Perfect storm caused Nelson flooding

On February 1, Fehi caused storm surges along New Zealand’s west coast, impacting coastal communities in many locations around the country. In Nelson whole streets were flooded in low-lying areas, causing emergency services to evacuate residents. Coastal residents were also evacuated in other areas including Taranaki and West Coast.

“The flooding was caused by an unlucky combination of factors,” explains oceanographer Dr Rafael Soutelino. “A very low pressure system coincided with king high tides, large waves and strong winds, resulting in very high coastal water levels in several areas around the country’s west coast.

“Storm surges occur when the sea rises as a result of wind and atmospheric pressure changes associated with a storm. The surges build up over time and will worsen when a low pressure system lingers.

“On this occasion, our models predicted a storm surge of up to 50 cm in Hokitika, and just over 35 cm in the Nelson region. This may not sound like much but when added to a king tide the impact can be devastating. The wind direction is important too. In this event, the trajectory of the tropical storm caused north-northeasterly winds which acted to push water towards the shore, further raising coastal sea levels.  

 Storm surges of up to 35 cm above astronomical tide coincided with high winds in Nelson on 1 February 2018.

Storm surges of up to 35 cm above astronomical tide coincided with high winds in Nelson on 1 February 2018.

“Strong winds are common along the west coast of the South Island, but systems like Fehi that have tropical origin cause a much bigger pressure drop than normal winter storms. Such pressure drops cause sea level to rise, a phenomenon known as the inverse barometer effect.

“Storm surges of up to 50 cm are not unusual in New Zealand, and thankfully they most often do not coincide with king tides. However, occasionally the worst possible combination of events will occur, and at such time good forecasting become very important.”

Storm surge forecasts can predict dangerously high water levels up to seven days in advance, providing valuable alerts for for low-lying coastal locations. MetOcean Solutions frequently collaborates with emergency services and local authorities to provide forewarning when large storm surges are predicted.  

“Storm surges can be reliably predicted,” continues Rafael. “MetOcean Solutions forecasts storm surges nationwide using a complex high definition 3D hydrodynamical model with 5 km resolution.The model computes the atmospheric effects on coastal water levels. It combines this with baseline water levels generated by open ocean eddies and water column expansions and contractions caused by the spatially variable vertical density distribution.

“Waves also significantly modify water levels and consequent impacts of storm surges. We are currently working on improving our models to include waves in storm surge predictions for vulnerable locations around New Zealand. Hopefully such models will contribute towards keeping coastal communities safe when the next perfect storm hits.”

 The 35 cm storm surge coincided with king tides and winds pushing water ashore, resulting in widespread flooding in Nelson.

The 35 cm storm surge coincided with king tides and winds pushing water ashore, resulting in widespread flooding in Nelson.