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 this will change in the future.
Urgency: The return period is not known, Hawkes Bay has a calculated return time of 10 years. The Gisborne District would be similar to that but the effects would not be as severe - in the short term. Return period for this district is more likely to be >20 years but <200years.
Growth: The impacts of a drought are likely to increase due to predicted climatic changes. The effects of climate change are expected to make the district generally drier, with winter rainfall varying between -5% & 5% on present levels, (although rainfall in the Te Araroa & Hick Bay areas will increase (summer) by 5 – 10% by 2080) this will be a gradual trend over the next 80 years.2 However the population exposed to the effects of drought are not likely to change significantly. The biggest impact from climate change could be more frequent and more intense droughts and floods, as these require different mechanisms for coping than a gradual increase in rainfall or temperature.
The East Cape has a history of “hits” from decaying tropical cyclones causing widespread disruption and flooding. There are also other events that are more localised and are a result a weather system dumping a large volume of water in a small area. This phenomenon appears to becoming more frequent.
Most areas north of Tolaga Bay have a higher average rainfall than the south of the District and can cope with higher rainfall events. The Waikura valley for example can have 200mms in a 24 hr period without adverse effects. The 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 e.g. Ngatapa flood in 1985 & the Glenroy flood in 1977 can also cause substantial damage.
In the city a number of river bank properties are at risk. Extensive flooding depends upon the tides, potential overflow from the Waipaoa, storm surges and the flood peak times of the Taruheru and Waimata Rivers. The threat to Te Karaka and the Flats is increasing with the aggradation of the Waipaoa riverbed but stopbanks are to be raised at Te Karaka to lessen this effect. Upgrading of the WRFCS is due to start and will be ongoing for up to 10 years.
Urgency: 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 had a return period of between 1:60 – 1:100, inland from Tolaga Bay it was probably a 1:80 rainfall event.
Growth: The predicted climate change indicates a generally drier climate with more intense rainfall events. This will mean more erosion and more widespread flooding.
Current planning processes require buildings in flood hazard areas to have a floor height of Bola + 10%. Regulations also control the location /or not of habitable buildings in flood prone areas. These measures will ensure no increase in population exposure to the hazard.
Strong wind events do occur in the District on a scale that may cause major damage. This is normally from the effects of a cyclonic event, but could also happen during a north westerly or a southerly storm.
Urgency: The return period is probably similar to a major flooding event of (7–10 years) but the likelihood of one of these events resulting in an emergency is small. The return period for an extreme event is unknown but is likely to be between 20-200 years.
Growth: Climate change may affect the frequency and intensity of windstorms if they are related to cyclonic storms. The magnitude of events does not warrant any restriction over settlement patterns. There is unlikely to be any significant increase of population in local areas prone to the effects of windstorms.
Climate change in itself is not technically a hazard, but is included in these descriptions so that there is an understanding of its impacts and implications to the other hazards. It is an established fact that it has started and its changes over the next 80 years are fairly well understood.
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Sea Level Rise: will vary around the world but it is possible the rise could be up to half a metre by the end of the century. Sea levels rising will likely cause increased coastal erosion & storm surges.
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Temperature: the mean temperature is expected to rise between 1.8 - 2 degrees, summer temperatures will rise faster than winter. Fewer frost days in winter & more hot days in summer.
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Rainfall: The district will be generally drier, with winter rainfall varying between -5% & 5% on present levels (although rainfall in the Te Araroa & Hick Bay areas will increase (summer) by 5 – 10% by 2080) this will be a gradual trend.
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Gradual increase of temperatures and sea level over the next 80 years will occur.
The westerly winds will become more predominate. The frequency of cyclonic events may increase, and will be more intense.
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Tsunami & Storm Surges: Increased areas of inundation.
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Flooding: Increased areas of inundation. Increase in speed of/and areas of erosion.
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Drought: Longer periods of nil rainfall, which will have an effect on all vegetation in the area. Some water supplies will disappear.
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Rural Fire: Increase in the fire risk.
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Biosecurity: Pests & diseases could spread in range & severity.
The projected decreases in rainfall combined with increased temperatures will likely lead to reduced runoffs from rivers of 10 - 40%.
The biggest impact from climate change could be the more frequent & more intense droughts & floods, as these require different mechanisms for coping than a gradual increase in rainfall or temperature.
Impacts expected from more intense precipitation include flooding, landslides, increased soil erosion, increased pressure on govt & private flood insurance schemes & disaster relief.
Ash fallout has the potential to be the most disruptive disaster in the District.
A 700 year Okataina event could last for 3 years. Past ash falls have ranged from mms to half a metre.
Wind direction & speed could bring ash to this district in less than one hour.
| Source |
Thickness (mm) |
Frequency (years) |
| Okataina |
1 – 150 |
700 – 5000 |
| Okataina |
1 – 1500 |
700 – 3000 |
| White Is. |
1 – 10 |
1000 – 5000 |
| Ruapehu |
1 – 5 |
100 – 500 |
| Taupo |
1 – 5 |
1300 – 1600 |
| Taupo |
1 – 50 |
1300 – 1600 |
| Taupo |
1 – 600 |
2500 – 5000 |
| Taupo |
100 – 4000 |
5000 - 10000 |
| Egmont |
1 – 2 |
1300 – 1600 |
There has not been a significant event from Okataina for ~ 700 years or any event from Taupo for ~ 1850 years. The last volcanic event to affect the Gisborne District was in 1995 when 1-2mm of ash fell from Ruapehu. For the purpose of this exercise we would plan for a 15-30mm or greater event. The return period for such an event is >200 years.
Growth: It is unlikely that climate change will impact on volcanic activity, and there is unlikely to be any increase of population prone to the effects of volcanoes.
The Gisborne District sits close to the boundaries of 2 crustal plates. It runs parallel to the coast about 50kms offshore. The Pacific Plate is subducting beneath the Australian Plate which leaves the district susceptible to large earthquakes. 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. Onshore and Offshore earthquakes have the potential to cause tsunami.
Urgency: Return period for the Gisborne District Years
|
MM Scale |
Return Period (yrs) |
| MM6 |
2.5 – 8.5 |
| MM7 |
13 - 42 |
| MM8 |
58 - 160 |
| MM9 |
210 - 530 |
| MM10 |
710 - 2300 |
There have been 5 MM7’s and 5 MM8’s in the last 90 years. A MM9 did affect the top of the East Cape in 1914. An emergency event would likely be an MM9 with a return period of >200 years.
Growth: Population growth is expected to remain static in the medium term and climate change will not impact on the likelihood of an earthquake event.
The entire coastline of the Gisborne District is subject to tsunami both from a distant and local source. There have been seven “hits” on the East Coast since 1840, the highest frequency in New Zealand. Four of these have been local and three distant. Highest known wave heights – local events up to 10m, distant - 4m. The potential for local waves greater than 10m is a reality.
The dates of known events are: 1868,1877,1883,1926,1947, 1947,1960,1970.
The mechanisms for causing tsunami are large earthquakes, slow earthquakes, volcanic events, undersea landslides and mud volcanoes. For the purpose of developing plans and defining the issues a large event is defined as being 10m or more on the coast or more than 4m in Poverty Bay.
Urgency: The available data indicates there is a 5% chance in any one year of a hit by a wave of unknown height. The likely return period is 20 years for a wave of unknown height to occur at some point in the district. The return period for a wave of >10/4m or greater is probably >20 years but is more likely <200 years.
Growth: The impact of a tsunami is unlikely to increase due to predicted climatic changes and associated sea level rise and the population exposed to the effects of tsunami are not likely to change significantly, although the minimum lot size for areas such as Lloyd George Road Wainui were halved in 1997, doubling the potential residential development in that low-lying area.
Coastal Communities, particularly Wainui Beach, Te Araroa, Hicks Bay, Loisels (camping period), Whangara and Anaura Bay are the areas mostly likely to be affected by an emergency scale storm surge. The Waipaoa and Turanganui Rivers will also be affected adversely if they are in flood with increasing problems up stream. Anaura Bay is the only one to have experienced a surge in recent history which impacted on people. This was in the summer of 1996/1997.
Urgency: The return period for such an emergency event is thought to be around 20 years or greater.
Growth: The impact of a storm surge is likely to increase due to predicted climatic changes. However the population exposed to the effects of storm surges are not likely to change significantly.
The impacts of erosion & instability will be localised in heavily capitalised areas i.e. Te Puia, Waimata riverbank, Tokomaru, Te Puia, Tuahine Point, Wainui Beach, Makorori Headland, Kaiti Hill, Hospital Hill. There will be widespread surface slipping on pastoral hill country.
Urgency: The expected return period for an emergency event may be between 20-199 years.
Growth: Climate change causing either sustained heavy rain causing saturated soils or intense storm events of relatively short durations are likely to increase erosion activities & instability problems. However the population exposed to the effects of erosion/instability are not likely to change significantly.
Oceanic mud volcanoes are more likely to result in an extreme emergency event in the Gisborne district than one on land. The eruption or sudden raising of the sea floor has the potential to cause Tsunami. The number of events that have caused Tsunami in the past will never be known as there is no way to differentiate between a sea floor slip and an eruption. The only known sea events are the 1931 sea floor uplift at Sponge Bay and evidence of “pock mark” like features on the sea floor in Poverty Bay.
On land the known events occurred in 1927 and 1930 in the Waimata Valley and 1931 at Hangaroa. The 1930 and 31 events were extremely violent. Large volumes of gas are also released with the risk of fire and explosion. There is also a risk of debris from an event blocking a watercourse and making a dam.
Urgency: The return period are unknown but would be greater than 20 years but probably less than 200.
Growth: Climate change is unlikely to impact on mud volcanic activity. Population exposure is not likely to change significantly.