,water and sunlight. These create tiny droplets of acidic water and tiny particles ofsulphate minerals which interfere with light rays from the sun, producing haze andsmog (Houghton et al., 1996).

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Figure 5: The interaction of volcanic gases during an eruption (afterJohnston, 1997a).

3.10.1 Mitigation Measures for Volcanic GasesFace masks need to be designed for toxic gases as well as respirable volcanic dusts,so that people can be protected from the volcanic gas hazard (Blong, 1984).

It may be necessary to evacuate populations where there is the potential forsuffocation, or toxic levels of gases are present. Evacuations were carried out duringthe 1973 Eldfield eruption in Iceland, when carbon dioxide levels exceeded acceptablelimits at night time (Thorarinsson, 1979).

3.11 Tsunamis and SeichesTsunamis are seismic sea waves of long period caused by disturbances on the seafloor (Allaby and Allaby, 1990). Tsunamis can be produced by a submarineearthquakes, by debris avalanches and by underwater volcanic eruptions(Thorarinsson, 1979). A number of waves may be produced and they may travel longdistances to far-off shores (Blong, 1996). The height of a tsunami varies and may beaffected by the bathymetry, resonance effects and other factors (Harbitz, 1991;Kowalik and Murty, 1992; Pelinovsky and Mazova, 1992; in Siebert, 1996). Inexceptional circumstances, waves may be produced up to 30 metres high (Blong,1996).

Tsunamis have produced the second highest toll of deaths during eruptions (Gregoryand Neall, 1996; Blong, 1996). While the initial tsunami causes many of the deaths,there are also deaths from the health hazards that result following flooding by the wave(Gregory and Neall, 1996).

Large earthquakes before or during a volcanic eruption from a vent in a lake, maygenerate seiches on the lake (Froggatt, 1997). The mass entry of volcanic debris intoa lake from an eruption itself may also create seiches (Froggatt, 1997; EnvironmentBOP, in prep.). Low-lying land on the edge of a lake would be flooded by a seiche.Seiches may also travel down rivers that flow from the lake (Froggatt, 1997).

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3.11.1 Mitigation Measures for Tsunamis and SeichesTsunamis and seiches affect structures and life situated along coastlines or lake edges.In the event of an approaching wave, evacuation from those areas is essential(Froggatt, 1997).

Hazard maps can be used to identify areas of potential risk from tsunami. Modelling oftsunami paths using detailed bathymetry can be used to construct hazard mapsshowing predicted tsunami arrival times (Aida, 1975; Kienle et al., 1987; in Siebert,1996).

3.12 FloodingFollowing a volcanic eruption, the deposition of volcanic sediment in valleys maydisrupt normal stream or river flows. Channel aggradation, and the increased lateralmigration of channels and bank erosion may occur. These conditions can causedamage to structures and worsen normal seasonal flooding (Pierson, 1989).

Volcanic material will cause stream blockages and pond temporary lakes (Blong,1996). After the 26,500 yr B.P. Oruanui eruption, and again after the 181 A.D. Taupoeruption, temporary lakes were created when volcanic material blocked the normal flowof the Waikato River. The largest of these lakes was situated in the Reporoa Basin(Tilly, 1987; Manville, in press; Manville, in prep; Manville et al., in prep.).

Erupted volcanic material will sometimes also block an outlet to an existing lake or rivercausing a body of water to build up behind the dam. When this dam collapses dueto erosion or is overtopped, large scale catastrophic flooding can occur. Following theTarawera eruption, a tephra bank was created at the outlet to the Tarawera River, andthe lake rose 12 m behind the bank. Following heavy rainfall, the tephra bank waswashed away and a breakout flood occurred down the river. The flood wasaccompanied by the silting up of the Tarawera River in the following years, as the floodwaters eroded the countryside that they flowed over (Nairn, 1991; White et al., 1997).

A similar situation occurred after the 181 A.D. Taupo eruption when Lake Tauposoutlet became choked with volcanic material from the eruption. The result of theblockage was that the level of Lake Taupo rose to a mean height of 34 m above itsnormal level. As the lake overtopped the ignimbrite barrier blocking the lake outlet, itbegan to erode the barrier away. Catastrophic collapse occurred, creating a breakoutflood down the Waikato River (Tilly, 1987; Manville, in prep.; Manville et al., in prep.).

3.13 Hydrothermal EruptionsTectonic or volcanic movement can trigger a decrease in fluid pressure, thus inducinginstability and boiling in the shallow portions of hydrothermal systems, and causingexplosions to occur. However, there are some hydrothermal eruptions