Interim tsunami evacuation planning zone boundary mapping for the Wellington and Horizons regions defined by a GIS-calculated attenuation rule

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Leonard, G.S.; Power, W.L.; Lukovic, B.; Smith, W.D.; Langridge, R.M.; Johnston, D.M.; Downes, G.L. 2009 Interim tsunami evacuation planning zone boundary mapping for the Wellington and Horizons regions defined by a GIS-calculated attenuation rule. Lower Hutt, N.Z.: GNS Science. GNS Science report 2008/30 18 p.

Abstract: Wellington and Horizons regions have engaged GNS Science to provide indicative evacuation zones. The selected method for defining evacuation zone boundaries is an attenuated elevation-distance relationship calculated in ArcGIS Workstation as an AML script (detailed in Section 4). The primary input datasets are a digital elevation model, river lines, river polygons and coastline. There are limitations to this method and local checking of the accuracy of outputted zones is needed. Over time it is recommended that zones are revised as the science is improved. A more robust simulation model could in future be used to calculate the above. Further in the future it may be possible to draw an envelope around all inundations from multiple/many well-tested computer models – this is the ideal method. The probabilistic wave height with a 500 year return period, from regional and distant sources (>1hr travel time away), is used to define the orange evacuation zone. This is the zone which we may reasonably expect to provide official warning for now or in the foreseeable future. The probabilistic wave height with a 2500 year return period (i.e. maximum credible event) from all sources is used to define the yellow evacuation zone. This zone must encompass all credible tsunami, including those for which there will only be enough time for natural or informal warning. Zones are capped at 35 m above mean sea level along the coast because inundations above this elevation are extremely rare from subduction zone sources, and to take this remote possibility into account would cause over-evacuation in the vast majority of situations. The 84th percentile wave height is used from the probabilistic model to allow a margin of safety. The off-shore near-shore modelled height was doubled to define the evacuation zone, because run-up can be up to double the arriving wave height through momentum. The orange zone height is rounded to the nearest draft GeoNet ‘threat level’ boundary, to aid pre-planning for calling official evacuations. The GIS model allows for attenuation by reducing the maximum potential run-up by 1.0 m every 200 m as the tsunami travels inland, 1m every 400m up significant rivers and 1.0 m every 50 m away from rivers. It has been tested against limited available data from real tsunami and against other models. (auth)