Power, W.L.; Henderson, A.; Lukovic, B.; Heron, D.W.; Wang, X. 2023 Agent-based tsunami evacuation modelling of the Kapiti coast. Lower Hutt, NZ: GNS Science. GNS Science report 2023/34. 29 p.; doi: 10.21420/229J-3K15
Abstract
The coasts of the Greater Wellington region are at risk from tsunami, which in some cases could arrive within minutes of a large earthquake. The primary means for saving lives is to educate the public to self-evacuate from tsunami-prone areas if they feel a long or strong earthquake: ‘Long or strong, get gone!’. In densely populated areas Civil Defence and Emergency Management (CDEM) encourage people to evacuate on foot if they can. Mass evacuations can pose many problems in urban areas, as the urban infrastructure is typically not designed to handle such large volumes of pedestrians. In rural areas, these problems can be identified using ‘tsunami evacuation hikoi’ exercises, where the community evacuates as if a large earthquake has occurred. This is not practical for large urban areas with tens of thousands of residents and workers. Instead, we have used a computer model of people evacuating on foot to try to identify some of the problems that are likely to occur and their potential solutions. In this report, we describe the application of this model to the Kapiti coast. Though the tsunami hazard to the Kapiti coast is relatively low in comparison to other regions of Greater Wellington, the challenges of evacuation are accentuated by the relatively flat terrain and absence of large hills close to the coast, which can lead to significant distances needing to be walked to exit the tsunami evacuation zone. In some locations a direct path inland is not available which further adds to the distance and time required to evacuate. Our modelling suggests that the population densities in most parts of the Kapiti coast are sufficiently low to avoid major congestion problems for people evacuating on foot (though this could be more challenging if some routes were to be blocked, for example due to fallen debris). It is instead those areas that are furthest in terms of walking distance that face the greatest challenge in achieving a timely evacuation to safety. Tsunami caused by earthquakes on the southern part of the Hikurangi subduction zone may occur on average once in approximately 500 years. In most studies such a tsunami typically takes around 40 minutes to reach the Kapiti coast. Approximately the same timing is true of tsunami caused by earthquakes on other faults in the Cook Strait region. There are also some less-probable events, involving earthquakes on offshore Kapiti faults or Hikurangi earthquakes that go unusually deep, which could cause tsunami that reach the Kapiti coast more quickly; to mitigate the risks from these events, a prompt start to self-evacuation is important. The evacuation modelling makes several assumptions, notably that everyone evacuates on foot and that evacuation routes are in their normal condition and therefore progress is not additionally impeded by e.g. debris, abandoned vehicles or liquefaction. With these assumptions and the limitations of using a simplified model for a complicated situation, our focus is primarily on drawing qualitative conclusions regarding areas of congestion and the locations with the greatest distance to travel. Bearing these limitations in mind, we find that for the southern Kapiti coast, from Raumati South to Waikanae Beach North, that around 98% of agents (simulated people) in the evacuation zones are able to reach safety within 40 minutes in either day or night time if we assume that everyone self-evacuates and starts evacuating within approximately five minutes of the end of strong shaking. Using instead assumptions based on responses to a recent survey, the equivalent number reaching safety drops to under 60%, largely due to those who would not self-evacuate following a strong earthquake according to the survey responses. For the northern Kapiti coast, from Peka to Otaki Beach, we find instead that around 90% reach safety within 40 minutes under the assumptions of complete evacuation within approximately five minutes, falling to around 45% under the survey assumptions, in both day and night time. For some at-risk communities, it may be possible to reduce the evacuation time by enabling shorter, more direct, routes inland than those currently available. In the most rural parts of the Kapiti coast, it may also be useful to discuss the potential for the use of cars to evacuate, even among the able-bodied, where evacuation distances are large and the risk of traffic congestion is low. (auth)
