“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.
Tropical Cyclone Debbie made landfall along the Queensland coast Tuesday as a Category 4 storm on the Australian cyclone wind scale. Debbie produced maximum sustained winds of around 110 mph, which would place it at the threshold of a Category 3 hurricane in the Atlantic.
Debbie generated substantial coastal flooding for areas east and south of Bowen. Photographs coming from the impacted area have depicted storm surge and large waves pounding coastal buildings on Hamilton Island.
Near Midge Point, photographs depict compound flooding from storm surge and heavy rain, without the presence of wave action. Witnesses on the ground have reported around 1m (3.3 ft) of water above ground level, inundating buildings in the town. The region from Midge Point to Conway has a harbor that is open to the southeast, enabling prolonged winds to pile up storm surge in this region.
Storm surge levels exceeded 1m (3.3 ft) above normal astronomical tides from Shute Harbour to Mackay. The highest recorded water level was 2.75m (9 ft) at Laguna Quays, however, higher water marks may be found after the storm subsides and coastal surveys are conducted.
Unfortunately, the peak storm surge for many locations occurred near the time of high tide, enabling total water levels to exceed Highest Astronomical Tide (HAT). HAT represents the highest high tide under "normal" (non-storm) conditions, taking into account astronomical tide levels.
This was due, in part, to Debbie's slow forward speed, which enabled high water to persist for more than one day, ensuring high storm surge would occur near the time of high tide. Locals on the ground reported that one of the major differences between Debbie and Tropical Cyclone Ului, in 2010, was that Debbie's wind and storm surge lasted noticeably longer.
At Laguna Quays, the 2.75m (9 ft) storm surge occurred near the time of high tide, enabling total water level (storm surge + tide) to exceed HAT by around 80cm (2.6 ft).
Debbie generated the highest storm surge in Queensland since Tropical Cyclone Yasi produced a 5.33m (17.5 ft) surge in 2011. According to the U-Surge Project, Debbie generated the 14th highest storm surge in Queensland since 1880, or a storm surge level that should only be expected approximately every 10 years in that state.