The natural hazards of South Dunedin

This report describes the environmental and community setting of the South Dunedin plain. The physical characteristics of the plain include its low-lying topography, underlain by poorly-consolidated sediment (mainly sand and silt), its proximity to the ocean and harbour, a shallow water table with strong connectivity between sea and groundwater, and an exposure to heavy rainfall events. The plain is vulnerable to natural processes which occur reasonably frequently (such as coastal storms),¹ and also events which occur rarely but have significant consequences (such as major earthquakes on nearby faults).² The South Dunedin plain is densely populated, with approximately 10,000 permanent residents, and contains infrastructure and other assets which are important at a local, district and regional level. The physical characteristics of the plain mean it can be affected by water ponding on the surface (i.e. flooding) and it is this hazard which poses the greatest risk to community wellbeing, infrastructure and other assets. The most recent example of major flooding was in June 2015, as a result of heavy rainfall, surface runoff, and a corresponding rise in groundwater. However, this report shows that there are a number of naturally occurring physical processes and human activities which together, or separately, could affect flood hazard on the South Dunedin plain. These are listed below, along with a summary of observed trends, future predictions, and interdependencies with other factors. A summary table on the following page highlights the wide range of factors which can influence flood hazard, and that future changes in mean sea level, climate and groundwater level are the processes most likely to exacerbate the effects of this hazard. This report shows how these effects will vary across the plain, with some areas likely to be affected sooner than, or to a greater extent, than others