Author(s)

PHILIP J. GLASSEY

Abstract

The southern part of Dunedin City, New Zealand, is built over an estuarine marsh, close to sea level and is vulnerable to increased flooding from raised groundwater levels, which will only worsen with predicted sea level rise. Limited data indicate that this area may also be subsiding via either sediment consolidation or tectonism. The underlying sediments amplify seismic waves with the potential to liquefy during strong earthquake shaking. Dunedin is a hazardous place to live and about 2,600 homes are less than 50 cm above mean sea level. Past mining of a dune barrier led to inundation of parts of the city and without the re-established dune barrier, this community would be unprotected. In June 2015, a moderate rainfall event caused NZD 26 million damage through flooding, primarily related to deteriorating and poorly maintained drainage infrastructure. However, the groundwater level here is near the surface (0–1 m depth) and shows tidal fluctuations, which influence the degree of flooding. Reduced storm water storage, related to the increased groundwater levels due to sea level rise, will result in more frequent flooding and of greater magnitude over time. Engineering solutions such as increased pumping capacity and creation of storage areas will be required at considerable cost and disruption to the city. There are difficult decisions to be made about the sustainability of this low-lying coastal community in terms of investment and protection or abandonment of this area. While fundamentally social and economic issues, such decisions should be informed by rigorous technical assessments and investigation and more extensive monitoring systems.

Keywords

flooding, groundwater, New Zealand, sea level rise, South Dunedin