It is important to know about the hazards that can affect our communities. The more we know about them the better we can plan for them causing an emergency.
Earthquakes present the most significant hazard to South Canterbury as well as other parts of New Zealand. We do not experience them very frequently but they have the most capacity to cause widespread damage.
Of all the earthquake sources that can affect Canterbury, the Alpine Fault is the most studied. It is capable of generating a very large earthquake which will affect much of the South Island. It will produce the strongest earth shaking experienced in South Canterbury since European settlement.
Of course, there are other known fault lines in South Canterbury (and probably some we do not know about). Earthquakes are a very real hazard and their occurrence must be planned for.
This is Canterbury's (and probably New Zealand's) most frequent emergency.
The Canterbury Plains were formed by alluvial gravels being deposited from the many rivers which cross them. Since European settlement, we have built communities on the plains and then tried to protect them from flooding by building stopbanks. Because the stopbanks are quite effective, more and more intensive land use of flood plains has occurred. That is okay until a flood exceeds the design of the stopbanks or they breach. Then people and property on the flood plain are in danger!
If you live, work, or your school is on a flood plain, you are at risk. You should inform yourself about that risk. Environment Canterbury has produced flood plain maps and staff there can tell you about the flood risk.
Many of the rivers in Canterbury build up to a flood over a period of time, ranging from hours to days. Some, however, flood with little or no warning. It is very important to understand the flood risk. You need to know what depth of water you could expect and how fast moving it could be (velocity). You also need to know the likelihood of getting flooded.
Declared Emergencies Caused by Flooding
|06.02.1997||Declaration for Timaru District.Opuha Dam failure while under construction.About 200 people evacuated.|
|14.12.1995||Declarations for Omarama (Waitaki District) and Hakataramea (Waimate District). Total of 52 people evacuated. Extensive damage to one lane bridges across the Waitaki River.|
|19.03.1994||Declarations for Timaru, Waimate and Mackenzie Districts. In total, appropriately 240 people evacuated. Approximately $5.5 million damages, including $3 million damage to state highways.|
|09.01.1994||Declaration for Mt Cook (Mackenzie District). 150 people evacuated.|
|11.08.1986||Declaration for Milford Riding (Strathallan County) and Temuka Borough. 65 evacuated. Agricultural assistance $4.79 million (1991).|
|16.03.1986||Declaration for Strathallan County, Waimate Combined District, Temuka Borough and Timaru City. 1,160 people evacuated from Pleasant Point and 200 people from Seadown. One person was killed. Insurance claims totalled $27.2 million (1994). Road and rail damage was estimated at $30.65 million (1991). Agricultural assistance estimated at $4.67 million (1991).|
Environment Canterbury can be contacted on: (03) 687 7800
You can check out river level and rain information on their website: www.ecan.govt.nz
The most likely meteorological event to cause wide scale emergency is a windstorm. These can be in two forms:
- A tornado can do great damage but is usually confined to a smaller area. These are pretty uncommon but have happened in the past.
- Cyclonic storms (usually from a southerly direction) and Fohn winds (north-westerly) can also cause damage and usually over a greater area.
Winds can damage buildings and overhead wires, uproot trees, blow vehicles off roads and cause debris to fly through the air. It is dangerous to go outside during a very high wind, so don't!
Canterbury can expect a damaging windstorm every 20 years on average. The last one happened in October 2000 and was mainly confined to Christchurch and Banks Peninsular. Prior to that, a severe windstorm affected all of Canterbury in 1974.
Because everywhere can be affected you should know what to do before, during, and after, a storm. Check out the Get Thru website - www.getthru.govt.nz
Some people know them as tidal waves but they are not caused by tides. Tsunamis are caused by undersea earthquakes, large scale subsidence of the sea floor (like a landslide) or volcanic activity. There are far field and near field tsunamis.
Far field tsunamis, as the name suggests, are generated a long way from New Zealand. In 1960, a far field tsunami was generated from a huge earthquake in Chile.
The Pacific Tsunami Warning Centre is based in Hawaii and can give us warnings that a tsunami has been generated from a seismic (earthquake) source anywhere around the Pacific. That is good!
The only time we get information about the size of the tsunami is when it approaches or passes a land mass. That is not so good!
In the open ocean, tsunamis travel at speeds in excess of 700kph. That is faster than a jumbo jet! The wave may be only a few centimetres high because all the action is under the water surface. As it enters shallow coastal waters, the wave slows down to 30-40kph but may increase in size. When it reaches shore, its destructive energy is able to do damage to anything it can reach.
Near field tsunamis are generated close to shore. The devastating tsunami which struck Papua New Guinea in 1998 was an example. This event generated waves in excess of 10 metres high. The only warning may be an earthquake, and could give you seconds or minutes warning.
Because the seismicity of the sea floor is not well understood, it should be assumed that there is a chance of receiving a near field tsunami. It is very important to get away from the coast if you feel an earthquake.
Do you remember all the fuss about the "Y2K Bug"? That was an example of a potential manmade hazard. We could plan so well for it because we knew when it may occur.
Timaru has had two declared emergencies for manmade events:
- 1975 when gas from the gas works leaked into the sewer.
- 1994 when a building containing grain threatened to explode because the grain got wet.
Many transport routes (roads, rail, ports and airports) convey hazardous materials. If something goes wrong (e.g. a chemical spill), it may require evacuation of many houses.
Other examples of manmade hazards are:
- Transport accidents involving many passengers (e.g. cruise liner, train)
- Transport and industrial accidents involving leaking chemicals or gas
- Widespread and prolonged disruption to services (e.g. power, sewer, water)
- Space junk falling to earth
Publish Date: 28 Mar 2014