Storm surge 

A storm surge is a higher than normal sea level, due to changes in atmospheric pressure and wind, which can result in inundation of roads and coastal property over an extended period.

 

Storm surge affecting Pounawea, April 2006. Photo: Mike Hilton (University of Otago)

A combination of all or some of the following components can result in higher-than-normal sea levels:

• Tides refer to the periodic rise and fall of the waters of the ocean, produced by the attraction of the moon and sun, and occurring about every 12 hours. The maximum tidal range along the Otago coast is approximately 2.5 metres, making it an important component of elevated sea level events.
• The storm surge component occurs as a result of low atmospheric pressure and persistent wind stress (or wind set-up). Low atmospheric pressure may result in higher than predicted sea levels, while strong onshore winds result in the sea “piling up” against the coast.
• Wave set-up and wave run-up can further elevate sea level. Wave set-up is the increase in the mean level of the sea due to the effect of waves. Wave run-up is the increase in sea level from waves breaking in the foreshore zone. The increase corresponds to each respective wave, so the effects of wave run-up occur over short periods. Since breaking waves are attenuated in sheltered areas such as inlets, wave run-up is generally restricted to open coast sites.

Modelled inundation extents and depths are available for 17 localities along the Otago coast. These include:

• Tautuku
• Papatowai
• Catlins
• Kaka Point
• Clutha River/Mata-Au Delta
• Toko Mouth
• Taieri Mouth
• Brighton
• Kaikorai
• South Dunedin
• Long Beach and Purakaunui
• Blueskin Bay
• Karitane and Waikouaiti
• Moeraki
• Hampden
• Kakanui and Taranui
• Oamaru

There are other settlements and unpopulated stretches of the Otago coast which are also subject to storm surge hazard. The lack of modelled storm surge information in these areas does not mean that this hazard does not exist, rather that ORC does not currently hold the same level of information about inundation extent and depth for these localities.

A range of other information and documents provides relevant information on storm surge hazards. A 2008 NIWA report titled 'Otago Regional Council Storm Surge Modelling Study' is referenced in the Natural Hazards Database stormsurge extents for return periods of 20, 50, 100 and 500 year with three increments of sea level rise (MHWS, 30cms and 50 cms) for 17 locations along the Otago coastline. There is also an older 2005 NIWA report entitled ‘Otago Extreme Sea Level Analysis’ which determines extreme sea levels with return periods of 2, 5, 10, 20, 50 and 100 years for eight locations along the Otago coastline.

An ORC report titled 'Community vulnerability to elevated sea level and coastal tsunami events in Otago' assesses the level of vulnerability at a number of communities along the Otago coastline. An updated assessment of coastal hazard for the Dunedin District was completed in 2014.