This section summarises the natural hazards that affect Christchurch City. Most of the information is from the Canterbury Regional Council report Natural Hazards in Canterbury. Natural Hazards which affect Christchurch are divided into the following types: Meteorological Hazards, Flood Hazards, Coastal Hazards, Slope Hazards and Seismic Hazards (Table 2.2).
Table 2.1 lists the types of severe storms and environmental risk associated with meteorological hazards. The greatest economic impacts result from events that have a regional impact such as heavy snow storms, drought and wind storms.
Christchurch does not experience the intensity of rainfall that some other parts of New Zealand do. Generally 24 hour rainfall with return periods of five years (6) produces 50 -100 millimetres on the plains and 100 - 150 millimetres on the Port Hills. The effect of heavy rainfall is covered in the Flood Hazard and Slope Hazard sections.
Significant snow storms have occurred in Christchurch in 1895, 1896, 1901, 1918 1945 and 1992. Snow storms cause damage to buildings and power lines, and disruption to traffic and communications within urban areas and stock losses in rural districts. A secondary effect is snowmelt flooding.
The 1992 snow storm killed over one million stock in Canterbury and damaged buildings in the Christchurch and surrounding areas. Its overall economic impact was estimated to be somewhere between 50 and 100 million dollars. The 1992 snowstorm had a recurrence interval of about 50 to 100 years.
The most severe winds in Christchurch result from orographically reinforced winds from the north west. Severe events have occurred in 1945, 1964, 1975 and 1988. The peak wind in 1975 was 193km/hr, which exceeded the 100 year return period. Extreme wind events cause personal injury and death, and extensive damage to buildings, vegetation and infrastructure.
Three significant tornado events have been recorded in the Christchurch area in the last 36 years - in 1962 (Christchurch), 1975 (Governors Bay) and 1983 (Halswell).
Electrical storms are relatively infrequent with, on average, five days of thunder per year on the Canterbury Plains. These tend to occur between September and March.
Canterbury has, on average, one significant hail storm a year, with the highest frequency occurring in October to March.
The region has one significant drought about every six years. Since 1970 severe droughts have occurred in 1977-78, 1985, 1988-89 and 1992. Drought is generally considered a rural problem, but effects on the urban environment include increased water consumption and the economic impact of reduced cash flow from the rural sector. The drought of 1992 also significantly affected hydro-electric power generation, resulting in power shortages in Christchurch.
Flooding is the most common and most significant natural hazard in Canterbury. Both urban and rural areas are affected.
The most significant flood hazard to the Christchurch urban area is posed by the Waimakariri River, although this has been substantially mitigated by river control works. Breakouts of floodwaters from the Waimakariri River caused damage to urban Christchurch in December 1865, October 1866, and February 1868. The Waimakariri River still threatens a greater concentration of life and property than any other New Zealand river.
The Halswell, Avon and Heathcote Rivers cause the most frequent flooding in Christchurch. Flooding from these rivers is the consequence of long-duration, moderate-intensity easterly storms.
Areas of the Avon River most susceptible to flood damage are the Lower Wairarapa Stream in the upper catchment and the five low-lying areas of New Brighton, Avondale, Horseshoe Lake, Hulverstone Drive and Bexley. Since 1883 over 30 different floods have caused damage from the ponding of stormwater and breakouts from tributary streams. Damage from flooding was increased by unusually high tides in July 1955, March 1957 and August 1992.
Flooding of property and roads occurs along much of the Heathcote River Channel between Woolston and Hoon Hay. Floodwaters have reached heights above house floor levels on four different occasions since the mid 1960s (1968, 1975, 1977 and 1980).
The shoreline position of Pegasus Bay is generally stable or prograding (7) . However, rivermouth sites such as South Brighton and Kairaki/Brooklands at the mouth of the Waimakariri River may be subject to rapid landform changes due to erosion or accretion.
Assessment of the Tsunami hazard for Christchurch is subject to considerable uncertainty. A tsunami could be a serious hazard and damage a wide area, perhaps as much as 30 percent of the urban area. It is believed that during a tsunami all openings in the coastal dune system would become conduits and result in widespread inundation. There could also be large-scale flooding associated with extreme water levels entering the Avon-Heathcote Estuary.
The Port Hills are at significant risk of landslides and rockfalls caused by earthquake or severe rainstorm.
Earthquakes can produce a rockfall hazard on the Port Hills, or facilitate slope failure if they occur during mid-winter to early spring when soil moisture levels are high enough to reduce the cohesive strength of loess soils.
Rainstorm-induced landslides tend to be associated with prolonged wet periods, for example two or more consecutive wet winters, or after a major snowstorm, when soil moisture levels are close to saturation. These conditions only require a minimal increase from moderate frequency storms to initiate movement. There appears to be a frequency of 10-20 years between significant events.
Apart from the slope hazards mentioned above, the greatest hazard from a major earthquake (with a return period of 150 years) to the Christchurch area is widespread liquefaction, especially in the fine-grained loose sediments around the Avon-Heathcote Estuary. Liquefaction results from ground shaking which does not permit pore fluids to escape. As a consequence loose saturated silts and sands lose strength and tend to flow, resulting in failure to support structures, soil movements, and the floating of buried structures such as pipelines.
Table 2.3 lists the natural events that have caused damage in Christchurch City during the year to June 1997. High winds during a strong north-westerly airflow resulted in property damage and loss of power during October 1996. Snow resulted in the brief closure of Christchurch Airport during June 1997.
The predominantly La-Nina conditions, which prevailed over New Zealand for much of the year to June 1997, tended to reduce the number of extreme events. The change to El Nino conditions during 1997 is expected to increase meteorological hazards such as strong westerly winds and drought conditions on the east coast of the South Island. The combination of these two conditions will also increase the risk of fire in rural areas.