Evacuations are, and most likely will continue to be, the most common and efficient emergency management strategy when a hazardous event threatens and puts at risk the safety of those within the area (Moriarty et al. 2007). Evacuations are also becoming increasingly frequent worldwide as humans continue to develop in hazardous areas and improved technology in many countries allows for prior warnings and the movement of people before a disaster strikes (Sparks 2003, Woo and Grossi 2009). However, evacuations can produce long-term negative effects such as psychological trauma, and disruption of community cohesion and employment and economic continuity (e.g. Mileti et al. 1991, Lindell and Perry 1992, Cola 1996, Tobin and Whiteford 2002, Perry and Lindell 2003). Poorly managed evacuations tend to lead to a strong resentment of government which, in turn, decreases the ability of emergency management organisations to act effectively in the future (MCDEM 2008). Therefore, effective planning of an evacuation is essential.
Volcanic eruptions are capable of producing a spectrum of hazards which are harmful to humans. These hazards range from highly destructive phenomena such as pyroclastic density currents, debris avalanches, lava flows and lahars that typically destroy everything in their path, to less destructive yet highly disruptive phenomena such as ash fall, volcanic tremor and gas release. As many volcanoes and volcanic regions around the world are already heavily populated, the most effective means of risk reduction will be to identify the most hazardous areas and evacuate the population from the danger zone prior to an eruption (Marzocchi and Woo 2007, Lindsay et al. 2011, Sandri et al. 2012).
The city of Auckland, New Zealand, with a population of ~1.5 million as of December 2012, is built within the 360 km2 potentially active basaltic Auckland Volcanic Field (AVF) (Figure 1), with the last eruption occurring just 550 years ago (Needham et al. 2011). Auckland’s geography poses significant constraints for evacuation planning. The city is located on an isthmus bounded by the Waitemata Harbour to the northeast and Manukau Harbour to the southwest. As a result, all land-based transport into and out of the city is constricted through narrow stretches of land serviced by four motorway bridges which form critical links in Auckland’s transportation network (Figure 1). To date, there has been no major modelling-based study conducted on the mass evacuation capacity of Auckland. Previous evacuation planning has been mainly strategic and lacking in geospatial analysis and physical evacuation procedures that can be used operationally (Auckland CDEM Group 2008a, Auckland CDEM Group 2008b, Tomsen 2010).
In this study we adopt a novel, non-specific approach (Shulman 2008) in considering the spatial and temporal distribution of population and transport networks across Auckland and how they affect mass evacuation planning. Spatial network analysis is used to determine the geographic functionality of major transport origin and destination points and we determine the relative vulnerabilities of the key motorway bridges to new AVF eruptions. We then assess micro-evacuation vulnerability by combining spatial network analysis with population evacuation demand to calculate evacuation capacity ratios for individuals, households and vehicles. Finally, we employ modelling using dynamic route and traffic assignment to measure evacuation attributes at a macro-scale and forecast total network clearance times. This quantitative study thus serves to fill the informational void and provides emergency management officials with a more holistic understanding of the local variations in susceptibility to mass evacuations, particularly those related to volcanic activity in the AVF.
Evacuation planning
There is a broad body of literature on effective evacuation planning. Many studies have attempted to classify evacuations into various types (e.g. Baker 1991, Ketteridge et al. 1996, Wolshon et al. 2001, Marrero et al. 2010) and others have focussed on emergency response activities (e.g. Cova 1999, Cutter 2003, Marzocchi and Woo 2007, Moriarty et al. 2007, Shaluf 2008). The core components to this evacuation planning can be summarised as: 1) conditions under which an evacuation may be necessary; 2) ‘at risk’ people/communities who may require evacuation; 3) evacuation routes and destinations; and 4) the resources and time required to evacuate ‘at risk’ people/communities (MCDEM, 2008).
Identifying when an evacuation is necessary
Evacuation can be classified as an “organised, phased, and supervised withdrawal, dispersal, or removal of civilians from dangerous or potentially dangerous areas, and includes their reception and care in safe areas” (U.S. Department of Transportation 2006, p.2-1). Evacuation becomes necessary when the benefits of leaving significantly outweigh the risk of other options, such as ‘sheltering-in-place’. In a volcanic context, evacuation is a response strategy – an effort to preserve human life (Marzocchi and Woo 2007, Auckland CDEM 2013). In order to assist with evacuations, plans are created in advance, identifying key personnel, areas at risk, and mitigation measures to enact (Moriarty et al. 2007). In New Zealand, the Mass Evacuation Plan (MCDEM 2008) is the key sub-national level plan which aims to detail a range of considerations and actions for the mass evacuation of people from a hazardous environment to a relative place of safety (Auckland CDEM Group 2008b). The Auckland Volcanic Field Contingency Plan is more specific and includes planning arrangements for evacuations resulting from an eruption within the AVF (Auckland CDEM 2013). According to the plan, an evacuation will be called by the Auckland CDEM Group if hazard assessment indicates urban or strategic areas may lie within 5 kilometres of the inferred eruption centre and/or there is a potential risk to life.
‘At risk’ people and communities
There is varied focus in the literature about which group or groups tend to be the most ‘at risk’. Low-income populations are studied in detail by some (e.g. Morrow 1999, Chakraborty et al. 2005), while others (e.g. Bascetta 2006, Dosa et al. 2007) focus on the elderly and disabled. The low-mobility population (i.e. those without access to a private vehicle), however, are discussed by many and we examine this group further as they will require public modes of transport (Leonard 1985, Hushon et al. 1989, Wolshon et al. 2001). Ideally, people within an evacuation zone evacuate and people resident outside the zone shelter in place. However, evacuations are typically far more complex, with some choosing to remain within a zone, and others outside the evacuation zone voluntarily evacuating (termed shadow evacuation, Baker 1991).
Uncertainty as to who will stay and go (the population evacuation demand) makes it difficult to establish credible time estimates for those evacuating, although this is fundamental for evacuation planning. The population evacuation demand is dependent on numerous variables including external conditions such as weather, location of the hazard source and time, as well as human behavioural characteristics inherent in the population (Wolshon 2006, Tomsen 2010). Research on evacuation response rates for hurricane-based evacuations in the United States found that evacuation rates ranged between 33-97% during the same hurricane, with an average of 47.5% (Baker 1991). People in high-risk areas, on average, were found to be more than twice as likely to evacuate when compared to low-risk areas. This was attributed to two factors: people residing in high-risk areas are aware of the hazardousness of their location and/or public officials go to greater lengths to evacuate the residents of these areas (Baker 1991). Less data is available for non-compliance to shelter-in-place orders, a factor that often causes emergency management officials the most difficulty. Two recent surveys regarding such unofficial evacuees reported nearly 60% of respondents leaving before evacuation orders were given during Hurricanes Lili and Katrina (Lindell et al. 2005, Lindell and Prater 2006).
Evacuation routes and destinations
Evacuation route choice is a complex decision-making process. Some researchers believe that in emergency situations, evacuees will take any possible egress route (Moriarty et al. 2007). However, others contend that people will take the most familiar routes (predominantly motorways), which often become overloaded while capacity on alternative routes remains unused (Prater et al. 2000, Dow and Cutter 2002). During Hurricane Katrina, drivers were more influenced by familiarity with the route than traffic conditions they experienced en route (Lindell and Prater 2006). As stated in the Mass Evacuation Plan for New Zealand (MCDEM 2008, p.56), “the planning process should decide upon primary and secondary evacuation routes from an anticipated affected area”, and “evacuation routes should be designed with due consideration to local area hazard maps to ensure that selected routes are appropriate for anticipated hazards”. Any potential bottlenecks in traffic movement should also be identified (MCDEM 2008). Many studies in the U.S. have shown that, despite the enormous demand during hurricane evacuations, many roads carry flows well below the predicted maximums (Wolshon 2008). However, contraflow systems are frequently used for evacuations in the U.S. and plans are often well engineered and publicised (Wolshon 2002). Indeed, although studies on hurricane-based evacuations provide valuable information, many differences in characteristics such as risk perception, familiarity and cultural geography, mean that the results cannot be easily extrapolated to other hazards or locations (Marrero et al. 2010). Transportation modelling can be used to help with specific planning objectives, and allow the testing of various assumptions and alternatives.
During evacuations people tend to favour temporary re-location in second homes, hotel/motel accommodation or with family and friends, rather than seeking public shelter (Quarantelli 1985). However, in a mass evacuation, many of the low mobility population and those without social networks or financial resources will require assistance with accommodation from emergency management authorities. For smaller events, ‘all-in-one welfare facilities’ may be all that are required to service evacuees. However, when the volume of evacuees is likely to be large, separate evacuation and recovery centres may need to be established (MCDEM 2008).
Evacuation resources and time
The ability of a community to respond to a disaster and cope with its consequences largely depends on its level of preparedness. However, the impact on an evacuated community is reduced when evacuation is carried out in a well-managed and organised manner. During a mass evacuation, transportation networks are the most critical components of a region’s infrastructure network, as they facilitate the mobility of the human population. In developed countries, private vehicles have often been the predominant form of mass evacuation (Quarantelli 1980, Drabek 1986, Lindell and Perry 1992, Tierney et al. 2001, Cole and Blumenthal 2004). This is likely due to their prominence in today’s society, the flexibility of route and destination choice they allow, as well as their asset value, which many evacuees seek to retain. A survey conducted in 2008 by the New Zealand Ministry of Civil Defence and Emergency Management with regard to evacuation behaviour in Auckland, confirmed this tendency. Of the 2,050 people in the survey, 91.3% would choose to leave with their own vehicle if required to evacuate due to an AVF eruption (Horrocks 2008b). Alternative forms of transportation such as trains and buses can also be used for evacuation purposes and are particularly beneficial to the low mobility population who may strongly rely on their provision.
When considering the time and resources required for evacuation, it is important to acknowledge the regular diurnal population shift which occurs in most developed countries when people travel to places of work and learning during the day and return home again at night. A national telephone survey conducted by Klepeis et al. (2001) across the U.S. demonstrated that while more than 90% of people are at home and indoors between the hours of 11 pm and 5 am, less than 35% are there from 10 am to 3 pm. We expect similar trends to occur in New Zealand, particularly in city environments, although there is little data for comparison at present. In addition to the standard diurnal shift, other spatio-temporal movement patterns exist in urban areas. On weekends and during school holidays, when many residents leave for recreational activities and travel, the population in the urban area sinks compared to its weekday highs. At other times, such as during major concerts, sporting events and conventions, the urban population may grow substantially. When evacuation time estimates are available, emergency management officials can determine how far in advance evacuation orders should be issued. This allows authorities to balance the competing demands of enduring public safety and unnecessary costs associated with imprecise or unnecessary evacuations, i.e. false alarms. Because running evacuation drills is difficult due to the large areas and populations involved, computer simulations based on various traffic analysis models offer the next best option (Franzese and Liu 2008). Current emergency management planning in Auckland assumes that a major evacuation (such as for an impending volcanic eruption) would require 48 hours for authorities to implement (this includes a pre-evacuation-call planning period). This was illustrated in the lead up to the simulated evacuation during a major 2008 exercise based on an Auckland Volcanic Field eruption, Exercise Ruaumoko, when civil authorities wanted to know when the 48 hour ‘time window’ before outbreak had been entered (Lindsay et al. 2010).
The Auckland Volcanic Field
When considering evacuation planning for a volcanic field eruption the following factors related to the hazard must be considered: the likelihood; the number of vents expected; the location(s) of the new vent; the area impacted by volcanic hazards (hazard footprint), which is dependent on the style and size of eruptive activity produced during vent opening; and how much warning will be provided by volcano monitoring systems. The two most important factors are the hazard footprint and the location of the eruption, which together allow determination of the necessary spatial extent of the evacuation zone. This section reviews the past known eruptive history of the AVF with particular focus on these two factors.
The AVF (Figure 1) is a geologically young, generally monogenetic, intraplate volcanic field made up of over 50 small basaltic volcanoes, which has been active for 250,000 years with the last eruption ~550 years ago (Lindsay et al. 2010, Needham et al. 2011, Shane et al. 2013). Being generally monogenetic in nature, each vent is typically only active for a single eruption sequence and new eruptions usually occur in a different location from those before. To date there have been no spatio-temporal trends identified for vents in the AVF. Recent algorithmic analysis by Bebbington and Cronin (2011) has discounted earlier studies that suggested spatio-temporal clustering in the AVF. Instead the spatial and temporal aspects appear independent; hence the location of the last eruption provides no information about the next location.
Previous AVF eruptions have typically been small in volume (<0.1 km3, Allen and Smith 1994), However the last two eruptions, Rangitoto (2 km3) and Mt. Wellington (0.17 km3) are two of the largest in volume, suggesting a possible change in future eruptive behaviour (Lindsay 2010).
The eruption style during vent opening is typically phreatomagmatic, due to rising magma interacting with groundwater and/or seawater (if a vent occurs in the ocean). Some eruptions cease after this stage, leaving broad maars or explosion craters typically 1-2 kilometres in diameter. Where the eruptions continue beyond this stage, subsequent activity is of magmatic Hawaiian style, which produces scoria cones and lava flows. The explosive phreatomagmatic AVF eruptions have generated volcanic hazards such as base surges, a type of pyroclastic density current (denser-than-air flows which can travel at 200-300 km h-1 and be >200 °C; Browne 1958, Belousov et al. 2007), shock-waves and ballistics (material >64 mm erupted from the vent) which are highly destructive to areas up to 3 kilometre radius of the vent. Secondary hazards, such as earthquakes, tephra fall and gas release, would also be noticed throughout the entire region. The footprint of these hazards from previous AVF eruptions and those of other analogous volcanoes have been used to calculate evacuation zones for future events. Current contingency and mass evacuation plans call for areas of 3 kilometres (“Primary Evacuation Zone”) and 5 kilometres (“Secondary Evacuation Zone”) radius from the erupting vents to account for base surges (Beca Carter Hollings and Ferner Ltd. 2002, Auckland CDEM Group 2008b, Auckland CDEM 2013).
Finally, It is likely that civil authorities and area residents will only be provided with at most a few weeks and as little as a few days of warning time prior to an eruption (Beca Carter Hollings and Ferner Ltd. 2002). This is based on the expected fast magma ascent rates (1-10 cm s-1) that basaltic volcanic fields are known to exhibit (Blake et al. 2006, Sherburn et al. 2007). Early detection of precursory activity is therefore critical. However, the factors contributing to a decision by emergency management officials to call a mass evacuation bring significant levels of uncertainty to mass evacuation planning in Auckland. Furthermore, the exact vent area is likely to be unknown until shortly before outbreak (Blake et al. 2006). One attempt to address this uncertainty was presented by Sandri et al. (2012), who developed a cost-benefit analysis model for evacuation planning by weighing the cost of issuing evacuation warnings for geographic areas (represented by lost work potential, warning costs, movement costs etc.) against the benefit of evacuating (represented by the number of lives saved). When the benefits of evacuating a certain area exceed the associated costs, an evacuation is deemed warranted. The point of changeover is the ideal boundary to use as the time to call for evacuation. Sandri et al. (2012) calculated probability threshold values for a range of magnitudes: small effusive, moderate phreatomagmatic and large phreatomagmatic eruptions. Evacuation radii for these three scenarios were established at 3.5, 5 and 8 kilometres respectively. These are somewhat consistent with the primary (3 km radius) and secondary (5 km radius) evacuation zones in the AVF contingency plan, but crucially suggest an additional larger radius of 8 km should be considered in evacuation demand analysis. The evacuation area was also found to change in size with time in the lead-in period, due to a reduction in the uncertainty in the vent location and increase in the probability of an eruption. Thus, there is a trade-off between these two factors (area and time) that dictates which cells must be evacuated, and when (Sandri et al. 2012). Given the uncertainty in vent location (and the subsequent need to wait until close to outbreak to define the evacuation area) it is likely that the evacuation will need to be carried out quickly, i.e. within the 48-hour evacuation time required by civil authorities.