Power WL, Lukovic B, Wang X, Heron DW. 2022. Agent-based tsunami evacuation modelling of Porirua and the Wellington south coast. Lower Hutt (NZ): GNS Science. 35 p. (GNS Science report; 2022/38). doi:10.21420/S9RA-6J05.
Abstract
The coasts of the 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 like Porirua and the Wellington south coast, 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 hīkoi’ 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 Porirua and the Wellington south coast. Most coastal suburbs of Porirua that are within the tsunami evacuation zones have high ground relatively nearby; the main exception to this is the City Centre. Our modelling suggests that the population densities in most parts of Porirua are sufficiently low to avoid major congestion problems for people evacuating on foot. While the city centre is highly populated during working hours, the relatively large open spaces between buildings largely avoid the ‘bottleneck’ congestion problems seen in our earlier modelling of the Wellington central business district. Compared to suburbs on the ocean coast, the Porirua City Centre also benefits from the relatively slow propagation of a tsunami through the shallow waters of Porirua Harbour, which takes about an additional 15 minutes. In the scenarios that we consider to be the most likely to require tsunami self-evacuation, a pedestrian evacuation can occur quickly enough to remove most of the risk according to our modelling, and we recommend that the priority for public education should focus on ensuring widespread participation. There are some less-probable events, involving earthquakes on offshore Kāpiti faults or Hikurangi earthquakes that go unusually deep, which could reach the Porirua coast more quickly; to mitigate the risks from these events, a prompt start to self-evacuation is also beneficial, especially for suburbs like Tītahi Bay and Plimmerton that are on the ocean coast. The Paremata bridge is not currently recommended for use in tsunami self-evacuations because of concerns over its safety after a strong earthquake. Our modelling suggests that up to about 100 residents could evacuate more quickly, though typically only by a few minutes, if the bridge was strengthened such that it could be used. However, as approaching the bridge from the north takes people into areas of greater tsunami risk, more consideration of factors beyond the scope of our current modelling would be required before this could be recommended. The Wellington south coast is directly exposed to tsunami generated in the Cook Strait region by Hikurangi subduction zone earthquakes and other faults linked to the subduction interface, such as the Wairarapa Fault. A tsunami from these sources could reach some areas of the south coast in under 10 minutes. Using a ‘worst case’ Hikurangi tsunami scenario, our model suggests that an evacuation proceeding according to the public intentions described in the survey by Dhellemmes et al. (2016, 2021) would only reduce the overall casualties by slightly less than half of what could be achieved with a prompt evacuation of the whole at-risk population. This suggests prioritising public education focusing on both achieving high levels of participation in self-evacuation and preparedness so that self-evacuations can start within a few minutes of the end of strong shaking. There are still some areas at the base of coastal cliffs, notably in Breaker Bay but also some areas between Owhiro and Houghton Bays, where the distances that need to be travelled before it is possible to go up to high ground are such that even a quick start to the evacuation is not enough to ensure safety. We recommend that CDEM work with these communities to see if alternative earthquake-safe routes to high ground can be provided. We also recommend consideration of how to accommodate up to around 2000 people on Rongotai Hill and how to ensure a good flow of people across the evacuation zone boundaries here. (The authors)
