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)

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:

  • Tutuku
  • Papatowai
  • The Catlins
  • Kākā Point
  • Clutha River/Mata-Au Delta
  • Toko Mouth
  • Taieri Mouth
  • Brighton
  • Kaikorai (Kaikarae)
  • South Dunedin
  • Long Beach and Pūrākaunui
  • Blueskin Bay
  • Karitāne and Waikouaiti
  • Moeraki
  • Hampden
  • Kākaunui/Kakanui and Taranui
  • Ōamaru

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.

 

Publications

A range of other information and documents provides relevant information on storm surge hazards, including publications below.

A 2008 NIWA report, Otago Regional Council Storm Surge Modelling Study, shows storm surge extents for return periods of 20, 50, 100 and 500 years with three increments of sea level rise (MHWS, 30cms and 50 cms) for 17 locations along the Otago coastline. This report is referenced in the natural hazards database.

There is also an older 2005 NIWA report entitled ‘Otago Extreme Sea Level Analysis’, commissions by ORC, to determine extreme sea levels with return periods of 2, 5, 10, 20, 50 and 100 years for eight locations along the Otago coastline.

The eight locations are:

  • Oamaru
  • Kakanui River Mouth
  • Shag Point
  • Otago Heads (Spit Jetty)
  • Otago Harbour (Port of Dunedin)
  • Taieri River mouth
  • Clutha River mouth
  • Catlins River mouth

The focus is on extreme sea levels to be used as ocean boundary conditions for river flood modelling.

 

 

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

The Otago coastline extends 480km from Chaslands in the south to the mouth of the Waitaki River in the north.

Approximately 124,000 people (64% of Otago’s population) live within five kilometres of this coastline. A number of the communities situated along the coast have a level of hazard exposure to elevated sea level (or storm surge) and tsunami events.


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