Earthquake-altered flooding hazard induced by damage to storm water systems

Abstract

Major earthquakes can cause extensive transformations to the land underlying cities, leading to decreased capacity in natural and built drainage systems and, as a consequence, to Increased Flooding Hazard (IFH). This phenomenon causes some areas, which previously were not exposed to flooding, to have the potential to flood, and already flood-prone areas to likely experience increased flood depth during the next rainfall events. This scenario occurred in Christchurch city, New Zealand, after the 2010–2011 Canterbury Earthquake Sequence (CES). The IFH was observed in many urban areas during a series of rainfall events occurred in the years after the CES. This paper proposes a method for analysing and assessing to what extent the earthquake-induced damage to storm water pipelines and the consequent impacts on the connectivity and capacity levels of the pipeline storm water network could contribute to the IFH. A probabilistic analysis, through a Monte Carlo simulation, is suggested for the proposed method so that the uncertainty affecting several key parameters can be accounted for. The proposed probabilistic method for IFH was implemented as an additional module within a recently developed open-source simulation tool, OOFIMS. Results from the added OOFIMS module are presented in terms of maps and cumulative distribution functions of increased flood height and flooded area, impact metrics that can be useful for emergency managers and infrastructure owners. The effectiveness of the proposed method to assess earthquake-altered flooding hazard and the relative OOFIMS-added module are tested using Christchurch as a case-study.

Keywords:

• Monte Carlo simulation
• probabilistic analysis
• overflow
• pipeline storm water system
• hydrological models

Acknowledgements

The authors gratefully acknowledge: the European Commission for financial support through grant agreement No. 244061 for the SYNER-G collaborative research project; the Christchurch City Council for providing the data on the city’s pipeline storm water network; Dr Deirdre Hart and Ms Su Young Ko from the University of Canterbury (Christchurch, New Zealand), for their contribution to the initial part of this research.

Notes

¹ Conventionally risk is expressed by the notation Risk = Hazard × Exposure × Vulnerability. The final goal of all risk assessment methodologies is to compute risk for primary systems, such as buildings (residential, commercial, or industrial), representing the inhabited space and arena of socio-economic life in a city. Therefore, the vulnerability concept is usually referred to the susceptibility of buildings or a community to the impact of hazards. In this paper, on the other hand, the focus is on the computation of flood height, intended as flood intensity measure, analogously to, for instance, spectral acceleration as seismic intensity measure. This is the reason why the acronym IFH, instead of IFV, is used.

² Some hyetograph models are not appropriate for the case-study area and therefore were excluded. As an example, consider triangular hyetographs, which are particularly suitable for arid and semi-arid regions where flash floods (characterised by rapid intensity rise and very short durations) are likely to occur.