Kathleen Kozyniak
Team Leader Air & Land Science, Hawke’s Bay Regional Council
kathleen@hbrc.govt.nz
Cyclone Gabrielle rivalled in severity with one of Hawke’s Bay’s most infamous storms, Cyclone Bola. Impacts included loss of life, flooding, landslides, property and infrastructure damage, and the deposition of sediment and woody debris. The infrastructure damage included parts of Hawke’s Bay Regional Council’s (HBRC’s) environmental monitoring network and disrupted lines of communication from some telemetered rainfall and climate sites. The data remained logged at the sites and was retrieved when communication was restored. This enabled the magnitude of the weather event in Hawke’s Bay to be documented and compared to historical storms.
Cyclone Gabrielle originated near the Solomon Islands, developed into a Category 4 storm in the Coral Sea and tracked to New Zealand. It’s moisture holding capacity was boosted as it crossed warmer than average seas enroute to Hawke’s Bay. It arrived when the region had experienced 6 months of above normal rainfall, including from Cyclone Hale the month prior, leaving soil moisture levels near or at field capacity. This meant the land was ill-placed to absorb more moisture and reduce runoff to waterways. Additionally, wet soils are susceptible to shallow landsliding and render trees vulnerable to wind-throw.
The storm struck Hawke’s Bay overnight on 13-14th February, bringing extraordinary rainfall to a region that was already saturated. Rainfall of 546 mm, half a year’s worth, fell at Glengarry in the Esk catchment, with 501 mm falling within 24 hours, 372 mm in 12 hours and 56 mm in one hour. It is possible 700 mm fell higher in the catchment. The one-hour rate at Glengarry nears the 60 mm measured in the Napier floods in 2020, while 63.5 mm fell at Glengarry when the Esk River flooded in 2018. Rainfall during the event contributed to a February total in the Esk catchment that was 600% of the long-term average.
Cyclone Gabrielle brought high rainfall to other parts of Hawke’s Bay. The eastern area of Wairoa and high ground in the Tūtaekurī and Ngaruroro catchments received 450-500 mm, some of the highest falls in the region. Residents living at Sherenden and Waiwhare reported to HBRC that 700-750 mm fell there.
The region’s February rainfall was more than 450% of the long-term average. Nearly all HBRC’s rainfall sites recorded their highest February rainfall and it boosted rainfall for the 2022-23 hydrological year (July to June) to the wettest year in the records of nearly all HBRC’s sites.
The short-interval (1-18 hours) intensity of Cyclone Gabrielle’s rainfall was greater than Cyclone Bola’s. At 24-60 hour intervals, Cyclone Gabrielle’s rainfall was higher at most sites that existed at the time of Cyclone Bola, except some in the Wairoa District.
Easterly winds gusted 120-150 km/h around Mahia, eastern Wairoa and Cape Kidnappers. Vegetation that is structurally adapted to the country’s prevailing westerly wind are more susceptible to damage from high winds in other directions. Gusts around Napier reached 89 km/h compared to 78 km/h during Cyclone Bola but were not as strong as the 117 km/h reached during Cyclone Bernie in 1982. Cyclone Bernie is known to have downed up to 40 ha of trees in the Urewera National Park.
The broadness of the weather system allowed sufficient fetch to generate large swells and wind waves. Satellite measurements of sea surface winds showed gale to storm force easterly winds blowing over approximately 600 nautical miles east of Hawke’s Bay. The estimated significant height of combined wind waves and swell from modelled ERA reanalysis data exceeds 7 m at a location near Mahia Peninsula and is the fifth highest in the more than 80-year record. It is the highest in the record at the same and other locations in Hawke Bay for waves and swell coming from the easterly quadrant.
Cyclone Gabrielle was a severe storm that proved challenging for environmental monitoring and communication infrastructure, compromising real-time data collection from sites. Complementary rainfall estimation methods, such as weather radar and modelling, undercooked totals in key catchments in the absence of rain gauge data to calibrate them. It highlights the benefit of HBRC’s efforts to build further resilience in its gauge network.
The storm also demonstrates the difficulty urban and rural landscapes have assimilating rainfall with intensities approaching 60 mm in one hour, 150 mm in three hours and 200 mm or more in six hours, such as observed during Cyclone Gabrielle, the Napier flood and the 2018 flooding in the Esk Catchment. This is especially the case when soil moisture is already high.
Links:
https://www.hbrc.govt.nz/environment/state-of-the-environment/three-yearly-report/