Decision support framework for post-earthquake restoration of sewerage pipelines and systems

Abstract

A sewerage system transfers the wastewater from toilets, kitchens and laundries through an underground network to treatment plants. Because most of the system components are buried underground; they are extremely vulnerable to earthquakes. In 2010 and 2011, the Canterbury earthquake sequence struck Christchurch city, causing extensive damage to the Christchurch sewerage system. 41 % of the sewerage pipelines and 136 pump stations were damaged as a result of the Canterbury earthquakes. This research aims to facilitate rapid yet reliable decisions on post-earthquake restoration for sewerage pipelines and systems.

There are three main steps: one, assess physical damage; two, evaluate functional impact; and three, predict the time to restore the service. For step one, the damage data on the Christchurch sewerage system were collected and the fragility functions for different pipe materials have been developed. These functions can provide a fast estimate of the damage levels of sewer pipelines. Then step two. A set of post-earthquake performance indicators to evaluate the loss of service due to the earthquake-induced physical damage has been developed. These performance indicators could examine the functional consequences that might arise from the physical damage. Last step, statistical models to predict restoration time of sewer pipelines have been proposed, based on the information provided from the last two steps. These models could estimate how long it will take to return the wastewater service to customers. Based on this information, decision-makers can better allocate repair resources and distribute portable toilets. Furthermore, residences can better prepare for the lack of sanitation services.

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Technical Abstract

During seismic events both transient ground motion and permanent ground deformation can induce physical damage to sewerage system components, limiting or impeding the operability of the whole system. The Canterbury Earthquake Sequence (CES) in 2010-2011 imposed widespread damage to the Christchurch sewerage system (CSS), causing a significant loss of functionality and serviceability to the system. The malfunction of the CSS resulted in the pollution of nearby waterways through the discharge of untreated sewage, posed a public health threat by preventing the use of appropriate sanitation facilities, and caused serious inconvenience for rescuers and residents.

Numerous initiatives to create platforms/tools aiming to, on the one hand, support the understanding, management and mitigation of seismic risk for infrastructure prior to disasters, and on the other hand, to support the decision-making for post-disaster reconstruction and recovery, have been promoted worldwide. Despite this, the CES in New Zealand highlighted that none of the existing platforms/tools are either accessible and/or readable or usable by emergency managers and post-disaster recovery decision makers. Furthermore, the majority of existing tools have a sole focus on the engineering perspective, while the holistic process of formulating recovery decisions is based on system-wide approach, where a variety of factors in addition to technical considerations are involved.

This research develops a decision support framework for sewerage pipe and system restoration after earthquakes, building on the experience and learning of the organisations involved in recovering the CSS following the CES in 2010-2011. The proposed decision support framework includes three modules: 1) Physical Damage Module (PDM); 2) Function Impact Module (FIM); 3) Pipeline Restoration Module (PRM). The PDM provides seismic fragility matrices and functions for sewer gravity and pressure pipelines for predicting earthquake-induced physical damage, categorised by pipe materials and liquefaction zones. The FIM demonstrates a set of performance indicators that are categorised in five domains: structural, hydraulic, environmental, social and economic domains. These performance indicators are used to assess loss of wastewater system service and the induced functional impacts in three different phases: emergency response, short-term recovery and long-term restoration. Based on the knowledge of the physical and functional status-quo of the sewerage systems post-earthquake captured through the PDM and FIM, the PRM estimates restoration time of sewer pipelines by use of restoration models developed using Random Survival Forest technique and graphically represented in terms of restoration curves.

The development of a decision support framework for sewer recovery after earthquakes enables decision makers to assess physical damage, evaluate functional impacts relating to hydraulic, environmental, structural, economic and social contexts, and to predict restoration time of sewerage systems. Furthermore, the decision support framework can be potentially employed to underpin system maintenance and upgrade by guiding system rehabilitation and to monitor system behaviours during business-as-usual time. In conjunction with expert judgement and best practices, this framework can be moreover applied to assist asset managers in targeting the inclusion of system resilience as part of asset maintenance programmes.

Published Paper:
Miao Liu, Sonia Giovinazzi, Piet Beukman, Post-earthquake performance indicators for sewerage systems - Proceedings of the Institution of Civil Engineers - Municipal Engineer, Volume 169 Issue 2, June, 2016, pp. 74-84; DOI: http://dx.doi.org/10.1680/jmuen.15.00028