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Hazards

Hazards in our region   

The Civil Defence Emergency Management Group is required to consider any and all natural and man-made hazards that may lead to a declaration of a state of local emergency in the Gisborne district.

NOTE

To avoid the potentially misleading conception created by using terms such as a 1:100 year event to describe event frequency, the hazard descriptions below use the chance (the annual probability) of the event occurring in any one year.  To help put these statistics into perspective, the following example compares the likelihood of winning the lower divisions of lotto in any one year.

4th division             0.013%

5th division             0.206%

6th division             0.275%

Biosecurity  |  Drought  |  Earthquake  |  Erosion  |  Flooding  |  Human pandemic  |  Information systems failure  | Tsunami  |  Volcanic

Biosecurity  

New organisms arriving in Tairawhiti may impact on the economic, environmental, socio–cultural and human health values.

New pests that damage horticultural production or affect animal health can have direct effects on production and severely impact export flows. There is a potential for this to occur under the climate change scenario.

MAF Biosecurity (MAFBNZ) leads and coordinates responses to new organisms, except if they are affecting humans when MoH will lead.

Drought 

The present district climate trends towards a seasonal drought rather than a long-term condition that will cause severe problems.

The current threat of drought in the Gisborne district is more one of inconvenience than of widespread disruption. There is an expectation that dry years may be more frequent in future, but the decline in pasture production on the East Coast may be less than for other eastern regions due to more rain forecast during the summer months.

The Ecoclimate team predicted that pasture production on the East Coast north of Napier would increase over summer.

In fact the changing distribution of rainfall with a reduction of 10% in winter and spring and an increase in summer/autumn rainfall may enable more hay and summer feed crops to be grown for use over autumn and winter.

The chance of a drought in any one year is currently 5% but this is expected to rise to 10% by 2030.

Earthquake 

Earthquake 20 December 2007  l  Earthquake research 

This report has been prepared by the GNS Science for Gisborne District Council in April 2012 (see disclaimer in report)
Review and update of earthquake return periods for the Gisborne Region (1MB 24 pages)

Earthquakes are measured by two different scales - they are reported using the Richter Scale which is the amount of energy released at the epicentre.  The other scale is the Modified Mercali and this is the felt intensity at a point  away from the epicentre and gives an indication on the likely effects on people, buildings and the environment.

The Gisborne district sits close to the boundaries of 2 crustal plates. It runs parallel to the coast about 60-90kms offshore.

The Pacific Plate is subducting beneath the Australian Plate which leaves the district susceptible to large earthquakes, although how large is unknown. The subsequent movement of the plates causes uplift of the East Cape, this activity has caused many faults to develop both offshore and onshore.

Identification of these faults has come from research but many are still to be found. It is possible that many will only ever be found when next they rupture as there is no visible evidence to suggest that they exist. These are commonly called blind faults.

Offshore and and to a lessor extent, by triggering undersea landslides, large onshore earthquakes have the potential to cause tsunami.

An MM8 event occurring at certain times of the day will potentially cause large numbers of casualties and will severely test the ability of responders to cope. This type of event is probably at the limit of what the organisation can plan for. Mitigation through building codes will help limit the number of casualties.

Liquefaction is a significant effect of earthquakes with much of the city area and Poverty Bay Flats being susceptible. During large earthquake certain soil types release ground water and the surface looses its strength allowing heavy structures to topple and roads become deformed.

Return period for earthquakes across the Gisborne district

Annual Probability (%) (these are new statistics from an update done by GNS in April 2012, there are some significant differences from the last data set with reductions in the return periods.  The difference is due to updates in technology and a better understanding of our active faults).

MMI

6

7

8

9

10

Gisborne

11.1%

2.9%

0.7%

0.1280%

0.0100%

Tolaga Bay

11.1%

2.6%

0.6%

0.1325%

0.0080%

Tokomaru Bay

11.1%

2.4%

0.4%

0.0475%

0.0030%

Te Puia

11.1%

2.4%

0.4%

0.0400%

0%

Ruatoria

11.1%

2.1%

0.3%

0.0225%

0%

Tikitiki

11.1%

2.1%

0.3%

0.0160%

0%

Te Araroa

10.0%

1.9%

0.2%

0.0115%

0%

There have been 5 MM7 and 7 MM8 experienced at various locations around the district in the last 90 years. A MM9 did affect the top of the East Cape in 1914.

A significant emergency event would be an MM8 occurring in the city during the day, during school hours and with a busy CBD area – or an event greater than MM8.

The Modified Mercalli Scale (MM) is the measurement of the felt intensity at a specific place where as the Richter measurement is the amount of energy released at the epicenter.

Erosion 

The Gisborne district is geologically unstable and apart from earthquake impacts, weather events and time can cause landslip and erosion. Several populated areas are at risk and they include Te Puia Springs, Tokomaru Bay, Tuahine Point, Wainui Beach, Makorori Headland, Kaiti Hill and Hospital Hill. There will also be widespread surface slipping and deep seated movement on hill country destroying productive land and adding sediment to river catchments. This in turn will contribute towards future flooding events. In rural areas there are some buildings close to steep hill country which may be at risk.

It is not practical to have an annual probability for an event that could result in a significant emergency response for a landslip event. The chances of one happening though are increasing with more development being allowed on slopes around the city.

Flooding 

Flood photos 30 June 2009

The East Cape has a history of “hits” from decaying tropical cyclones causing widespread flooding and disruption. There are also other events that are more localised and result from a weather system dumping a large volume of water in a small area. A recent study showed that there is a trend of less of these events, but when they occur more rain falls.

Most areas north of Ruatoria have a higher average rainfall than the south of the district and can cope with high rainfall events. The Waikura Valley for example can get 200mms in a 24 hour period without any adverse effects.

The populated areas likely to be worst affected in a large event are the Poverty Bay Flats, Te Karaka and the area just north of Tolaga Bay. More intense localised events such as the 1985 Ngatapa flood and the 1977 Glenroy flood also caused substantial damage.

Most of the Poverty Bay Flats, during inundation, is not subject to fast flowing water but huge areas are subject to ponding, which after the water recedes, will leave thick layers of silt damaging pasture and fences.

In the city a number of riverbank properties are at risk. Extensive flooding depends upon the tides, potential overflow from the Waipaoa River, storm surges and the flood-peak times of the Taruheru and Waimata Rivers.

The stop-banks at Te Karaka have been raised to give protection up to an event of 0.5% excedance, but this does not mitigate the need to evacuate in a significant event as they get isolated early and forecasts can be wrong.

Upgrading of the Waipaoa River Flood Protection Scheme was due by 2012 but is still being consulted on via the 10 Year Plan process.

The July 2008 events are also a warning to watch multiple events that occur within days of each other.

A major rainfall event is expected somewhere in the district every 10-15 years. The return period for an event requiring a declaration is likely to be less than 20 years.

The Cyclone Bola flood in the Waipaoa River has a 1.4% annual probability (although this is potentially increasing with aggradation of the berms).

Human pandemic 

There is a significant difference between normal influenza and a pandemic. As a normal occurrence, hundreds can die around the world in any one year through flu pandemic. A pandemic is more significant mainly because it is a new strain of virus, so less people will have immunity. Consequently most people could get sick if they come into contact with the virus. With the mobility of today’s population the virus can be around the world before an outbreak becomes evident at its source.

A pandemic influenza epidemic is probably the event that will cause the most significant community disruption. This scenario is potentially the most disruptive emergency scenario the Group could face, not only because of the impacts locally, but because of the potential for the epidemic to have national and international impacts leaving only a local response with no support. One factor in the Groups favour is the relevant isolation and the potential to stop the disease entering the district. But there is also the potential for an event to spread globally before any response is possible as witnessed in the 2009 Swine Flu pandemic.

Other viral, bacterial or other influenza pandemics may trigger emergency response.

Information systems failure 

The primary reason for an Information Technology (IT) failure is power loss. This can affect phones, computers, Eftpost, hubs, wireless links and modems. Failures can also occur due to hackers, viruses, hardware failure and operating system failures. Most IT failures are isolated to individual organisations but can be widespread under some scenarios.

The most probable scenarios for a major IT failure that could cause widespread disruption is a loss of telecommunications to a significant number of emergency responders, utility providers or businesses at the same time, or a widespread power outage of more than 2-3 days duration.

People are becoming more and more reliant on technology, so when it’s not there, it doesn’t take long for disruption to start causing problems.

Tsunami 

The entire Gisborne district coastline is at risk from a tsunami originating from a distant or local source. The only real threat from a distant event is the west coast of South America – Chile or Peru. The only other real threat is from a local event generated within 60-90 km off the coast. Since 1832 there have been 25 “hits” on the East Coast. Eleven of these have been from local events and 14 from distant events. The highest known wave-heights from local events were up to 10 metres and distant event were up to 1-2ms.

None of these events have caused serious inundation but that does not rule-out a life threatening event in the future.

The most common triggers for tsunami are large undersea earthquakes, slow earthquakes, volcanic events, undersea landslides, mud volcanoes and certain weather related conditions.

For the purpose of developing evacuation plans, worst case credible events a (maximum run-up at high-tide) large event is taken as 10 metres on the coast and 6 metres in Poverty Bay.

It is critical that everyone understand the difference in management requirements for distant and local events. A locally generated event has the potential to cause a significant number of casualties with the only warning being an earthquake (either a very large felt earthquake or an earthquake of any size that shakes for over a minute). Whereas a distant event will have between 11-14 hours warning and give authorities time to evacuate those in the potential risk areas.

The available data indicates there is a 5% annual probability of a hit by a wave of unknown height, but it is unlikely to be more than a few centimetres. There is a 0.125% chance of a 6 metre event in Poverty Bay but the worst case impacts occur at high-tide so the chance of significant inundation is further reduced. The annual probability of a 10 metre event is unknown on the coast but it falls within the range of 1.4% - 0.02%.

Volcanic activity 

Ash fallout has the potential to be the most disruptive disaster in the district. A 700-year Okataina event could erupt ash intermittently over a 3 year period. Past ash falls have ranged from millimetres to half a metre. Wind direction and speed could bring ash to this district in less than 2 hours.

Source Thickness (mm) Frequency (annual probably %)
Okataina 1-150 0.14% - 0.02%
Okataina  1-1500  0.14% - 0.03%
 White Island  1-10  0.1% - 0.02%
 Ruapehu  1-5  1% - 0.2%
 Taupo  1-5  0.08% - 0.06%
 Taupo  1-50  0.08% - 0.06%
 Taupo  1-600  0.04% - 0.02%
 Taupo  100-4000  0.02% - 0.01%
 Egmont  1-2  0.08% - 0.06%

There has not been a significant event from Okataina for approx 700-years or an event from Taupo for around 1850 years. The last volcanic event to affect the Gisborne district was in 1995 when 1-2mm of ash fell from Ruapehu. There is a 0.5% chance in any one year of an event of 15-30 millimetres occurring.

Volcanic activity outside of the district that does not result in ash-fall within it, especially in the Auckland field, can still have a significant affect on the district through economic disruption and hinder distribution of essential resources.

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