Impacts of volcanic ash on road transportation: considerations for resilience in central Auckland

Abstract

Transportation networks are critical infrastructure in urban environments. Before, during and following volcanic activity, these networks can incur direct and indirect impacts, which subsequently reduces the Level-of-Service available to transportation end-users. Additionally, reductions in service can arise from management strategies including evacuation zoning, causing additional complications for transportation end-users and operators. Here, we develop metrics that incorporate Level-of-Service for transportation end-users as the key measure of vulnerability for multi-hazard volcanic impact and risk assessments.

A hypothetical eruption scenario recently developed for the Auckland Volcanic Field, New Zealand, is applied to describe potential impacts of a small basaltic eruption on different transportation modes, namely road, rail, and activities at airports and ports. We demonstrate how the new metrics can be applied at specific locations worldwide by considering the geophysical hazard sequence and evacuation zones in this scenario, a process that was strongly informed by consultation with transportation infrastructure providers and emergency management officials. We also discuss the potential implications of modified hazard sequences (e.g. different wind profiles during the scenario, and unrest with no resulting eruption) on transportation vulnerability and population displacement.

The vent area of the eruption scenario used in our study is located north of the Mangere Bridge suburb of Auckland. The volcanic activity in the scenario progresses from seismic unrest, through phreatomagmatic explosions generating pyroclastic surges to a magmatic phase generating a scoria cone and lava flows. We find that most physical damage to transportation networks occurs from pyroclastic surges during the initial stages of the eruption. However, the most extensive service reduction across all networks occurs -6 days prior to the eruption onset, largely attributed to the implementation of evacuation zones; these disrupt crucial north-south links through the south eastern Auckland isthmus, and at times cause up to -435,000 residents and many businesses to be displaced. Ash deposition on road and rail following tephra-producing eruptive phases causes widespread Level-of-Service reduction, and some disruption continues for >1 month following the end of the eruption until clean-up and re-entry to most evacuated zones is completed. Different tephra dispersal and deposition patterns can result in substantial variations to Level-of-Service and consequences for transportation management.

Additional complexities may also arise during times of unrest with no eruption, particularly as residents are potentially displaced for longer periods of time due to extended uncertainties on potential vent location. The Level-of-Service metrics developed here effectively highlight the importance of considering transportation end-users when developing volcanic impact and risk assessments. We suggest that the metrics are universally applicable in other urban environments.