Gisborne District Council (GDC) is interested in taking a risk-based approach to managing its natural hazards. GNS Science has produced results from its national probabilistic tsunami hazard model (Power, 2013), which estimates the size of the tsunami at the coast for specified probabilities. This information has been used here as the basis for developing a probabilistic understanding of the tsunami hazard inland in Gisborne City and Wainui.
The Tsunami Land-Use & Evacuation Planning Workshop was held in Gisborne in October, 2014 to consider the implications of the GNS Science Review of Tsunami Hazard in New Zealand and to determine best practice to address inconsistencies in the application of tsunami science for evacuation and land use planning. One of the issues identified in this workshop was the modelling level requirements for different purposes (e.g., evacuation, land use planning) and circumstances (e.g., remote coastlines or high-density urban populations). The subsequently revised Director’s Guidelines for Tsunami Evacuation Zones recommended the use of “Level 3 or 4” probabilistic mapping to provide results with sufficient accuracy for land use planning purposes. In this report GNS Science presents results from a “Level 3” tsunami inundation study conducted for Gisborne City and Wainui Beach. The “Level 3” approach consists of identifying tsunami scenarios consistent with the tsunami hazard on specified timeframes, modelling of these scenarios, and combining the results into a map of tsunami hazard. From this maps of expected inundation flow depths were produced at Average Recurrence Intervals (ARIs) of 100, 500, 1000, and 2500 years. A further analysis was conducted, using only the scenarios arising from local tsunami sources. These were combined with models of population fragility to assess the level of tsunami risk in terms of annual fatality probability assuming no mitigation.
