Multi-hazard assessment of increased flooding hazard due to earthquake-induced damage to the natural drainage system

Major earthquakes can extensively modify the natural and built environment, possibly leading to Increased Flooding Hazard (IFH). This term indicates greater flood heights and/or extents in response to future rainfall events compared to pre-quake scenarios, and therefore higher probability for impacts on buildings and lifelines. This work illustrates a probabilistic multi-hazard framework for assessing to what extent earthquake-induced damage to the natural drainage system (i.e. rivers and open channels) of a city, in terms of change of riverbed, damage to levees and ground subsidence/uplift, could contribute to the IFH phenomenon. The pro- posed methodology, which allows modelling earthquake-altered pluvial and fluvial flooding, was implemented as an additional module within a recently developed open-source simulation tool. Results are presented in terms of cumulative distribution functions for increased flood height, flooded area and overflow volume, as well as inundation maps to be used for emergency management and mitigation planning purposes. The effectiveness of the proposed methodology to assess IFH is reality-checked using Christchurch, New Zealand, as a test-bed, focussing, in particular, on a portion of Christchurch’s natural drainage system.