Natural Hazards Update - No.7 2004

Storm story

February 2004 will live long in the memory of many New Zealanders as a result of a series of intense weather systems that passed over the country. While the systems that passed on 14–16 February caused widespread damage, they were not the only weather events to cause concern. In total, 47 severe weather warnings were issued, many covering a large part of the country. On 27 and 28 February the only areas of the country not covered by severe weather warnings were Southland, Otago, and Canterbury, though they had had their turns as well. Indeed, there were only 8 days in which no severe weather warnings were issued. February is meant to be a summer month! This is a reminder that our weather can present hazards at anytime of the year.

The weather during February gave heavy rainfall that led to flooding, storm force winds, hail, and lightning. Emergencies were declared in Taranaki, Manawatu, and Marlborough, and response action was initiated in 11 districts.

  • 14–16 February
    • Deep low develops east of N.Z.
    • Heavy rainfall and flooding (over 200 mm in many places)
    • Storm force southeasterly winds (over 120 km/h)
  • 17 February
    • Thunderstorms lead to flooding in Picton
  • 21 February
    • Strong northwesterly winds ahead of front (over 160 km/h)
  • 28 February
    • Depression passes across the North Island from the Tasman Sea
    • Heavy rainfall and flooding at Turangi

This list represents only the most major events. There were other events of note; for example, the Waikanae River nearly over-topped its banks on 12 February.

14–15 February event

The rainfall led to stopbanks being overtopped, bridges damaged, roads closed, gas lines ruptured, power cut off, water reticulation systems broken, and an estimated 2500 people displaced. The gale force winds led to many trees being toppled, including many that were mature, and power lines blown down. One of the significant features was the large amount of debris carried by the rivers, leading to blocking of flow under bridges and culverts.

All up, the economic impact was estimated to be close to $300 million, but the social impacts will continue to be felt for some time.

Helping communities cope

Practical advice and support will be more useful to flood victims in the next few months than counselling, says Associate Professor of psychology Kevin Ronan of Massey University. Dr Ronan says people have natural coping systems and, despite extreme stress and loss, are able to cope and eventually heal. In the immediate aftermath of a traumatic event, psychological issues come second to physical needs.

‘Most people recover naturally, but many need to be reassured that they will do so, and part of this process is to try to maintain regular activities, to stick with their families and community, and, of course, try to look after yourself, to eat well.’

Citing the aftermath of 11 September in the United States, Dr Ronan says the number of people diagnosed with post-traumatic stress disorder was less than expected. Most recovered from symptoms over the following four months. Research on the effectiveness of victim support and early counselling showed mixed results. Dr Ronan says the natural coping process means people may be reluctant to accept assistance from health and counselling professionals, and professionals should be careful not to ‘help too much’. People cope and help themselves by acknowledging and expressing grief and anger, through the re-establishment of routines and community contact, and by helping their families and friends. Dr Ronan has been working with GNS on their social research programme for several years.

To assist communities, clinical psychologist Dr Rob Gordon from Melbourne, who specialises in working with communities affected by disasters, visited the lower North Island and Marlborough in early March. Dr Gordon was brought to New Zealand through a joint initiative by Child, Youth and Family, Ministry of Social Development, and the Community Employment Group, with support from the Ministry of Civil Defence & Emergency Management. Dr Gordon talked to communities, local authorities, government agencies, non-governmental organisations, and volunteers working directly with people affected by the floods during his five day visit. He discussed social processes that can alert agencies and community members to issues arising at different stages of recovery.

Over the next year research will continue to assess the medium and longer term community impacts of the flooding to help agencies that have responsibility for planning for and responding to such events.

Contact: d.johnston@gns.cri.nz

Natural hazards in summer 2003-04

Southland floods

The February without a summer

The storms that lashed the lower North Island from 14 to 28 February were the worst in several decades. They caused the most devastating floods in 100 years in some areas, leaving many families homeless and farms and roads destroyed by flooding and landslides. By the end of February the damage bill was estimated to be close to $300 million (roads $65 million; farms $159–189 million; private claims about $100 million – The Dominion Post).

More than four to six times the usual rainfall for February was recorded in Wanganui (239 mm cf 40 mm normal), Feilding (334 cf 57 mm), Palmerston North (299 cf 60 mm), Paraparaumu (361 cf 60 mm), Upper Hutt (403 cf 64 mm), Lower Hutt (482 cf 68 mm), and Wellington (364 cf 62 mm). Much higher rainfall was recorded in the Tararua (1288 cf 300 mm) and Ruahine Ranges (621 cf 217 mm). Judging by the severe flooding in the lower Wanganui, Whangaehu, Turakina, and Rangitikei valleys, similarly high rainfall fell in the steep hill country northeast of Wanganui as far as Raetihi and Taihape. A deep and complex depression located to the east of the North Island during the 14th, to 16th, caused much of the damage, bringing very heavy and persistent rainfall and gale force southeasterly winds.

During the event of the 14–16th, heavy rain on both sides of the ranges led to the Manawatu River overtopping its banks. The peak flow of the Manawatu River was the second largest on record, the largest being recorded in 1902. The peak of the Rangitikei River was the third largest on record since 1897 (Ministry of Civil and Emergency Management).

In the Wanganui–Manawatu area, 4 bridges were destroyed, and 21 seriously damaged, several showing signs that masses of trees and other debris, mainly from river bank collapses, lodged against piers had contributed to their failure. Major highways were blocked, the Manawatu Gorge road was closed until the end of April, and 2500 people were displaced.

Along with the mayhem caused by the rain and floods, significant damage was done by the strong winds that occurred during the 14–16th, event, and again on the 19th, and the 21st, . Again, the 14–16th, event caused the most concern, with 230 km/h gusts recorded in the Tararua Ranges, 163 km/h at Castlepoint, 161 km/h at The Brothers, and 156 km/h at Mt Kau Kau. This was the most severe southerly storm since May 1992 in Wellington and the most severe for February since 1967. The winds generated 9–10 m swells in Cook Strait, cancelled ferry sailings, and felled trees and caused power cuts in the southern North Island.

The event on the 21st, of February brought gusts of 178 km/h at Mt Cook Village, 167 km/h at Mt Kau Kau, 163 km/h at Castlepoint, 146 km/h at Baring Head, 139 km/h at Paraparaumu Airport (the highest since November 1982), 137 km/h at Kelburn (the highest for February since at least 1971), and 135 km/h at The Brothers.

One of the notable impacts was the large number of landslides caused by the rainfall in the Wellington area, Wairarapa, and Manawatu and Wanganui hill country. Overall the landsliding in hilly areas north of Palmerston North was probably as severe as, but possibly more extensive than, that during Cyclone Bola in March 1988, with very extensive areas of coalescing shallow soil slides and flows in some areas. There was also good evidence in many areas of the effective slope protection that native bush and scrub and exotic trees offer to slopes in Tertiary mudstone (papa) hill country. As a rough initial estimate, 4000–6000 km2, of hill country northeast to northwest of Palmerston North had been moderately to severely affected by landslides, with the damage extending as far north as Taihape and Raetihi, and over to the Wanganui and Waitotara rivers.

The largest and most spectacular landslide in the greater Wellington area occurred during heavy rain on 17 February 2004. A large landslide (estimated volume 200 000–300 000 m3, ) on the old eroded Wellington fault line scarp dammed the Hutt River and diverted it through the Te Marua golf course, creating a new ‘water hazard’ through the fairway.

Work is now being undertaken to gather data and understand the causes and impacts of these events. Aerial photographs have been taken to back up the analysis of high-resolution satellite data on the extent and character of the landslides. Information gained from studies of landsliding caused by this flood event will help to evaluate and quantify landslide and sedimentation dangers for use in future economic and land use planning studies, and for flooding and landslide hazard assessments.

Preliminary conclusions on landsliding in the Manawatu–Wanganui area

  • The most severely damaged areas were in the Turakina, Whangaehu, and Mangawhero valleys, where thousands of smaller (10–1000 m3), shallow (1–2 m deep) soil and regolith slides and flows occurred, along with some larger (1000–100 000 m3) deep-seated landslides in Tertiary mudstone, some forming small dams in streams. The landslides that blocked the larger rivers were then moved by the floods.
  • Most of the landslides occurred on steeper (25–40°) pasture-covered hill slopes, gullies, and terrace edges. Gully and river bank failures, however, contributed considerable sediment and trees to flooded rivers, with the latter causing some bridge failures.
  • Landslide occurrence was clearly related to land use. Hill slopes covered with bush and exotic forest were much less affected by landsliding, with only a few isolated landslides observed compared with grassland areas. Forest cover clearly provided good protection against landsliding during this event. However, most recently milled forest areas were severely affected by landsliding.
  • Damage to farmland was extensive across the region, and many roads were severely damaged and closed by slips. SH3 in the Manawatu Gorge was closed for two months by several large landslides. Some slips came close to houses and buildings, but, as far as we are aware, none were significantly damaged.

Warren Gray (w.gray@niwa.co.nz)
Graham Hancox (g.hancox@gns.cri.nz)

Keynote speaker confirmed – Professor Dennis Mileti

The 6th Natural Hazards Management Conference in Tauranga on 10–11 August, has confirmed Professor Dennis Mileti as one of the keynote speakers. He is Professor and Chair of the Department of Sociology and Director Emeritus of the Natural Hazards Research and Applications Information Center at the University of Colorado at Boulder, USA.

Professor Mileti is the author of over 100 publications, most of which focus on the societal aspects of mitigation, preparedness, response, and recovery for hazards and disasters. His book, Disasters by Design, published in 1999, involved over 130 experts assessing knowledge, research, and policy needs for hazards in the USA. His address is entitled ‘Towards a resilient society: turning research into policy and programmes for sustainable hazard mitigation.’

For further information contact:
Dianne Tilyard (d.tilyard@gns.cri.nz)
or visit www.naturalhazards.net.nz/conferences

Manukau City Council – leading the way in hazard guidelines

Manukau City Council has undertaken a project to implement the Auckland Regional Council Hazard Guidelines in order to fulfil its obligations set out in the Auckland CDEM Plan, to meet other statutory obligations that relate to hazard management (e.g. the RMA), to improve their knowledge of hazards specific to Manukau, and to seek more efficient and effective ways of implementing hazard management practice into everyday activities.

Working closely with the Natural Hazards Centre, the project has identified current hazard treatment options and gaps, and sources of hazard information (both at the Council and externally). It has also looked at improving hazards information capture and quality assurance, and how to store and maintain hazards information. Recently the Council held a workshop with staff to identify information and training needs, and identify key external stakeholders. On 2 June 2004 the Council will run a workshop to prioritise hazards with the aim of focusing resources towards key hazards in the future.

For further information about this project, contact:
Janine Kerr, j.kerr@gns.cri.nz

Upcoming events

New short courses: www.naturalhazards.net.nz/courses