By Thomas Mortlock, Niyas Madappatt and Matalena Tofa
Severe Tropical Cyclone Debbie made landfall near Airlie Beach on the north Queensland coast at midday on 28th March 2017. It was a slow-moving category 4 system that generated wind gusts exceeding 160 km/hr, heavy rainfall, large waves and storm surge at the coast. 27 hours later, after having tracked inland over the townships of Proserpine and Collinsville, it was downgraded to a Tropical Low. A team from Risk Frontiers travelled to the landfall site and surrounds a day later to assess the damage.
Surface water and river flooding
From the air, the most obvious impact of TC Debbie was floodwater. The slow forward speed of the cyclone (average 9 km/hr) contributed to surface water flooding from large amounts of accumulated rainfall on Tuesday 28th while river levels at the coast peaked a day later as the inland movement of the (then) ex-tropical cyclone released large amounts of rainfall on the hinterland. Speaking to local residents in Mackay, the delayed flooding of the Pioneer River (Figure 2 A) came at a time when most were reeling from the initial passage of the Cat 4 system.
From our observations, most main road and rail lines remained free from floodwater because of the appropriate elevation of both above the surrounding floodplain. The Bruce Highway – the main arterial between Mackay and Proserpine – was temporarily cut at Gunyarra but good gullies and freeboard (visible in Figure 2 B) ensured this was short-lived.
Wind gusts of up to 260 km/hr were recorded at Hamilton Island (50 km due east of Proserpine) and 160 km/hr at Proserpine on 28th March. Further south at Mackay, the maximum gust speed was considerably less (95 km/hr).
Driving north on the Bruce Highway from Mackay the visible signs of wind damage increased. Fallen power lines and trees were common from Seaforth north. Around Proserpine and Hamilton Plains, fields of sugar cane – usually 2 to 3 metres in height – were bent knee-high, demonstrating the severity of near ground-level winds in this area. The importance of local topography was also obvious, with steep relief exhibiting worse impacts than adjacent valleys (Figure 3 A).
With the exception of trees fallen on houses, there were few external signs of wind-induced building damage. The design gust wind speed for typical buildings in this region is 248 km/hr. Most post-1986 buildings around Proserpine, for example, experienced ~75 % of the design ultimate wind speed and faired well (JCU, 2017).
The only area where we witnessed significant structural damage was flying over South Molle Island (Figure 3 B). While this resort was half-abandoned prior to Debbie, it demonstrates what can happen with significantly stronger winds and sub-standard building condition.
Figure 3 A) Contrasting impacts of exposed relief (hill in background, trees stripped) and adjacent valley (foreground, foliage and power lines intact), B) Destruction at South Molle Island.
Storm surge is the rise in the ocean water level associated with strong onshore winds and low atmospheric pressure during a cyclone. The ‘surge’ element is additional to the normal tide. On top of the tide and surge (collectively known as the ‘storm tide’), waves can further contribute to coastal flooding locally by increasing the surface water level (wave setup) and providing momentum for an up-rush of water up the beach (wave runup). At the Mackay wave buoy, for instance, waves reached 8 m around 5 am on the Tuesday morning.
Overall surge impacts were limited because: 1) the strongest winds occurred two hours after the morning high tide and 2) the cyclone made landfall over a section of the coast partially shadowed by the Whitsunday Islands. If Debbie had made landfall two hours earlier, an extra 1 metre of storm tide may have been possible.
As it happened, water levels were able to breach most beaches on Tuesday morning as sheet flow rather than causing any significant inundation. This was largely because of the delayed landfall of Debbie after the highest state of the tide. The largest recorded storm tide was around 4.1 m at Laguna Quays (1.6 m surge), and 2.7 m at Shute Harbour (0.9 m surge). The worst affected areas that we observed in terms of surge and erosion damage were Seaforth, Midge Point, Laguna Quays, Airlie Beach and Shute Harbour.
At Seaforth, the beach was stripped away and waterfront property damaged (Figure 4 A). At Midge Point there was no surge-related damage because property was well set-back from the shoreline and the mangrove swamps dampened inundation velocities and extents. At Laguna Quays, boats were damaged and the storm tide large enough to break concrete-clad pontoons from their moorings and deposit them above the marina on adjacent high ground. At Airlie Beach and Shute Harbour, several boats were washed up onto the beach (Figure 4 B) and sand and saltwater extended onto the promenade.
Figure 4 A) Storm surge damage at Seaforth, B) boats washed up onto Airlie Beach.
Debbie has probably caused more damage after her transition to a Tropical Low than during her time as a Severe Tropical Cyclone. While witnessing the impacts of Debbie first-hand was impressive, the most striking thing we saw was the positive attitude and community spirit of Queenslanders to clean-up and get-by under very stressful circumstances.
Thanks to residents at Seaforth for providing valuable information on surge impacts in the middle of their clean-up operation. Thanks also to the Cyclone Testing Station group at James Cook University for sharing their Severe Wind Hazard Preliminary Assessment, and RegionalAir charter flights from which all aerial photographs were taken.