Natural Hazards Update - No.6 2004

The economics of resilience

In partnership with economists from the New Zealand Institute of Economic Research (NZIER) and consulting firm Future Impacts, GNS and NIWA are extending their hazards research to determine the resilience of the New Zealand economy to natural hazard events. The centrepiece of the new research will be a computable general equilibrium (CGE) model of the New Zealand economy, which will be an extension of models previously developed to examine issues of national and regional economic importance. CGE models are commonly used to assess the full economic impact of some ‘shock’ to an economy, whether that shock is a policy change, a change of New Zealand‘s position in international markets, or a natural hazard event. This model will provide the basis for assessing, in practical measures (e.g., GDP, numbers employed, price levels), the economic impact of natural hazard events.

New research is focused on direct and indirect economic costs so that we can better understand and model the scale of losses and the change in the flow of goods and services within the economy. As well as quantifying the magnitude of loss attributable to a natural hazard event, the CGE model output will also determine the recovery path, and hence the recovery time, of the economy following a natural hazard event. A current project due out in mid 2004 is an assessment of the economic impacts of the 2002 ‘weather bomb’. Preliminary results show that the event had little effect on the regional economy, but large losses were experienced by a few individuals in the affected communities.

Mark Walton (MarkW@nzier.org.nz)

The vulnerability of water supplies to volcanic ash

Over the past few years a number of studies have evaluated the volcanic risk to Auckland’s infrastructure. GNS, Massey University, and the University of Canterbury are working with Auckland’s Watercare Services and Metrowater on a new study to determine the potential for physical and chemical contamination of the city’s water supply from volcanic ashfalls. This work, commissioned by the Auckland Engineering Lifelines Group, aims to develop appropriate mitigation options and public information messages to manage supply before, during, and after an ashfall.

Experience worldwide has shown that volcanic ashfall can cause physical and chemical changes in natural surface waters and water supplies. The extent of these changes depends on the composition and thickness of the ashfall, and the surface area and volume of the receiving water body. The most commonly reported problem is an increase in turbidity due to the suspension of ash in water. Turbidity is not a health problem in itself, but it affects the appearance of the water, making it unacceptable for water supply, and, more importantly, can inhibit disinfection of pathogenic micro-organisms. In addition, freshly fallen ash releases readily soluble components into water that it comes into contact with, such as rain or surface water. After the initial washing, prolonged exposure of the ash to weathering may cause slow release of elements from within the structure of constituent minerals, or in solid solution in glass. This can affect the water chemistry.

Indirect impacts may include ash causing physical damage to filters at intake structures and treatment plants, increased wearing of plant, interference with electrical equipment, and increased water demand for cleaning up after the ashfall.

The study is due for completion in mid 2004.

David Johnston (d.johnston@gns.cri.nz)

Natural hazards in winter 2003

Landslides	Coastal hazards	Weather extremes
Weather extremes - Rainfall	Floods and droughts	Earthquakes and volcanic activity

Data sources for these maps: GeoNet, NIWA, MetService, Regional Councils

This quarter in history ...

Awatere earthquake

On 16 October 1848, at 1:40 a.m., settlers in the upper South Island and lower North Island experienced a major earthquake, probably magnitude 7.5. A major fault running up the Awatere Valley ruptured over a length of 105 kilometres, with land moving up to 8 metres horizontally. Aftershocks continued for at least 10 months.

Damage in Wellington was severe, with nearly 80 buildings seriously damaged. A man and his two children were killed by a falling brick building during an aftershock. Wooden buildings survived well, but so many brick buildings collapsed that they were later rebuilt in wood.

Spotlight on ...

Landslides, gully erosion, and debris floods

Landslides, gully erosion, and debris flood effects during the 3 October 2003 Paekakariki floods: summary of a report prepared for Kapiti Coast District Council by Graham Hancox

The heavy rain on Friday, 3 October 2003 affected many parts of the Wellington region, the Kapiti coast being one of the worst affected areas. Although it rained for much of the day, it was the rain from several small intense raincells that fell on the Paekakariki area during the evening that caused severe flooding and extensive landsliding, gully erosion, and debris flood damage (Figure 1). Totals were over 100 mm in 24 hours. The area of greatest ground damage coincided with the area of maximum rainfall accumulation (Figure 2), a narrow band about 10 km wide extending from just south of Kapiti Island directly across Paekakariki towards Upper Hutt. NIWA rainfall data suggest that the average return interval of rainfall in the most damaged area (over 82 mm in 4 hours) is over 125 years.

The areas most affected by landslides and debris floods were around the junction of Paekakariki Hill Road with State Highway 1, and across the hills about 2 km south of Paekakariki, especially in the Fly-by-Wire gully above the BP Service Station and Belvedere Motel and the Hill Road gully 700 m to the south. About 3000 m³ of gravel from the flooded Fly-by-Wire gully was deposited around the Belvedere Motel buildings and across SH1 at the bottom of Paekakariki Hill Road. All of the flood water and gravel was passed through the culvert under the hill road, which was not affected at this site. The lack of damage to buildings suggests the gravel was deposited gradually as a debris flood over several hours, not as a single debris flow. The buildings on the east side of SH1 at the bottom of Paekakariki Hill Road are built on an old debris fan formed at the gully exit. This makes it a potentially dangerous site for future flooding, debris floods, and possibly the more damaging debris flows.

Geomorphic evidence suggests that locally the flood was a very rare event with a return period of more than 100 years, possibly several hundred years. Much gravel was deposited in the stream channel and will continue to be transported downstream by future floods. This problem could be minimised by erecting steel catch fences in the upper reaches of the stream or by constructing a stream bypass with a debris fence and accumulation area at the culvert entrance just above the motel site.

Damage to the northern end of Paekakariki Hill Road was the result of flood water and gravel deposition as a debris flood, rather than debris flow. Damage to road cuts on the hill road was generally minor, with only a few small rock and soil falls observed north of the summit. Gravel trapped behind fences and gates above the road suggests that this material accumulated slowly. Most of the silt and sand was carried away in flood water, which flowed over the road and washed out road edge fills and severely undermined the road in several places. This mechanism suggests that steel catch fences above the road and culvert entrances could be used to trap gravel during floods, while allowing water to pass through culverts. The control of debris in the head of gullies would potentially reduce gravel inundation problems for Paekakariki Hill Road, and for SH1 and the main trunk railway line.

Hancox, G.T. (2003). Preliminary report on landslides, gully erosion, and debris flood effects in the Paekakariki area as a result of the 3 October 2003 flood. Institute of Geological & Nuclear Sciences Client Report 2003/120, 19 p. Report prepared for, and made available courtesy of, the Kapiti Coast District Council.

Recommendations from the International Workshop “Tsunamis in the South Pacific”, Wellington, September 2003

This Workshop, jointly convened by the IUGG Tsunami Commission and the International Coordination Group of the Tsunami Warning System for the Pacific (ICG/ITSU), the Institute of Geological & Nuclear Sciences (GNS), and the National Institute of Water & Atmospheric Research (NIWA), produced the following recommendations.

1. Historical data

There is a wealth of historical data on Pacific tsunamis in many government, institutional, and historical archives scattered in numerous publications and not readily available. The workshop recommends continuing efforts to search for historical data on regional and local levels, and to encourage their integration into the Pacific-wide historical database.

Geological information on paleotsunamis can provide important estimates of the long-term tsunami risk for coastal locations where the historical record is short. Systematic paleotsunami research in these areas should be encouraged and supported.

Systematic searches of recent sea level records for tsunami signals are further recommended to improve our knowledge of the areas at risk. These data should be saved and copies sent to the International Tsunami Information Center (ITIC) for archiving.

2. Regional tsunami warning system for South-West Pacific and Indian Oceans

As Indonesia has recently decided to create a national tsunami warning system, the Workshop recommends that Indonesia and other countries of the South-West Pacific proceed in the development of the Regional Tsunami Warning System for the SW Pacific and Indian Oceans with reference to the ITSU Master Plan for Further Development of the Pacific Tsunami Warning System. This will benefit the countries of the region and also enhance the ability of the Pacific Tsunami Warning Center (PTWC) to locate smaller earthquakes in this area and to assess their tsunami threat.

3. Deepwater tsunami observations

The Workshop recommends that every opportunity should be used to further expand the network of deepwater instruments and to integrate the data into the operations of the PTWC. The development of methods for interpreting these data should be encouraged.

4. Availability of sea level data for operational warning

Given the recent considerable improvements in the sea level networks in New Zealand, the Workshop recommends increasing the use, through the Internet, of the real time or near-real time sea level data from New Zealand stations and others in the South Pacific by PTWC and others in the scientific community.

5. Report on the panel discussion

The panel discussion on the responses of warning centres and emergency agencies to the issuance of tsunami warning messages for a major M8.7 earthquake off the northern Chile coast was very useful and productive. The Workshop recommends that the records of this discussion be converted into a technical report by GNS and NIWA, in collaboration with ITIC.

6. Far-Field Tsunami Workshop in 2004

Pacific-wide tsunami are rare in Tsunami Warning System operations. However, the risk of failure of prediction and the cost of false alarm is very high due to the many countries and territories put into a warning status. The Workshop recommends a special ITSU workshop to discuss issues related to Pacific-wide TWS operations in 2004, possibly in October in conjunction with the VIIIth Earth Science Congress in Santiago, Chile.

7. Public education and awareness

Workshop participants recommend developing and maintaining archives of tsunami public education and emergency management products for sharing and international consultation, and that ITIC be consulted to facilitate an international archive of these products.

For further information on tsunami research in New Zealand, contact:
Gaye Downes (g.downes@gns.cri.nz) or
Roy Walters (r.walters@niwa.co.nz)