Planning timeframes for natural hazards

While the NZCPS provides some guidance on timeframes for assessing coastal hazards (i.e. at least 100 years under policies 24, 25 and 27) (Department of Conservation, 2010), there is no national regulatory standard approach for deciding what timeframes should be used for other natural hazards. This has led to an inconsistent approach being adopted throughout the country, and has the potential to lead to an increase in risks if appropriate planning timeframes for natural hazards are not included in planning processes.

Choosing the appropriate timeframe as the basis for land use planning is difficult for communities, planners, and politicians (who tend to focus on outcomes within political cycles, rather than long-term) alike (Deyle, et al., 1998; Ericksen, 2005). The decision of what return period [1] should be used often represents a value judgement that may be difficult to deal with in the political arena. At one end of the scale are hazards that produce modest levels of damage on a relatively frequent basis, generally with a recurrence interval of less than 20 years; at the other end are catastrophic events that recur less frequently, only once every 2,500 years or more, but produce devastating levels of damage and consequences (Deyle, et al., 1998; Ericksen, 2005). These high consequence, low-likelihood events are the most difficult hazards to manage (Slovic, Fischhoff, & Lichtenstein, 2000) due to a lack of understanding and awareness of the consequences of these events; and the ‘it won’t happen in my lifetime’ view. It is essential that any decisions on these types of events are made with community and scientific input, to ensure support and understanding of risks and consequences.

In the Environment Court case Save the Bay v Canterbury Regional Council (C6/2001), the Court considered that there needed to be a greater recognition of catastrophic natural events, stating that 90% of damage to the environment caused by natural hazards occurs in 10% or fewer of events. The Court suggested that “authorities should recognise this inverse relationship in the preparation and wording of their plans”. This case, and Bay of Plenty Regional Council v Western Bay of Plenty District Council [2002] A27/02 EnvC, and Skinner v Tauranga District Council [2002] A163/02 all provide discussion regarding the appropriate risk period to plan for when preparing regional and district planning documents. These cases point to a 100-year planning horizon for hazards such as coastal erosion and coastal flooding, but should not be a benchmark for other natural hazards.

Flooding, coastal erosion, landslide hazards and tsunami risk are likely to be influenced by transient end points (Health & Safety Executive, 2001; Johnston & Paton, 1998), instigated by climate change, which may change their risk profile. For example, it is imperative that coastal erosion time frames of 100 years are adjusted with time to incorporate climate change – what was a ‘100 year event’ in 1990 may be less than that in 2020. To ensure the 100 year return period remains at 100 years over time, the effects of climate change must be regularly monitored and incorporated into any planning timeframe. Tsunami risk is also impacted by climate change; whilst not the trigger of a tsunami, it has the potential to increase coastal erosion, which can erode previously stable beach dunes, allowing a tsunami to have a greater run-up and inundation (and impact) on land.

There is no consistent all-hazard return period/AEP for land use planners to use as a basis for planning for natural hazards events in New Zealand (see Saunders, 2010). While some perils ‘share’ return periods, not all are equal, in part due to forecasting and warning capabilities, as outlined in Table 3.4 and discussed below. For example, high rainfall events can be forecast, flood warnings can be given, and evacuation of communities at-risk is possible.

Table 1. Comparative land-use planning return periods for selected natural hazards in New Zealand

Planning timeframe (years) Case law Forecast possible Warnings available Map extents Affected by Climate change Likelihood Consequence
Flood 20-100 ~ Yes Yes Yes Yes Almost certain Minor/ Moderate
Coastal erosion 100 Yes Yes Yes Yes Yes Likely Minor/ Moderate
Active faults </= 20,000 No ~ No No Yes No Rare Moderate/Major
Tsunami (local and distal) </+ 2,500 Yes Yes (distal only) Yes (distal only, natural warning for local source) Yes Trigger not, but dune health is Possible Moderate/Major
Landslide </+ 2,500 No? Yes (in some circumstances) No Yes Yes Possible Moderate

For floods and coastal erosion, forecasts of impending weather are available via the MetService. Through tsunami modelling, a forecast of wave height can be provided for distant-source tsunami, although only natural warnings are available for local-sourced tsunami events. For other hazards such as earthquakes and some landslides, warnings are not possible due to the sudden onset of such events. Floods, coastal erosion, and tsunami inundation have the potential to be affected by climate change, due to increased severity of rainfall events, sea level rise, and associated impacts (e.g. decreased dune health).

For effective risk reduction, hazardous areas should be avoided, as even with warning and evacuation, property is still affected (with subsequent social and economic consequences). A balance needs to be reached between allowing a land use to proceed in an at-risk area; constructing buildings to withstand hazards; and having emergency management procedures in place when required. Once a land use has been permitted, and buildings have been constructed, the land use will carry on indefinitely beyond the 50 year default timeframe of buildings under the Building Act. Planning within a sustainability context, which implies planning for future generations, needs to extend beyond 50, and even 100 years. The influence of the Building Act is outlined in the following section.

4.1 The influence of the Building Act 2004 on timeframes under the RMA

Often there is reliance on timeframes under the Building Act for land-use planning, in particular the 50 year timeframe. Under the Building Act, buildings have a minimum intended lifetime of 50 years, and are constructed to withstand a 475 year return period earthquake (i.e. a 10% probability of occurrence in 50 years). Critical facilities are constructed to withstand a 2,500 year (2% chance of occurring in 50 years) earthquake event. Based on this approach, the timeframe of 50 years has become, in some districts, the default planning timeframe for flooding. However, if correctly used for flooding, the 475 year return period, with a 10% chance of occurring in 50 years, not a flat 50 year return period, should be used.

There is also a reliance on the Building Act to protect people’s health and safety, rather than land use provisions. Within RMA case law from the Environment Court (Petone Planning Action Group Incorporated v Hutt City Council, W020/2008), it is stated that:

“… the performance of the structure and the safety of people in earthquake events, is to be left to compliance with the Building Code and Standard … risks to safety from earthquake shaking, liquefaction and tsunami would be appropriately addressed and mitigated in the Building Code process and assessment in accordance with NZS1170.5:2004” (New Zealand Standard Structural Design Actions Part 5: Earthquake Actions).

The decision was summarised as follows:

… we conclude that the consenting to the proposal on condition of compliance with the Building Code and NZS1170.5:2004 would enable people to provide for their safety against risks from earthquakes and other natural hazards.

However, NZS1170.5:2004 only considers earthquake, not other natural hazards such as tsunami, landslide, or flood, leading to the conclusion that the Environment Court was questionable in its judgement that other natural hazards are provided for in this standard, and consequently peoples’ health and safety is not provided for. Under the Building Act, only the consideration of other hazards is required. The implication of this is that planners should adhere to the purpose of s5 of the RMA and provide for people’s health and safety. It is recommended that this includes planning beyond the default 50 year planning horizon of the Building Act. In summary, reliance on the Building Act is too restrictive in its timeframes, and does not allow for consideration of consequences beyond a 50 year timeframe for buildings (excluding critical facilities).

[1] For a discussion on terminology and definitions on the terms return period, annual exceedance probability (AEP), probability of occurrence and likelihood, see Saunders (2010).


Department of Conservation. (2010). New Zealand Coastal Policy Statement. Wellington: Department of Conservation.

Deyle, R. E., French, S. P., Olshansky, R. B., & Paterson, R. G. (1998). Hazard assessment: the factual basis for planning and mitigation. In R. J. Burby (Ed.), Cooperating with nature: confronting natural hazards and land use planning for sustainable communities (pp. 119-166). Washington D.C.: Joseph Henry Press.

Ericksen, N. J. (2005). Hang-ups in flood hazard planning (Part I). Planning Quarterly(158), 24-28.

Health & Safety Executive. (2001). Reducing risks, protecting people: HSE's decision making process. Sudbury: HSE Books.

Johnston, D. M., & Paton, D. (1998). Social amplification of risk: transient end-points. Paper presented at the Risk assessment of environmental end points, University of Auckland.

Saunders, W. S. A. (2010, 20-23 April 2010). How long is your piece of string - are current planning timeframes for natural hazards long enough? Paper presented at the Planning pathways to the future, Christchurch.

Slovic, P., Fischhoff, B., & Lichtenstein, S. (2000). Cognitive processes and societal risk taking. In P. Slovic (Ed.), The perception of risk (pp. 32-50). London: Earthscan Publications.