Performance of New Zealand Wharves Under Tsunami Loading

Abstract

New Zealand's position along the Pacific "Ring of Fire" exposes it to various natural hazards including volcanoes, earthquakes, and tsunami. Tsunami have gained special attention in recent decades as a result of several large events which have highlighted their destructive potential including the 2004 Indian Ocean Tsunami, 2011 Tohoku Tsunami, and 2010 and 2014 tsunamis off the coast of Chile, among others. While large-scale, destructive tsunami have not recently been generated in the vicinity of New Zealand, it does have a history of large local source tsunami occurring and resulting in significant runup heights along various coasts. Furthermore, the lack of obstructive island chains and orientation between New Zealand and South America leaves New Zealand exposed to distant source tsunami from one of the most regular sources of Mw 8.0 and larger earthquakes in the world. Although tsunami have the potential to penetrate inland, the first and most severe damage will occur in coastal cities and communities. Among that infrastructure that is most exposed to damage from tsunami is ports. While some ports are situated on rivers or in protective harbours, many also lie along exposed coastlines, some of which may exhibit great exposure to tsunami threats. Given the variation regarding the structural response of port infrastructure witnessed in many of these recent large-scale tsunami, it is uncertain how New Zealand infrastructure would respond to similar events. Assessing both the exposure and resilience of port infrastructure to tsunami is essential given the critical nature of these facilities. Statistics New Zealand suggests that approximately 95% of imports and exports depart or arrive through seaports, the vast majority of inter and intra-island bulk cargo transfers occur via seaports, and the Civil Defence and Emergency Management Act of 2002 designates ports as critical lifelines which must be operational following natural hazards. Thus this research seeks to assess the structural resilience of port structures to tsunami loading. Since ports are complex and diverse in terms of the types of infrastructure they incorporate, all of which cannot be accounted for in a single study, the focus here is on pile-supported wharves, one of the most basic types of structures and the dominant type of berth in New Zealand. This goal was achieved by way of three principal steps. The first involved developing tsunami loading time series for both horizontal and uplift components. Elevated structures such as pile-supported wharves are exposed to high magnitude uplift loads which have not been investigated in great detail and thus series of laboratory tests were carried out in flumes at the University of Auckland with the purpose of quantifying these uplift loads on scale model wharf structures. The tsunami loads were then applied to structural wharf models in the second phase of research. Generic structural models were developed to encompass structural, geometric, and geotechnical characteristics of existing New Zealand wharves which would provide the best possible damage estimates. A number of broad damage states were defined which described the resulting conditions of the structures. The third phase of research was meant to connect these generic wharves to specific port locations via a series of tsunami propagation models. The result of this modelling was the expected maximum tsunami amplitudes and current velocities for each port. Thus the level of damage expected from a given tsunami can be devised through a combination of the damage state results and the predicted tsunami amplitudes.