Storm Surge Forecasting during Cyclone Gabrielle
This week, Cyclone Gabrielle brought significant heavy rain and strong winds that, combined with energetic ocean swells, caused significant storm surges to New Zealand. Surges occurred along the northern and eastern coasts of the North Island as well as parts of the South Island, as the system moved away to the east. Storm surge is the abnormal rise of water generated by a storm, over and above the predicted astronomical tides. These surges, caused by the wind, waves and atmospheric pressure changes associated with a storm, exacerbate the effect of waves on coastal areas.
Map showing the locations of tide gauges used for the model verification.
During Cyclone Gabrielle, significant damage and destruction affected large parts of the country. Coastal inundation and flooding were amplified due to storm surge associated with the event – either by directly overwhelming the coastal area - or elevating the ocean’s localised hydraulic head* and reducing the ability of rivers to discharge the increased volume of water in the river networks during these events.
These extreme events provide us an opportunity to validate our in-house storm surge models. Validation is when we compare the model results directly with real life measurements and allow us to assess and refine our models to ensure they accurately predict the phasing and amplitude of these events. Storm surge forecasts can predict dangerously high-water levels up to seven days in advance, providing valuable alerts for low-lying coastal locations.
In the figures below, we present the time series comparison of measured and forecast storm surge.
We are delighted that the timing of the events was spot-on and the magnitude acceptable for an open-coast extreme. It is very encouraging to see the models perform with such confidence under these extreme storm conditions
Time series of the storm surge as measured by the tide gauges (source: LINZ) and the Metocean Solutions forecast values.
Sea surface height anomalies. Sea surface height anomaly data can characterise small- and large-scale changes over weeks and years.
Model forecast wind and rain (LHS) on the left and predicted storm surge (RHS) from Cyclone Gabrielle on the right.
Measured storm surge at sites around New Zealand illustrating that storm surge reached over 0.4 m in places.
For more information contact us at enquiries@metocean.co.nz
*Hydraulic head is the height above a datum such as sea level.