Natural Hazards Update - No.2 2003
Working together for resilient communities
New Zealand faces significant, but varying, risks from an array of natural hazards, and increasingly from exposure to technological hazards. More effective ways to manage hazards through a comprehensive risk management approach are now seen worldwide as the best practice, requiring changes to civil defence legislation.
The Civil Defence Emergency Management (CDEM) Act 2002 came into force on 1 December 2002. It updates and redefines the duties, functions, and powers of central and local government, emergency services, lifeline utilities, and the general public.
The basis of the new act is understanding our hazards and their potential consequences as a starting point for planning, leading to measures to improve the four Rs – reduction, readiness, response, and recovery. This is different from the 1983 Civil Defence Act and will require some significant shifts in our thinking.
The CDEM Act 2002 improves and promotes:
- reduction of risks through partnerships with communities;
- reduction of community disruption from avoidable hazards and risks;
- reduction of risks from the costs of disruption;
- more effective and efficient emergency readiness, response, and recovery through the integrated activities of responsible agencies and relevant disciplines;
- the act promotes a culture with processes and structures that encourage and enable people and communities to undertake risk management, and to build operational capabilities for response and recovery from emergencies.
According to John Norton, Director, Ministry of Civil Defence & Emergency Management, there are already some excellent relationships being built between emergency management and science providers.
“There is excellent capability in New Zealand to understand our natural hazards. The challenge is to convert that into action. We need to better understand what we want and need from each other”, he said.
“The Natural Hazards Centre is an important development to build on this foundation. It will focus and promote the relationships which are necessary for agencies to work together more effectively to convert knowledge into action, so we can achieve our overall aim of resilient communities in New Zealand”.
For more information on the new act see: www.civildefence.govt.nz
Mt Adams landslide: lessons on preparing for large earthquakes
Gravel from the landslide dam changed the course of the Poerua River, inundating farmland and resulting in the collapse of this milking shed. (Photo taken in August 2001)
In October 1999 about 15 million cubic metres of rock debris fell from Mt. Adams, south of Harihari, Westland, into the gorge of the Poerua River, forming a 120-m high natural dam. The dam failed 6 days later, releasing 3–4 million cubic metres of water. Although the discharge peaked at about 3000 cumecs, there was surprisingly little immediate damage to farms.
Since 1999, sediment from the dam has progressively moved down the river on to farmland in the Poerua Valley. An alluvial fan has formed at the head of the valley, making one farm valueless and damaging others. Other farmers are building stopbanks to protect their land from flood inundation and the build-up of sediment deposits. These stopbanks constrain the riverbed, causing it to accumulate more sediment. This in turn helps the river carry sediment farther downstream in high flows, threatening the State Highway 6 bridge with severe sediment build-up. The sediment accumulation process is still not over.
Major earthquakes are likely to cause large landslides which might affect rivers in the same way. Such effects must be considered during planning for large earthquakes, and the reconstruction of infrastructure can cause serious difficulties by slowing the response time. In some instances an earthquake may present an opportunity to resite vulnerable infrastructure out of reach of landslide-induced river aggradation.
Natural hazards in spring 2002
Landslides |
Weather extremes |
Coastal hazards |
Floods and droughts |
Earthquakes and volcanic activity |
Volcanic fury vented in 1995
Mt. Ruapehu erupting
This quarter in history marks 7 years since the 23 September 1995 eruption of Mt. Ruapehu. Hundreds of skiers were on the mountain to witness a spectacular eruption from the Crater Lake at the mountain summit. One lahar narrowly missed a T-bar ski lift as it surged downhill. Blocks of rocks were blown as far as 1.5 km from the crater, and steam and volcanic ash rose 12 km into the sky.
Inundation
Wanaka Flood, 1999
Alexandra Flood, 1999. (The Southland Times)
The drama of heavy rainfall can make great viewing on TV, but the reality of water flowing through homes and businesses is tragic. When rivers swell beyond their banks or overtop their defences, the flow of water through the adjacent low-lying areas is called inundation. To evaluate risk to life and property, what we need to know for a flood is the potential depth of floodwater, not just the peak river flow rate of “so many” cumecs. Thus, of fundamental importance to flood prediction are the processes that influence the relationship between flow in flooded rivers and the flood depth on adjacent land. The processes that influence the level of floodwater include: river channel geometry; the physical roughness of the bed of the river and its floodplains (vegetated or inhabited); how much scouring of the riverbed, banks, and adjacent land occurs; and, conversely, where sediment or debris is deposited.
Vegetation and buildings impede the floodwater, and this resistance to the water flow controls the inundation depth. The roughness and shape of the river bottom, and the riverbank, also control the flow. The rougher the bottom is, the more the water banks up before it overcomes the resistance to flow.
Defining the geometry and heights of a riverbed and adjacent floodplains can be arduous and expensive. Conventionally, this has been done by small-boat and land-based survey techniques. Increasingly, we are turning to the skies, where aerial scanning methods such as airborne laser or aerial photogrammetry can cover much more ground in less time. The advantage of laser scanning is that it gives direct physical output and can also measure building heights and penetrate vegetation to measure the ground surface height.
Once the resistance can be quantified, the laws of fluid mechanics can be used in a computer model to predict flood depths.
NIWA is working on two levels of computer model – one where the adjacent floodplains are flooded “passively” as the river overtops its banks, and the other more complex approach where floodwaters around buildings and through breaches are dynamically simulated. The “dynamic” model is still under development, but is a promising tool for the future, particularly in the application to coastal inundation from waves, tsunami, and storm tides, as well as river floodplains.
We all take it for granted that stopbanks will protect us, but they need to be carefully designed. For example, the closer the stopbanks are sited on either side of a river, the higher the water level will be in a flood and the faster it will flow, and therefore the lower the level of sediment deposition in the channel. Conversely, when the stopbanks are farther apart, although the water level is lower, sedimentation will increase because the flow is slower.
However, stopbanks are not fail-safe. Often scenarios are needed to assess the risks from stopbank failure through overtopping, seepage, or structural failure. These studies show that care must be taken in building infrastructure on land nominally protected by stopbanks. For example, a stopbank may make an area 50% less likely to be inundated, but if the building of a stopbank encourages the development of the land to, say, 10 times its current value, then the financial risk (chance of inundation times the damage done) could easily be 5 times as much. The flooding of a paddock may be a problem, but the inundation of houses and businesses or the closure of roads may be catastrophic.
Once the potential inundation area for floodwater is known, the property and infrastructure at risk can be assessed. GNS are developing GIS-based software that overlays the locations of buildings, roads, and powerlines over topographical maps. The techniques that were developed to apply to inundation caused by tsunami or lahars are also applicable to river flooding.
One way for a community to be resilient to inundation is to have its people prepared for hazardous events. An area of combined research between NIWA and GNS is the assessment of hazard preparedness of at-risk communities. In the past GNS has looked at those at risk from volcanoes. More recently the combined work has surveyed the flood preparedness of Waikanae (see “Flood awareness in Waikanae”). This summer GNS and NIWA are combining resources to survey community perceptions of coastal hazards, including coastal inundation from tsunami, at several seaside communities.
Flood awareness in Waikanae
Sand-bagging at Waikanae flood. (Image courtesy of Flood Protection Department - Greater Wellington.)"
A combined NIWA/GNS study that looked at residents’ understanding of flood hazards in Waikanae highlights the complexities in improving community resilience to natural hazards. One thousand homes within and around the Waikanae floodplain zone were surveyed in 2001 on awareness of flood risk, previous exposure to, and preparedness for, a flood. Although the residents were aware of the flood hazard, they weren’t too concerned about the actual risk, and relied on local authority support in the event of a flood. The research provided insight into the relationship between risk awareness, degree of preparation, and flood management. We need to invest in science to improve our knowledge of hazards and develop tools for warnings, but ultimately communities need to be well prepared and resilient to hazards.
Wellington Regional Council’s Waikanae Floodplain Management Plan is at: www.wrc.govt.nz/fp/waikanae.htm
Volcano course to include other hazards
Mt. Ruapehu erupting
In October 2002 GNS ran its sixth annual Volcanoes & Society course at Wairakei, near Taupo. The course is part of a series developed by GNS to explore the relationships between the physical and social aspects of natural hazards and their management. It aims to present a state-of-the-art assessment of volcanic hazards in New Zealand and explore ways in which organisations can prepare for, and mitigate against, future volcanic crises. The Natural Hazards Centre is considering expanding the programme in 2003 to cover a broader range of natural hazards.
Communicating information on natural hazards
Regional councils
In October 2002, staff from NIWA and GNS, with support from the Hawke’s Bay Regional Council and Napier City Council, ran a workshop to improve the transfer of scientific knowledge to end-users working on natural hazards. The workshop was the second in a series of regional visits to be held over the next 12 months. The Natural Hazards Centre is commited to research and data being transferred effectively to end-users (e.g., planners, emergency managers, and asset managers) in a way that meets their needs. Each workshop will consist of a series of talks on GeoNet and hazards research by GNS and NIWA, with presentations from end-users identifying their research needs and issues.
Lifelines forum
GNS and NIWA staff from the Natural Hazards Centre presented the latest findings on natural hazards research to the annual National Lifelines Forum held in Napier on 12–13 November 2002. David Brunsdon, the National Lifelines Coordinator reiterated that the focus of Lifelines Project groups around the country is to identify the weakest links in utility networks (power, telecommunications, sewerage, roads, water, etc.) from a regional perspective and address them in a collaborative way.
Forecasting coastal flooding
Mt. Ruapehu erupting
Coastal communities are at risk from flooding, either from the sea or from rivers. A major contributor to coastal flooding around New Zealand is the high tide, which can be forecast well in advance. Most rivers and estuaries have protection against inundation from the tide itself, but if a storm surge or river flood occurs at the same time as a large tide, serious flooding of low-lying coastal areas can occur. Forecasts of “Red-Alert” days during the year when the potential for coastal flooding is greatest are posted by NIWA at www.niwa.co.nz/rc/hazards/dates. Very high tides, called perigean-spring tides, occur a few days after a full or new moon, when the moon is closest to the earth (in its perigee).
A major river flood or storm surge can cause coastal flooding at any time, regardless of the tide. Keeping an eye on weather systems (e.g., low pressure systems or high winds or waves) during Red-Alert dates can provide early warning of coastal flooding or wave-overtopping in low lying areas. Tide forecasts out to 2006 around New Zealand can be calculated by using NIWA’s new Tide Forecaster (www.niwa.co.nz/services/tides/).