Storm Surge and Beach Response in Southern New Zealand with Climate Change Considerations

Abstract

There is $22 billion (NZD) of coastal infrastructure in New Zealand, situated within 1.5 m elevation of spring high tide. In southern New Zealand alone, there are 4,000 homes, 13,000 people and 174 km of roading infrastructure susceptible to the positive feedbacks associated with climate change, most notably rising sea levels and intensification of storm events. This research examines the frequency and magnitude of storm surge and beach response to storm events in southern New Zealand. There is little understanding regarding storm surge in New Zealand and even less is understood in the southern region. Additionally, the research details the influence of climate change on the frequency and magnitude of storm surge and coastal resilience to rising sea levels. Ocean Beach, Dunedin is examined due to the amenity value and the protective role that the beach system provides to the densely populated South Dunedin community.

Extreme value analysis was applied to 21 years of sea level records, from three offshore sites around southern New Zealand, to determine the frequency and magnitude of storm surge. The ‘extRemes’ package in R was developed in 2018, specifically for short data sets. Beach response to storm events was explored through a comprehensive observational data set at Ocean Beach; determined by the examination of an array of meteorological, morphologic and oceanographic data sets. Probabilistic models and trend analysis was then applied to processes associated with storm surge to explore any increasing trends to date, and used to form predictions of what may occur with future climate change scenarios.

In southern New Zealand a one-in-one hundred-year storm surge event is between 0.91 m (Green Island) and 0.95 m (Dog Island). In comparison, the frequency and magnitude of storm surge is larger on the south west coast, with a one-in-one hundred-year event approximately 1.31 m. Nearly five years of data is missing from the Jackson Bay data set, thus the results must be interpreted with caution. No association between Southern Oscillation Index and the frequency and magnitude of storm surge in southern New Zealand is evident.

Storm ‘clustering’ and the coincidence of high storm surge with spring high tide are the primary causes of ‘significant’ erosion and management issues at Ocean Beach. The winter periods of 2007, 2009 and 2015 all resulted in ‘significant’ erosion at Ocean Beach, requiring remedial works by local authorities, including beach nourishment and expensive beach armouring. The cost of this work since 2007 is estimated to be in excess of $2 million dollars. Multiple storm events occurred in each of these three years, with little time for beach recovery between storms. In addition, the total surge intensity for all three years was between 30 – 460% higher than years where significant erosion did not occur. The duration of storm events in erosion years was the cause of the larger storm intensity and produced surges greater than 0.4 m. The years 2007, 2009 and 2015 also produced multiple storms coinciding with spring high tide, increasing the total water level above mean sea level.

The likelihood of an event where spring high tide, large offshore wave heights, and high storm surge occurs simultaneously has increased since 2003, and is predicted to continue increasing in the future. Such events contribute to erosion at Ocean Beach. Furthermore, since 2003 storm surge intensity, intense extra-tropical cyclonic events, and increased offshore wave energy has increased, probably as a result of climate change. Consequently, erosion will likely occur more frequently and with more intensity along the Ocean Beach coastline. Coastal retreat is inevitable.

To mitigate the impacts of climate change in the southern New Zealand coastal environment, future-focused and adaptive coastal management is required. This implies allowing for coastal retreat in some instances, and restoring the dynamic interplay between land and sea where possible, by avoiding ‘hard’ engineering structures.