“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 was bearing down on the coast of Queensland, Australia, Monday afternoon local time.
As of 10 p.m. Australian Eastern Standard Time (1200 UTC), maximum winds near the center of circulation were sustained at 110 mph, making it a Category 4 on Australia's tropical cyclone category system. Debbie is forecast to make landfall near Bowen as a Category 4 tropical cyclone around 9:00 a.m. local time on Tuesday. However, conditions will deteriorate throughout Monday night between Bowen and Mackay.
Storm surge levels were fluctuating Monday evening at several sites along the Queensland coast. Laguna Quays, Mackay and Dalrymple Bay all reported storm surge levels exceeding 1.6 feet (0.5m) early in the evening, but as of the latest update, only surge levels at Laguna Quay remained above that threshold.
Water levels should increase through the night as Debbie approaches the coastline, with storm surge exceeding 8.2 feet (2.5m) in localized areas between Bowen and Mackay.
Most locations in this part of the world have a high tidal range, with the difference between high and low tide often exceeding 13 feet (4m). Such large tidal ranges will affect the timing of maximum coastal flooding on the landscape. While storm surge refers to the difference between predicted (astronomical) tides and actual water levels, storm tides combine storm surge with astronomical tides to produce a total water level that is seen on the landscape.
Most areas will experience the greatest coastal flood impact within two hours of high tide, however, storm tide flooding is quite localized, and a difference of several kilometers can make substantial difference in water levels. The table below provides generalized information on tidal ranges and the timing of the greatest flood impact for selected locations.
|Location||Tidal Range||Max Coastal Flood Impact|
|Dalrymple Bay||5m+ (16.4ft+)||10PM Mon - 2AM Tue|
|Mackay||5m+ (16.4ft+)||10PM Mon - 2AM Tue|
|Laguna Quays||4m+ (13.1ft+)||10PM Mon - 2AM Tue|
|Shute Harbour||3m+ (9.84ft+)||10PM Mon - 2AM Tue|
|Bowen||2.5m+ (8.2ft+)||10PM Mon - 2AM Tue, 8-10AM Tue|
Much of the coast will experience the greatest coastal flood impact late Monday night into Tuesday morning, corresponding with the hours near high tide. However, some areas near or just south of Bowen could experience a secondary high water event later Tuesday morning near the time of landfall.
There should be a drastic difference in water levels and timing of high water between north- and south-facing coasts.
South-facing coasts, in areas such as Conway:
Water levels will gradually build as Debbie approaches, reaching maximum levels just before Debbie makes its closest approach.
North-facing coats, such as Airlie Beach:
Water levels may actually be lower than normal as Debbie approaches due to strong south winds. Just after closest approach, winds will suddenly blow from the north and could rapidly generate storm surge — flooding could come in as we typically think of a tsunami — almost in one sudden wave.
Do not assume the worst is over when the eye arrives, especially if you are on a north-facing coastline east or south of Bowen.
Fortunately, the coast of Queensland has been on high alert and flood/evacuation maps have circulated through many communities. For example, Townsville City Council circulated a map depicting the areas most vulnerable to storm surge flooding to help people make evacuation decisions.
The U-Surge Project has identified storm surge levels for 72 tropical cyclones that have struck Queensland since 1880, providing an updated database that builds off the foundational work from Needham et al. (2015).
Needham et al. (2015) provided a data-driven frequency analysis of storm surges in Queensland, finding that this region observes an average of 2.5 storm surges per decade exceeding 6.56 feet (2m) and an average of 1.4 storm surges per decade exceeding 9.84 feet (3m).
Tropical Cyclone Yasi was the last cyclone to generate a storm surge exceeding 6.56 feet (2m) in Queensland. This cyclone generated a storm surge of 16.4 feet (5m) near Cardwell in 2011.
A storm surge level of at least 8.2 feet (2.5) would tie Debbie for 15th place since 1880, according to Queensland storm surge records. This would make Debbie's storm surge around a nine-year flood event, or a flood level we should expect on average around every nine years. As of late Monday evening, Debbie's high water mark of 2.62 feet (0.85) ties it for 50th place all-time since 1880.
This blog will be updated frequently through the storm, enabling you to follow Debbie's peak surge level and see how high it ranks in historical context.