16.2 Natural Hazards affecting Northland

The types and extent of natural hazards that could potentially affect Northland has not changed in the last five years. A summary of the main natural hazards that could affect Northland is included below. For more detailed information refer to the natural hazards chapter of the 2002 State of the Environment (SOE) Report (NRC 2002).

Since the 2002 SOE report there has been a review of the information available on natural hazards for Northland. From this review three reports on coastal, weather and natural hazards have been produced.

This review found that Northland's level of exposure to natural hazards is relatively low, except for flooding and climatic events related to ex-tropical cyclones and severe convection storms (Gray 2003). Landslides are the most significant of the other potential natural hazards identified. Others include volcanic, earthquake, tsunami and mine subsidence hazards (Beetham et al. 2004).

Weather hazards

Ex-tropical cyclones have the potential to cause damage through extreme winds and heavy rain such as in cyclones Bola and Fergus (Gray 2003). Northland has, on average, one ex-tropical cyclone pass nearby every year, putting it more at risk from ex-tropical storms than the rest of New Zealand.

Severe convection storms tend to cause localised damage but they can catch communities unaware as the events are often not well forecast (Gray 2003). They can also be widespread, causing regional flooding such as what occurred on 27 March 2003 and twice in 2007.

The main risks associated with weather hazards are flooding from intense rainfall, which is covered in the surface water quantity chapter, and storm surges (coastal inundation), which is covered in the coastal hazards chapter.

Flooding

Flooding is the most common risk to Northland, threatening human life, disrupting communications and access, damaging property and reducing primary production. There is detailed information on flooding and the environmental effects of flooding in the 2002 SOE report.

Northland was hit with two significant storm events in 2007, causing severe flooding throughout the region. The first was in March and the second was in July. For more information on these floods refer to Case Study 1: Flood event 29 March 2007 in the surface water quantity chapter or the detailed NIWA and MetService reports.

Coastal hazards

A tsunami is a natural phenomenon that results when a large volume of water is displaced causing a series of waves to be generated, most commonly due to earthquakes causing sea floor displacement. Tsunamis can devastate coastal communities causing inundation, strong currents, contamination and other effects.

The post-historic record shows that New Zealand has been affected by more than 40 tsunami in the last 150 years. Four moderate tsunami inundation events have impacted on Northland's east coast in the last 150 years. The prehistoric record indicates at least one large event, or a series of large closely-spaced events, have affected Northland's coast in the last 600 years, plus others.

Tsunami hazard is considered a significant risk for Northland, especially among coastal communities. A generalised tsunami hazard-risk model for Northland indicates that a moderate hazard and risk exists for most of the northwest and east coast, a high hazard and moderate risk for the north, and a low hazard and risk for the west. The hazard is largely a function of tsunami source, intensity and return period. For more information, check out the coastal hazards chapter of this report.

Fire

Wildfires are generally seen as a greater threat in other parts of New Zealand but they are a growing problem in Northland. From June 2001 to June 2006 the number of wildfires attended by Northland's Fire Brigades and Rural Fire Forces increased from 265 to 411, a 55% increase. There are a number of possible factors contributing to this increase, including a growing population along with supporting buildings and infrastructure, an increase in malicious fire starts and drier weather patterns.

The east coast of Northland from North Cape to Mangawhai is particularly vulnerable due to the predominant vegetation types, larger and denser population and the traditionally lower rainfall. Also of particular significance are the Pouto, Aupouri and Karikari Peninsulas with large amounts of flammable vegetation – such as mänuka, gorse and exotic pine forests – lower rainfall and growing populations.

Land instability

Landslides can be a threat to life and property, with one fatality in Dargaville in 1998 and significant damage to property every year. In Northland the dominant trigger is intense or prolonged rainfall which initiates many landslides annually (Beetham et al. 2004). Earthquake-induced landslides represent less than 1% of the total landslide damage occurring in Northland.

There have been 17 significant (newsworthy) landslides in Northland between 1996 and 2003 (Beetham et al. 2004). These have resulted in one death, evacuation of houses and hundreds of thousands of dollars damage. There are four main types of landslide hazards in Northland: Debris avalanche, earth flows, greywacke slips and unstable mudstone. For more information on land instability, check out the land and soils chapter.

Volcanic

There are two areas of past local volcanic activity in Northland, one the Puhipuhi- Whangarei area and the other the Kaikohe-Bay of Islands area. Eruption centres of the Puhipuhi-Whangarei volcanic field are spread across the Whangarei District. The most recent eruptions in this field occurred more than 250,000 years ago and there is some doubt as to whether or not it should be considered active. The Kaikohe-Bay of Islands field may have been re-activated in late Quaternary times (up to 700,000 years ago) and could be considered as still active. An eruption from either of these Northland volcanic fields is likely to produce lava flows, scoria and ash fall.

The other volcanic hazard for Northland to consider is the effects of ash fall from a distant volcano, with the main threat being renewed activity within the Taupo Volcanic Zone and/or Taranaki volcano. There are several impacts from ash fall even when it is less than 1 mm thick such as irritation to lungs and eyes, visibility hazard (leading to airport closures) and possible contamination of water supplies.

Mine subsidence

Mine subsidence is included as a natural hazard event though it is clearly caused by human activity. Coal has been mined at several places in Northland Region, mainly at Kamo, Kiripaka, Hikurangi, Kawakawa and Avoca (near Dargaville) coalfields. The risk of surface subsidence due to historical underground coal mining is recognised and well studied at Kamo and Hikurangi but not at the other areas. The most significant risk is the sudden collapse of an old mine shaft. There is evidence of such collapsing near Kawakawa.

Earthquakes (Seismic hazard)

Earthquake risk in Northland is low with no active faults mapped and generally regarded as tectonically stable (Beetham et al. 2004). There have been about 12 earthquakes in the Northland region over the last 100 years, all of a Richter magnitude of less than five. There is a proven risk of small earthquakes that have caused slight damage in Northland. However the risk is lower than the rest of New Zealand (Beetham et al. 2004).

There is an estimated mean return period of 1000 years for an earthquake of six on the Modified Mercalli (MM) scale of intensities and 7000 years for an earthquake of seven on the MM scale in Whangarei, compared with nine and 42 years respectively for Wellington (Beetham et al. 2004). Intensities of six or more are those which may cause damage to some buildings.