Integrating tsunami inundation modelling into risk-based land-use planning: an update of guidance

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Beban JG, Gunnell SN, Saunders WSA. 2019. Integrating tsunami inundation modelling into risk-based land-use planning: an update of guidance. Lower Hutt (NZ): GNS Science. 47 p. (GNS Science miscellaneous series; 132). doi: 10.21420/6MGN-4T72

In 2011, GNS Science produced guidance aimed at land-use planners and decision makers on how tsunami inundation modelling can be incorporated into land-use planning to reduce the risk posed by this hazard (Saunders et al. 2011). However, land-use planning for tsunami hazard remains an underutilised tool in New Zealand, with most modelling completed to date only being sufficient to produce tsunami evacuation zone maps. In order to encourage and support the uptake of land-use planning methods to manage tsunami hazard risk, this updated guidance presents two examples of how a risk-based approach to address tsunami hazard has been developed at a regional and also district council level. This updated guidance retains the format and content of the original document where appropriate. The basics of tsunami are again provided, along with identification of the key pieces of legislation governing natural hazard risk in New Zealand, and how they can contribute to managing tsunami risk. The opportunity is taken to update the guidance to reflect recent changes to legislation in New Zealand and emphasise the latest approaches to the risk assessment process. Some of the key factors that need to be decided when considering tsunami inundation modelling are also discussed. The main message of this guidance is that the requirements of tsunami modelling need to be discussed and agreed with those undertaking the modelling before it is commissioned. This is to ensure that the scenarios modelled and the level of uncertainty in the results are suitable for the intended end-use, otherwise poor outcomes will arise. For example, tsunami inundation modelling to define evacuation zones in New Zealand is generally based upon the worst-case scenario (low probability but high potential consequences), as the purpose is to promote life safety. There is a high degree of conservatism in the results, as the modelling does not account for the full range of possible tsunami generating scenarios or tsunami wave behaviours. As such, this type of modelling should not be applied for land-use planning purposes to restrict private development rights. Guidance on tsunami modelling levels for evacuation purposes is available from the Ministry of Civil Defence and Emergency Management (MCDEM 2016), and this guidance continues to recommend that the modelling levels for land-use planning are based on the same approach. In general, Level 1 modelling is not recommended for use in New Zealand; Level 2 modelling is recommended primarily for emergency management readiness and inclusion into LIMs, with limited use for land-use planning; and Levels 3 and 4 are regarded as suitable for informing land-use planning decision-making. To assist decision-makers with incorporating tsunami inundation modelling into land-use planning processes an updated decision tree is presented. Regulatory and non-regulatory options for including tsunami hazard risk into land-use planning are outlined, including avoiding new development or intensification in high risk areas, mitigating the risk with tsunami evacuation structures, and ensuring that development facilitates evacuations routes. A risk-based approach to managing tsunami risk should be taken, which is consistent with recent amendments to the Resource Management Act 1991 that require consideration of both the likelihood of a natural hazard event and the consequences to land and buildings when assessing if risk is significant when the subdivision of land is being proposed. A risk based approach can be supported by precautionary approaches where uncertainty is high and must include the use of participatory approaches to ensure the outcomes reflect the expectations and risk tolerance of the affected community. An adaptive risk-based approach based on the work of Saunders et al. (2013) is outlined, which involves determining severity of consequences of an event; evaluating the likelihood of an event occurring relevant to the consequences; then determining the resource consent activity status based on quantified levels of risk. Resource consent activity status becomes more restrictive as the potential consequences increase. Two case studies are presented demonstrating how this framework has been adapted for use in the Bay of Plenty Regional Policy Statement, where the use of an Annual Individual Fatality Risk (AIFR) metric has been introduced, and the draft Porirua District Plan that proposes to manage tsunami risk based on the sensitivity of activities. (auth)