A Multi-Hazard Resilience Framework for Residential Building Stocks subjected to Floods and Debris Flows

Floodings and debris flows (mudflows) are the most frequent natural disasters provoked by heavy rainfalls, leading to significant physical damage to the built environment, severe economic losses, and social disruptions in communities. To promote disaster resilience of communities, sophisticated models are needed to support citizens in assessment of potential hazard losses and taking proper disaster mitigation actions. This study thus develops a framework to support the "multi-hazard resilience" of EU residential building stocks under rainfall-triggered floods and mudflows. It considers two levels of resolution for resilience assessment: at the local level of individual buildings versus at the level of building portfolios in a community. First, a typical EU residential building set was created and disassembled into components. The damage potential of each structural, nonstructural, and content component of the buildings was examined for various hazard depths and velocities of flooding and mudflow actions. Repair cost and time for each damaged component were obtained considering the individual failure limit of the components. Then, all components and associated variabilities were probabilistically assembled to estimate the total losses and repair times on residential buildings. Next, the developed impact models for individual buildings were extended into models for building portfolios, considering a virtual EU community under multi-hazard scenarios of flooding and mudflow. The effects of uncertainties associated with building and hazard properties were considered, and spatial correlations in hazard demand and common building configurations and practices were reflected in an aggregated impact and resilience assessment of building portfolios. Lastly, several retrofit actions were explored to understand their effects on flood or mudflow impacts for residential buildings. The "optimum retrofit strategies" were investigated for both individual buildings and building portfolios by performing benefit analyses on repair costs and times.

Results show that, while water/mud contact can cause severe damage to the interior building and content items, physical flood and mudflow load actions may cause significant damage to the exposed exterior building components. They may lead to high hazard losses and long repair times, especially for buildings with finished basements. The impact models developed for building portfolios show that neglecting spatial correlation in losses and repair times due to commonality in hazard demand and building performance may underestimate the overall loss and recovery time assessments for the lower probabilities of exceedance, the region of significance for public safety and insurance underwriting purposes. Moreover, benefit analyses on retrofit actions to reduce hazard impacts show that most of these actions cannot perform adequately on their own, but if grouped together as a package, they may be more effective and useful solutions. To conclude, this study brings an innovative resilience framework that greatly contributes to different stakeholders, including building owners seeking to mitigate their homes, reinsurance companies seeking to improve insurance portfolio risk policy, and government or research agencies seeking to improve disaster response and management plans.

ACKNOWLEDGEMENTS: One of the authors, Dr. Derya Deniz, acknowledges the support provided by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 893147.