Natural Hazards

Volcanic Hazards


Auckland is vulnerable to volcanic eruptions. Much of Auckland is built on the Auckland Volcanic Field (AVF), which covers 360km2 and contains at least 50 volcanoes. It is expected that these volcanoes won’t erupt again and any future eruptions will occur in new, unknown locations. Auckland has experienced volcanic ash falls from Mount Taranaki/Egmont, Taupo, Okataina, Tongariro and Mayor Island


 

The Auckland Volcanic Field (AVF)

Auckland’s volcanoes vary in shape, size and character. The earliest volcanic eruption in the AVF was an estimated 250,000 years ago. The last occurred about 600 years ago and formed Rangitoto. Māori living on Motutapu Island witnessed it. 

Past eruptions have sometimes started with a large explosion because of either ground or sea water coming in contact with rising magma. An eruption of this type is more likely to recur in Auckland due to the close proximity of many water sources. These eruptions can form large craters, which can subsequently fill with water such as Lake Pupuke, Orakei Basin and Onepoto Reserve. 

Continued eruptions often create volcanic or scoria cones such as Mt Wellington, Mount Eden, Browns Island and Three Kings. As the volcano continues to erupt it may produce extensive lava flows. Many have been mapped within the city, extending up to 10km from the source.

Likely hazards and effects of the next Auckland eruption depend on what type of eruption occurs and for how long. 

When and where future eruptions will occur is unknown. Based on the number and frequency of past eruptions it is estimated there is about a 1 in 1000 (0.1 per cent) chance an eruption could occur in any one year.

This means that there is an 8 per cent probability (1 in 12.5 chance) an eruption will occur in the AVF field over any 80 year period. This is based on the life expectancy of a New Zealand child born in 2008 being 82.4 years for females, and 78.4 years for males (Births and Deaths: December 2009 quarter - Statistics New Zealand). 

Here is a summary of the possible resulting hazards:

 

Hazard

Description

Base surge

Base surges are a hot blast of rock, gas and steam that travels across the land at hundreds of kilometres an hour. These surges flow outward from the vent and may significantly devastate an area up to 3km from the eruption location. 

Ash and debris

For the duration of the eruption, ash and debris will accumulate down wind of the volcano. The depth of ash and debris deposited will decrease further from the vent.

Fire fountaining

Fire fountaining includes molten rock and magma erupting from the volcano’s vent. Continued fire fountaining can lead to the formation of lava flows that will be slow moving and destroy most things in their path.

Shockwaves from explosions

Shockwaves from violent explosions will be stronger closer to the vent and can flatten trees and break windows.

Poisonous gases 

Gases are likely to be released closer to the volcanoes vent. As these gases can be much denser than the surrounding air, they often pool and become concentrated in low-lying areas.

Earthquakes

As rising magma moves upwards through the earth’s surface, it can generate earthquakes. Earthquake activity is likely to increase as an eruption becomes more likely and will continue for the duration of the eruption.

Tsunami

Volcanic eruptions occurring beneath the ocean or large water bodies can displace water creating a tsunami.

 

Impacts could include:

  • devastation of buildings and infrastructure within a 3km radius of the volcanic vent or in the direct path of lava flow
  • large economic losses due to clean-up costs, physical damage to buildings and infrastructure, closure of businesses, damage to horticultural and agricultural products and a decline in the region’s tourism
  • an increased risk of widespread fires from hot ash or disrupted gas supply lines
  • significant impacts on Auckland’s infrastructure from any of the listed hazards which may result in mass evacuations
  • disruption and restrictions to lifeline networks such as electricity, gas and water supplies, and waste and stormwater networks from any of the listed hazards
  • ash and dust will affect air-conditioning systems and some communication networks
  • roading networks within a 10km radius of the vent may be impassable or greatly affected
  • rail and air services stopping or greatly reduced
  • eye and lung irritation and poor sanitation causing increased health risk.

 


 

Central North Island volcanic sources

Auckland is at risk from ash fall from several large and frequently active volcanoes in the central North Island and the possible reawakening of volcanic activity in Northland. 

In contrast to the Auckland field, eruptions from New Zealand stratovolcanoes such as Mount Ruapehu and Mount Taranaki/Egmont occur, on average, every 50 to 300 years. Over the past 80,000 years, eruptions from distant volcanoes have deposited at least 82 different ash layers, greater than 0.5mm thick, in Auckland. 

Here is a summary of the source of these layers compared to those identified from the AVF over the same period.

 

Source

Number Of Identified Tephra Layers

Mount Egmont/Taranaki

52

Okataina and Taupo

21

Mount Tongariro

7

Mayor Island

2

Auckland Volcanic Field (AVF) 

24

 


Whether ash from a distant eruption will reach Auckland, and the thickness of any associated ash deposit, depends on factors such as the type of eruption, its location, duration and wind direction. 

Some of the impacts that may occur include:

  • eye and lung irritation increasing health risks
  • high economic costs due to clean-up, damage to infrastructure, temporary business closures and adverse effects on tourism
  • economic losses due to damage of horticultural and agricultural industries
  • disrupted electricity supply with power outages if the ash is wet
  • widespread disruption to transport infrastructure including the closure of roads and airport facilities
  • disruption of fresh and waste water systems
  • disrupted communication systems due to interference, overloading or direct damage.

 


 

Monitoring volcanic hazards in Auckland

GNS Science, through the GeoNet Project, monitors volcanic hazards across New Zealand. It studies changes in volcanic activity through earthquake and gas monitoring, ground deformation and the composition, including chemical make-up, of crater lakes. 

The health hazards of volcanic ash - A guide for the public.

Guidelines on preparedness before, during and after an ashfall.

http://www.aelg.org.nz/document-library/volcanic-ash-impacts/.

Coastal Erosion


Coastal erosion is a natural process that is generally only of concern when threatening human habitation or development. The extensive urbanisation of Auckland, a region with 3100 km of coastline, has resulted in exposure to hazardous coastal erosion and accretion processes and both 'soft' and 'hard' shoreline and cliff erosion.

'Soft shorelines' refer to sandy beaches and dunes made up of unconsolidated or very weakly consolidated materials. Sandy beaches in the Auckland region experience periods of accretion (net sediment accumulation) and erosion (net sediment decrease). On beaches experiencing a sustained period of erosion, sediment loss results from waves, currents and wind removing sediment from the beach faster than it is replaced.

Erosion and instability also happens from natural and human factors such as cliff height and modification. Geology is the main factor that affects the extent of coastal erosion in Auckland. Volcanic coasts are formed of much harder rock than sedimentary coasts and less likely to form bedding, joint and plane failures. Greywacke rocks exposed along the southeast coastlines are softer and contain rock defects making these cliffs more vulnerable to instability than those of volcanic rock.

 


Impacts of coastal erosion

Coastal erosion can pose a risk to residential developments, roads, lifeline utilities and coastal structures. Soft shorelines bordering the Manukau and Waitemata Harbours are particularly vulnerable. Sea walls are particularly vulnerable and have been modified by seawalls in many places in attempts to control erosion. The sandy beaches around Omaha and Orewa have developments very close to or on dune deposits meaning that soft shoreline erosion is a possibility.

Specific impacts of coastal erosion in Auckland include:

  • danger to life in the case of sudden onset landslide events
  • structural damage or destruction of buildings and infrastructure
  • damage or destruction of lifeline infrastructure such as water, sewage and gas pipes and roads
  • loss of land, resulting in coastal cliffs or shorelines retreating closer to other buildings
  • land instability at neighbouring slopes and properties
  • loss of beach amenity due to cliff collapse or sea wall construction.

 


Coastal inundation hazards

Coastal inundation, often resulting from storm surge, occurs in Auckland and can cause significant disruption to low-lying coastal areas. Coastal flooding is commonly associated with severe storm events and is often the result of a combination of factors including:

  • strong onshore winds
  • low barometric pressure
  • high astronomical tides
  • wave set-up
  • wave run-up.

 


Impacts of coastal inundation hazards

Impacts of coastal inundation may include:

  • isolated coastal communities
  • damage to properties and critical infrastructure from flooding and waves
  • evacuation of some coastal areas
  • corrosion of electrical devices and other metal objects
  • salinisation of flooded land affecting agriculture
  • secondary hazards such as land instability and possible fire.

Cyclones


The Auckland region has a sub-tropical climate with warm humid summers and mild winters.

Typical summer daytime maximum temperatures range from 22ºC to 26ºC and rarely exceed 30ºC. Winter daytime maximum temperatures range from 12ºC to 17ºC. Annual sunshine hours average about 2000.

Winter usually has more rain and is the most unsettled time of the year. In summer, storms of tropical origin may bring high winds and heavy rainfall from the east or northeast. The El Niño Southern Oscillation also has a large impact. Annual maximum rainfall during La Niña events is generally larger and more variable than the extremes during El Niño events.

 


 

Rates of climate change

Rates of climate change depend on future global emissions of greenhouse gases, which depend on global social, economic and environmental policies and development. You need to consider a range of possible features when assessing climate impacts and developing adaptation strategies.

Climate change will influence many of Auckland’s natural hazards. Reductions or increases in magnitude and changes to the location and frequency of hazards may be experienced.

The International Panel on Climate Change has provided climate projections for New Zealand for 2040 and 2090.

Their Auckland projections include:

  • Increase in the mean air temperature.
  • Increase in sea level due to thermal expansion within oceans and loss of ice sheets and glaciers on land.
  • Fewer periods of cold temperatures and an increase in the number and intensity of periods of high temperatures. We’ll have more days above 25ºC.
  • Decrease in annual mean rainfall.
  • Increased frequency and intensity of extreme rainfall events due to a warmer atmosphere.
  • Increased intensity of El Niño and a possible increase in El Niño frequency with an associated increase in the annual mean westerly wind flow.
  • Possibly more intense extra-tropical cyclones bringing torrential rain, strong winds and storm surges.

 


 

Impacts of climate change

We don’t know exactly how climate change will affect Auckland.

Various impacts would heighten social, economic and environmental challenges.

They include:

  • Health problems from extreme temperatures.
  • Damage to properties and critical infrastructure from more intense inland flooding and coastal inundation.
  • Coastal erosion from larger waves hitting the coastline.
  • Disruption to agriculture and horticulture from water supply issues and more severe drought.
  • Fire from increased drought frequency, windier conditions and fire suppression difficulties.

Severe Weather


Tropical cyclones

Tropical cyclones typically form close to the equator in the southwest Pacific Ocean. It is from the warm equatorial waters that cyclones derive their energy.  Most cyclones play out their whole life in the tropics. Sometimes though, a few cyclones migrate out of the tropics into the mid-latitudes, close to New Zealand. As they move south into cooler waters generally, their central core cools causing the cyclone to weaken. Some cyclones can re-intensify to become mid-latitude 'ex-tropical cyclones'.

Southwest Pacific cyclones tend to form between December and April with at least one ex-tropical cyclone passing within 500km of New Zealand most years. The severity of each storm depends on many factors including where the cyclone forms and the strength of the dominating La Niña or El Niño cycle. 


 

Cyclone classifications

Cyclone classifications differ between the northern and southern hemispheres.  In the Southwest Pacific, whether a storm is large enough to be classified as a cyclone generally depends on the strength of the storms winds. 

The table below shows tropical storm classification in the southwest Pacific and some of the likely effects.

 

Storm Type

Sustained Wind (Km/H)

Likely Effects

Tropical depression

<63

N/A

Tropical cyclone category 1

63-125 
(gales)
  • minimal house damage 
  • damage to some crops, trees and caravans.

Tropical cyclone category 2

125-164 
(destructive)
  • minor house damage 
  • significant damage to signs, trees and caravans 
  • heavy damage to some crops 
  • risk of power failure 
  • small boats may break moorings.

Tropical cyclone category 3

165-224 
(very destructive)
  • some roof and structural damage 
  • some caravans destroyed 
  • power failure likely.

Tropical cyclone category 4

225-279 
(very destructive)
  • significant roofing and structural damage 
  • many caravans destroyed and blown away
  • dangerous airborne debris 
  • widespread power failure.

Tropical cyclone category 5

>280 
(extremely destructive)
  • extremely dangerous with widespread destruction.

 

As tropical cyclones move into the mid-latitudes, where New Zealand is situated, they transform into ex-tropical cyclones as they move into cooler seas.  Generally Auckland is affected by such storms.

On average, about 10 or 11 tropical cyclones form in the southwest Pacific per year, although in any one season the number can range from about two to 16.  La Niña conditions are more favourable for cyclones to form in the Coral Sea instead of further east, which increases the chance of ex-tropical cyclones tracking towards Auckland.

Some of Auckland’s highest wind gusts have occurred from ex-tropical cyclones. Winds such as those resulting from cyclones Bola (March 1988) and Giselle (April 1968) are examples of tropical cyclones that transformed into mid-latitude systems, causing damage from extreme winds. Cyclone Bola produced gust speeds of 107km/h in Auckland.  In the last few decades, the worst storms affecting Auckland were Cyclone Fergus in December 1996 and Cyclone Drena in January 1997.

View footage of Cyclone Drena in Auckland on YouTube.  

 


 

Impacts of cyclones

Tropical and ex-tropical cyclones are generally associated with heavy rainfall, high winds, and storm surge. Ex-tropical cyclones are often the cause of Auckland’s most extreme weather leading to extensive flooding and wind damage. In addition, particularly during high tides, flooding of low-lying coastal areas is common because of storm surges and waves.

General impacts on property and people in Auckland include:

  • uprooted trees damaging buildings and utility infrastructure such as power lines
  • injuries to people from airborne debris
  • boats blown onto shorelines
  • disruption to air travel
  • land movement in the form of landslides, rockfalls and coastal erosion
  • flood inundation of properties in floodplains and low-lying coastal areas.

Drought


Drought

Auckland is vulnerable to the effects of different types of drought that could have severe implications on people, agriculture and the economy. Unlike most natural hazards, the onset of drought can be slow and go unrecognised. It usually occurs from a lack of precipitation, commonly rain, over a long period.  In Auckland, these conditions are more likely to occur under El Niño-dominated weather conditions like the summer of 1997-1998 when our region experienced water shortages. In general, two main types of drought can affect Auckland. They are agricultural drought or hydrological drought. 

 


 

Agricultural drought

Crops, including pasture, rely on moisture within the soil to survive. During an agricultural drought, soil moisture becomes so low that plants can no longer grow and start to die. As soil moisture decreases, plants become stressed and if the drought is prolonged, they will die. An agricultural drought ends when adequate amounts of rain restore soil moisture levels. 

The effects of an agricultural drought in Auckland can include:

  • agricultural productivity being reduced because of less breeding sheep and cattle
  • significant economic losses for the horticultural sector
  • considerable increases in the risk of fire resulting in the loss of crops.

 


 

Hydrological drought

A hydrological drought refers to a significant reduction in the amount of water available in rivers, lakes and groundwater (the hydrological system). This type of drought occurs when rainfall is well below expected levels in any large catchment area for an extended period. Hydrological droughts can result in water supply shortages.

In Auckland, the last most notable hydrological drought was in 1994. Advertisements promoting water conservation began that January. By late February, formal water restrictions were imposed to try to preserve water levels.  Water supply levels continued to fall in the following months with a total ban on the use of water sprinklers enforced in mid-April. The drought continued until September when Auckland received more than double the normal rainfall for that month. Using rainfall data, subsequent investigations indicated that there is a 4 per cent chance of this type of drought occurring in any given year  - a one in 25 year return period.

As a result of this drought and a dry summer in 1997-1998, Auckland’s water supply was redesigned in 2002 to cope with a hydrological drought with a 0.5 per cent chance of occurring in any given year (one in 200 year return period).

 


 

Drought impacts

Some of the severe impacts of a drought include:

Environmental impacts

  • Damage to plants and animals, wildlife habitat, and air and water quality.
  • Forest and range fires leading to a reduced landscape quality.
  • Loss of biodiversity and soil erosion.

Social impacts

  • Public safety and health.
  • Conflicts between water users.
  • Inequities in the distribution of impacts and drought relief.

Economic impacts

  • Economic impacts occur in agriculture and related sectors.
  • Losses in yields in both crop and livestock production.
  • Insect infestations, plant disease, and wind erosion.
  • Increase in the number of forest and range fires.
  • Reduced income for farmers and retailers and others who provide goods and services to farmers.
  • Increased prices for food, energy and other products as supplies are reduced.

Earthquakes


Earthquakes

While earthquakes may occur in Auckland, we are located in one of the lowest earthquake activity parts of New Zealand mainly because of the 300km distance from the Tonga-Kermadec subduction zone.

The Wairoa North Fault is the only identified active fault in Auckland although there is no evidence it has been active within the past 10,000 years. The Kerepehi Fault in the Firth of the Thames is a potential source of damaging earthquake.

Auckland’s strongest ever known quake was the 1891 Waikato Heads earthquake, which had a magnitude of between 5.5 and 6.0. Yet the only damage was some plaster and a chimney in Onehunga falling and some broken pottery and crockery. 

 


New earthquake reports for Auckland

The Regional Policy and Strategy Committee has endorsed Auckland Council's submission on the government's Earthquake Prone Building Policy.

For more information please read the following reports:

Estimated damage and casualties from earthquakes affecting Auckland (PDF 1.8MB)

Benefits of strengthening earthquake prone buildings in Auckland (PDF 78KB)

 


Historic earthquakes

The table below lists historic earthquakes felt in Auckland.

 

Earthquake Date

Location

Magnitude

Shaking Felt In Auckland

23 Jan 1855

Wairarapa

8.1-8.2

MM4

18 Oct 1868

Cape Farewell

7.0-7.5

MM4-5

23 Jun 1891

Waikato Heads

5.5-6.0

MM5-6

11 Feb 1893

Nelson

6.6-6.9

MM3-4

6 Oct 1914

East Cape

6.7

MM4

28 Oct 1914

East Cape

6.5

MM3

28 Jun 1921

Hawkes Bay

7.0

MM3

9 Mar 1929

Arthurs Pass

7.1

MM2-3

16 Jun 1929

Buller

7.8

MM3

21 Sep 1931

Bay of Plenty

6.75

MM2-3

20 Jul 1932

Taranaki

6.3

MM2-3

5 Mar 1934

Pahiatua

7.6

MM2-3

15 Mar 1934

Hawkes Bay

6.4

MM3

24 Jun 1942

Wairarapa

7.2

MM2-3

1 Aug 1942

Wairarapa

7.0

MM2

29 Sep 1953

Bay of Plenty

7.2

MM3

18 Oct 1953

Taranaki

5.3

MM3-4

30 Jan 1956

Bay of Plenty 

5.8

MM2-3

23 Jan 1962

Aria

5.5

MM3-4

23 May 1968

Inangahua

7.0-7.1

MM3

11 Feb 1975

Hen and Chickens Islands

4.4

MM3

2 Mar 1987

Edgecumbe

6.1

MM3

 


Ground shaking in Auckland

Ground shaking is the most common hazard experienced during an earthquake. In New Zealand, earthquake intensity at any specific location is measured by the Modified Mercalli Scale (MM).

 

MM Magnitude

Effects

1

Not felt in general.

2

Felt by a few on top of buildings.

3

Hanging objects may swing slightly.

4

Felt indoors by many, dishes rattle, walls creak.

5

People run outside, crockery dislodged from shelves, hanging pictures move.

6

Felt by everyone, heavy furniture moved, plaster cracks.

7

Frightens everyone, damage to weak buildings, difficult to stand up.

8

General fright and some panic, unreinforced chimneys fall, but only superficial damage to ordinary buildings.

9

Panic is general, some damage to strong buildings, ground cracks, some houses shifted off their foundations.

10

General panic, wooden buildings seriously damaged, landslides, rivers slop over banks.

11

General panic, broad ground cracks, soil slumps, great damage to underground pipes, few buildings remain standing.  

12

General panic, total destruction, objects thrown up in air.

 

The amount of ground shaking depends on a number of factors including the earthquake size and location. Auckland’s geology and soils vary considerably and will affect the intensity of ground shaking.

Shaking may be more intensive on softer soils such as those on the Manukau lowlands, floodplains running from the Waitakere Ranges and other low-lying floodplains or estuaries. It is likely to be less so on places built on hard bedrock such as much of the central city. 

 


Liquefaction

Liquefaction requires very specific ground shaking and soil conditions to happen. It occurs when waterlogged sediments experience earthquake shaking of enough intensity to cause the ground to become weak. This can cause heavy structures, such as buildings, to sink and light structures, such as underground pipes and tanks, to rise up to the ground surface. Once the shaking stops, the ground settles which squeezes water out of cracks or holes in the ground to cause flooding.

In Auckland, the geology and ground conditions are generally hard volcanic rock or ancient mud and siltstones, which are not very susceptible to liquefaction.  Localised areas that could be susceptible include reclaimed areas around the Ports of Auckland, floodplains of the Kumeu and Kaipara Rivers and the Manukau lowlands.

GNS Science have assessed Auckland’s urban liquefaction potential and concludes: “We expect neither liquefaction nor earthquake-induced landsliding to have significant impact on buildings in Auckland (a) because of the low levels of the intensities anticipated in future earthquakes and (b) because susceptible ground is uncommon in the built-up area.”

 


More information

Visit the GeoNet website for the latest information on earthquakes in New Zealand and for general information visit the GNS website.  

 
 
 

 

Fire


Emergency services deal with thousands of urban fires in Auckland every year as shown below. While many of these are structure and building fires, other fires are from chimneys, vegetation, human negligence, arson, chemical and gas sources.

The table below shows urban fires responded to by emergency services between 2005 and 2010.

 

Year

Number Of Urban Fires

2005/2006

6684

2006/2007

6668

2007/2008

6378

2008/2009

6233 

2009/2010

4387

 

Source: The New Zealand Fire Service – Emergency Incident Statistics Report (NZFS, 2010)

 

Wildfire in Auckland

There is a risk of wildfire in forested areas to the west (Waitakere Ranges), south (Hunua Ranges), northwest (Woodhill Forest), north (Mahurangi Forest) and east (Gulf Islands). 

Fires in these areas can be caused by agricultural burn-off getting out of control, arson, careless activities such as campfires in restricted locations, or natural causes such as lightning strikes. 

Weather contributes significantly to the risk of wildfire. Prolonged drought can provide tinder dry conditions and strong winds can buffet and spread wildfire easily. 

Auckland Council undertakes a Regional Wildlife Threat Analysis to assess the vulnerability.

If wildfire isn’t controlled it could result in:

  • evacuations
  • buildings destroyed
  • loss of life
  • physical loss and economic damage to the horticultural, agricultural and forestry sectors.

More information

 

See outdoor fires for information on fire seasons and permits, or visit the New Zealand Fire Service website

Flooding


Flooding is Auckland’s most common natural hazard. Most flooding occurs over a short period and affects relatively localised areas. Flooding is dependent on several factors including rainfall intensity and duration, soil conditions, local river levels and the physical characteristics of the catchment.

 


 

Flood conditions

The heaviest rainfall occurs during warm moist north or north-easterly wind flows, depressions from the north or northwest and slow-moving anti-cyclones to the east. These flows produce about 60 per cent of the annual rainfall. In contrast, south-westerly flows produce showery weather, especially in winter. Stormy westerlies also produce rainfall and may be accompanied by thunderstorms and sometimes tornadoes.

Localised falls can often occur during mid-afternoon in summer when sea breezes from the main east and west coasts sweep inland and converge over Auckland city sometimes producing thunderstorms. Heavy rainfall can also occur in moist and unstable northeast airflows.

Areas prone to flooding may include low-lying flood plains with streams or rivers, valley floors of steep river catchments susceptible to intense rainfall and ex-tropical storms, and low-lying areas near sea-level and the coast.

The conditions that lead to flooding in Auckland are often dependant on whether the catchments are predominantly rural or urban. Ground conditions influence flooding in rural catchments as run-off to streams and rivers occurs much faster if soils are already saturated. As much of Auckland’s urban catchments are concrete-lined or modified, flooding generally occurs from prolonged or sustained rainfall.

 


 

Recent flood events

The following are some of the recent Auckland flood events:

Auckland City flood: 1997

Heavy rain flooded a number of homes in Auckland’s western and southern suburbs during the morning of 24 May, and caused chaos on the roads. On average, the Fire Service recieves 15 emergency calls between midnight Friday and 1pm on Saturday. This time it got 230 calls. The meteorological maps for the event show a classic 'blocking' pattern. A broad ridge of high pressure extended from a high in the south Tasman Sea to another high east of New Zealand, cradling a shallow low in the central Tasman.

Pukekohe flood: 1999

In January 1999, 145mm of rain fell in six hours causing flooding in Pukekohe.

Floodwaters rose to 1.5m in some houses and resulted in:

  • flooding of residential homes and consequent evacuation of a number of Pukekohe residents
  • many roads being made impassable
  • sediment deposition causing extensive damage to land and buildings
  • water supply being contaminated due to the infiltration of sewer overflows causing a potential health risk to local communities. This continued for weeks.

The Leigh flood: 2001

On 29-30 May 2001, 132mm of rain fell in 24 hours over Leigh In an hour on 30 May, 109.4mm of rain fell, creating a new one-hour rainfall record for New Zealand. This event caused significant damage in the township and surrounding farmland. One family lost about 200 sheep and fences were flattened. Houses were flooded to a depth of well over one metre. On two properties, cars parked outside were swept some distance. A nearby footbridge across the Kohuroa Stream broke in two and there were large amounts of debris. For several nights, local fishermen didn’t put to sea because of the risks of running into whole trees just off-shore. The Rodney District Council estimated it caused $700,000 worth of damage mainly to the Leigh and Mangawhai districts.

Tropical Cyclone Wilma flood: 2011

On 28-29 January 2011, Tropical Cyclone Wilma passed across the northeast side of North Island. This was the first recorded storm to hit New Zealand with a ‘tropical cyclone’ status. Rainfall associated with the event reached 300mm in 24 hours in parts of northeast Auckland. This heavy rain, along with saturated ground from another storm event a few days before, led to flooding of properties and other infrastructure, particularly in low-lying rural areas near the east coast. The heavy rainfall also triggered several landslides in the region.  

 


 

Sandbagging your property
 

  • It is the responsibility of the home owner/occupier to protect their property from flooding.
  • During low level flooding, sandbags placed in the right locations around your home can reduce the impact of flooding.
  • Sandbags are not stored pre-filled as they begin to rot when damp. Keep sandbags dry and separate from sand until needed.
  • Sandbags require time and effort to fill and place, therefore they generally need to be filled and placed in advance of an event rather than in the middle of an event.
  • Alternative Solution
    Sheets of PVC are a practical way to waterproof around doors provided they are fixed in place adequately.


1. Where do I get sandbags?

Sand bags can be purchased from:

  • Gubba, Located in Albany sells unfilled bags for $1.00 each.
  • Sand is available from any garden centre.
    Sand bags can be made out of feed bags, hesian bags or plastic (polypropylene) sacks. If you have only the plastic open weave type bag, put one inside the other.  Don’t use kitchen rubbish bags as they will slip around in the water and may split.


2. How do I fill sandbags?

  • Use sand or sandy soil to fill bags. Dirt is not recommended.
  • Only fill sandbags two-thirds full
  • Do not tie or seal the sandbag
  • Take care when filling and lifting the sandbag, to avoid injury.


3. How to I lay sandbags?

  • Lay sandbags like brickwork. Stagger rows so that the joins to not line up.
  • Start at one end and work to the other end.
  • Ensure the unfilled part of the bag is covered by the next bag.
  • Tuck flap under bag at the end of the row.
  •  If the sandbag wall is going to be more than five bags high, you will need to lay two rows wide.


4. Where do I place the sandbags?

  • Place a small sand bag wall across doorways (for houses on a concrete pad). The number of layers will depend on the expected flood height. It is generally around two layers.
  •  If available plastic sheeting may be used under sandbags to reduce the seepage.
  • Make sure that you have at least one doorway that you can use to access or exit the building.
  • Cover drainage holes in the home e.g. toilets, showers, sinks to stop the back flow of water.


5. How do I dispose of sandbags?

  • Gloves should be worn when handling wet sandbags as they can contain chemicals, waste and diseases.
  • Sand and sandbags that have been in contact with floodwater need to be thrown away. They can be taken to a local transfer station. 

Watch this clip on YouTube for a sandbagging explanation.

 

6. Why is council not proactively sandbagging an area known to flood?

Maintenance crews employed by council are focused on maintaining the function and safety of key infrastructure such as roads, stormwater, water and sewerage.

Their priority is keeping critical services available for emergency services and the wider public.

Available resources are limited in times of crisis, crew will be prioritised to where they provide the greatest effect

 


 

More information

Visit the Metservice website for the latest information on weather in New Zealand and the NIWA website for general information on flooding in New Zealand.

Land instability


Coastal erosion is a natural process that is generally only of concern when threatening human habitation or development. The extensive urbanisation of Auckland, a region with 3100 km of coastline, has resulted in exposure to hazardous coastal erosion and accretion processes and both 'soft' and 'hard' shoreline and cliff erosion.

'Soft shorelines' refer to sandy beaches and dunes made up of unconsolidated or very weakly consolidated materials. Sandy beaches in the Auckland region experience periods of accretion (net sediment accumulation) and erosion (net sediment decrease). On beaches experiencing a sustained period of erosion, sediment loss results from waves, currents and wind removing sediment from the beach faster than it is replaced.

Erosion and instability also happens from natural and human factors such as cliff height and modification. Geology is the main factor that affects the extent of coastal erosion in Auckland. Volcanic coasts are formed of much harder rock than sedimentary coasts and less likely to form bedding, joint and plane failures. Greywacke rocks exposed along the southeast coastlines are softer and contain rock defects making these cliffs more vulnerable to instability than those of volcanic rock.

 


Impacts of coastal erosion

Coastal erosion can pose a risk to residential developments, roads, lifeline utilities and coastal structures. Soft shorelines bordering the Manukau and Waitemata Harbours are particularly vulnerable. Sea walls are particularly vulnerable and have been modified by seawalls in many places in attempts to control erosion. The sandy beaches around Omaha and Orewa have developments very close to or on dune deposits meaning that soft shoreline erosion is a possibility.

Specific impacts of coastal erosion in Auckland include:

  • danger to life in the case of sudden onset landslide events
  • structural damage or destruction of buildings and infrastructure
  • damage or destruction of lifeline infrastructure such as water, sewage and gas pipes and roads
  • loss of land, resulting in coastal cliffs or shorelines retreating closer to other buildings
  • land instability at neighbouring slopes and properties
  • loss of beach amenity due to cliff collapse or sea wall construction.

 


Coastal inundation hazards

Coastal inundation, often resulting from storm surge, occurs in Auckland and can cause significant disruption to low-lying coastal areas. Coastal flooding is commonly associated with severe storm events and is often the result of a combination of factors including:

  • strong onshore winds
  • low barometric pressure
  • high astronomical tides
  • wave set-up
  • wave run-up.

 


Impacts of coastal inundation hazards

Impacts of coastal inundation may include:

  • isolated coastal communities
  • damage to properties and critical infrastructure from flooding and waves
  • evacuation of some coastal areas
  • corrosion of electrical devices and other metal objects
  • salinisation of flooded land affecting agriculture
  • secondary hazards such as land instability and possible fire.
  •  

 


More information

 
 
 
 

Tornado


Tornadoes occur infrequently in Auckland but are much smaller than those that occur in the Midwest of the USA. 

They typically have narrow and short tracks about 10m to 30m wide and between 1km and 5km long. 

They are impossible to predict because of their small size, short lifespan and tendency to form offshore so people in a tornado's path are highly vulnerable to injury. 

Most tornado injuries or deaths in Auckland are related to airborne debris or building collapse.

 


 

Tornado damage

In Auckland, while tornado damage generally is localised, tornadoes have caused damage to:

  • buildings and power lines
  • fences and trees
  • vehicles by overturning or wind or hail damage
  • injuries to people but rarely death.

 


 

Recorded tornadoes

Some reported tornadoes in Auckland include:

3 May 2011
A tornado swept through parts of the North Shore with most damage around the Albany Mega Centre and Roseberry Avenue, Birkenhead. Significant damage was caused to the Albany Mega Centre, Westfield Albany shopping mall and residential housing resulting in one death and 14 people injured.

View footage of the tornado on YouTube.

4 July 2007 
A tornado caused damage to at least 30 homes in southeast Auckland. Wind gusts removed tiles from houses in Botany Downs, Golflands and Somerville. Two people were taken to hospital with injuries.

25 June 2005 
Two mini-tornadoes formed near Ardmore causing minor damage to some buildings.

22 December 2004 
Winds associated with a small tornado that occurred in Penrose and Mt Wellington caused damage to six houses, and were strong enough to move a stationary truck.

25 October 2002 
Tornado-like winds lifted roofing iron off a house and toppled fences in Blockhouse Bay.

31 October 2001
A tornado was observed in Auckland with recorded wind gusts of about 150km/h. At least 10 houses had damaged roofs and many trees were toppled.

May 1991 
A tornado swept through parts of Albany causing roofing iron to be lifted from homes and a small church, located in the southwest, was destroyed. A man died when debris from the tornado collided with him while driving a bulldozer but no other severe injuries were reported. 

 


 

More information

Visit the NIWA website.

Tsunami


A tsunami is a series of waves created by the sudden movement of the ocean floor caused by earthquakes or underwater landslides or volcanic eruptions. In deep water a tsunami therefore can travel at more than 500km/h at great distances in just hours. 

In the ocean, tsunami are generally small. When they enter shallow water, they increase in height and travel more slowly. Tsunami have been known to reach 10m and more in height. Typically, they have wave periods of 15 to 60 minutes, much longer than wind waves or swell. 


 

Tsunami sources

Local, regional or distant sources can generate a tsunami. Sources that could create a tsunami in Auckland include:

  • Local - Offshore faults, underwater volcanic eruption, large underwater landslides.
  • Regional - Tonga-Kermadec Trench, Southern New Hebrides Trench, large earthquakes near the southwest Pacific islands.
  • Distant - Any location around the Pacific Rim including, South America, Alaska, the Aluetian Islands, Kamchatka, Japan.

The size and nature of any tsunami affecting Auckland will vary on the source location and how it was created. 

Here are examples of tsunami observed here:

Year

Located Observed

Wave Height (M)

Source

Cause

1868

Great Barrier Island
Tamaki Estuary
Orewa
Port Charles

2.90
1.50
1.80
1.80 

Chile

Earthquake

1877

Auckland
Thames
Port Charles

0.20
0.90
3.60 

Chile

Earthquake

1883

Auckland
Thames
Coromandel

1.80
1.50
0.90 

Krakatau Volcano

Volcanic eruption 

1952

Auckland

0.10

Kamchatka, Russia

Earthquake

1960

Great Barrier Island
Auckland 

1.50
0.60

Chile

Earthquake

1964

Auckland

0.45

Alaska, USA

Earthquake

1976

Auckland

0.10

Kermadec Islands

Earthquake

1977

Auckland

0.10 

Kermadec Ridge

Earthquake

1982

Auckland

0.10

Kermadec Islands

Earthquake

1986

Auckland

0.10

Kermadec Islands

Earthquake

1993

Auckland

0.10

Kermadec Islands

Earthquake

1994

Auckland

0.10

Kuril Islands

Earthquake

2007

Aniwhata

0.12

New Georgia Islands

Earthquake

2009

Auckland

<0.5m

Samoa

Earthquake

2010

Auckland

<1.0m

Chile

Earthquake

2011

Auckland

<0.5m

Japan

Earthquake

 

 


 

Local source tsunami in Auckland

There is a low likelihood of a local source tsunami affecting Auckland, If one did, the effects would be significant due to the limited warning time - less than one hour. 

Auckland is one of the lowest seismically active parts of New Zealand. While large earthquakes aren’t common, the chance of a large tsunami-generating earthquake is possible. The Kerepehi Fault, in the Firth of the Thames, is possibly active offshore and could generate earthquakes up to magnitude 7.1.  Research suggests this fault does not represent a major tsunami threat to Auckland. 

An underwater volcanic eruption in the Hauraki Gulf or harbours surrounding Auckland could create a local source tsunami. If magma from a volcano suddenly comes into contact with water it can generate an explosive eruption called a phreatomagmatic eruption which can send large waves towards shore.

 


 

Regional source tsunami in Auckland

Regional source tsunami are created in locations where the wave will come ashore one to three hours after generated. The most common source is subduction zones, where tectonic plates collide. The most common source concerns those in the southwest Pacific.
 
The Tonga-Kermadec trench to the east and northeast of Auckland could generate earthquakes greater than magnitude 9.0. Auckland Council has modelled the effects. The most susceptible to tsunami are coastal communities in the north. Coast wave heights for other parts of Auckland vary considerably.

This modelling shows maximum coast wave heights for selected Auckland communities for 100 large earthquakes on the Tonga Kermadec Trench.

 

Location

Maximum Modelled At Coast Wave Height (Metres)

Omaha

6.2

Orewa

2.8

Whangaparaoa

1.5

Takapuna

2.1

Northcote

0.9

Point Chevalier

0.5

CBD

0.9

Tamaki River

1.0

Howick

0.8

Beachlands

0.7

Oneroa

2.3

Onetangi

2.7

 


The South New Hebrides Trench, Vanuatu and Samoa are among areas in the southwest Pacific that could generate tsunami to New Zealand. Travel time to Auckland would be 2 to 3 hours.

 


 

Distant source tsunami in Auckland

Distant source tsunami generally have travel times between three and 15 hours. Large earthquakes occurring anywhere around the Pacific Rim have the potential to generate tsunami that can affect Auckland. Distant source tsunami from Alaska, Russia and most commonly South America have all been observed in Auckland.

The most frequent distant source of tsunamis affecting Auckland is the west coast of South America. Tsunami generated here generally take around 12 to 15 hours to arrive and can affect both coasts, depending on the size and location of the earthquake. In August 1868, a large earthquake generated a tsunami that took 15 hours to reach Auckland and reached up to 2.9m on Great Barrier Island’s east coast. In May 1960, the largest earthquake ever recorded, magnitude 9.5, occurred in Chile and created a tsunami that reached 1.5m on Great Barrier Island.

The Krakatau volcanic eruption of May 1883 in Indonesia generated a tsunami of about 1.8m in Auckland. This event was rare as it is one of only a few tsunami to occur in Auckland not created by earthquakes.

 


 

More information

The GNS Science website, through the GeoNet Project, monitors the arrival of tsunami in New Zealand. 

The United States National Oceanic and Atmospheric Administration (NOAA) website maintains and operates deep ocean buoys that provide some warning of a tsunami being generated. 

 
 
 
 

 

 

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