Science to show impact of Cyclone Debbie

By Freya Jones. This article first appear in Issue Three 2017 of Fire Australia.

A team of Bushfire and Natural Hazards CRC researchers have investigated the damage caused by Tropical Cyclone (TC) Debbie in Queensland’s Whitsunday region during March. In the lead-up to and immediately after TC Debbie, the team  set up wind monitors and assessed building damage. The team members were Dr David Henderson, Dr Geoff  Boughton, Dr Daniel Smith, Ms Debbie Falck, Mr Mitch Humphries and Mr Korah Parackal of the Cyclone Testing Station at James Cook University, and Mr Thomas Kloetzke of the University of Queensland. 

TC Debbie made landfall near Airlie Beach on 28 March 2017. The subsequent invesitgation will inform the CRC project Improving resilience of existing housing to severe wind events. Issues investigated by the team included:

• performance of contemporary buildings, including wind-driven rainwater ingress
• assessment of older housing 
• storm tide damage to buildings.

The team installed six Surface Weather Information Relay and Logging Network (SWIRLnet) towers to measure wind speeds, direction and pressure in the potential path of the cyclone: from Ayr, just south of Townsville, through to Proserpine. The 3.2-m-tall towers used anemometers to continuously collect data in the affected areas before, during and after landfall. The highest wind gust recorded by the SWIRLnet towers was 125.7 km/h in Bowen.

On 30 March, the team headed south to Proserpine and Bowen. In Proserpine, they conducted street surveys and observed minor structural damage of contemporary housing, while some older housing and commercial properties suffered damage to sections of roof structure.

The team noted partial separation of roof members, even though no major structural roof damage was initially obvious from the outside. After seeing similar damage—such as battens partially  separated from rafters—in investigations following TC Larry (2006) and TC Yasi (2011), the researchers recommended inspections by qualified builders, building surveyors or structural engineers of the inside roof structure of buildings (typically older houses) in areas where structural damage to other buildings has occurred (i.e. Bowen, Proserpine and Airlie Beach).

In Airlie Beach, the investigation focused on issues relating to water ingress of contemporary housing and strata buildings. The issues identified were primarily associated with windows and doors, though other water paths were noted, such as loosened or bent barges, removed flashings and blocked gutters. 

Nearby Dingo Beach and Hideaway Bay reportedly experienced the eye of TC Debbie, but showed fewer signs of structural damage. The team spoke to residents who described a two-hour lull during the passage of the eye. 

“Despite experiencing severe winds during the event, most houses at Dingo Beach and Hideaway Bay had no obvious signs of damage,” Dr Henderson noted. “The majority of houses observed were relatively new, built since 2000.” 

Along the Whitsunday coast, at Conway and Wilson Beaches, the team noted wind and storm tide damage to properties. In Wilson Beach, water levels and wave action saw one older house lifted off its piers. Water levels throughout houses varied from less than 300 to 1,100 mm.

“Residents who remained during the cyclone reported that the rising seawater came from over the beach dune and from the mangroves and channel to the rear of community,” Dr Henderson explained. 

With the absence of substantial wave action on most properties, structural damage was minimal. However, inundation damaged most of the linings, cabinetry and wiring. 

Hamilton Island, along with the other Whitsunday Islands, bore the brunt of TC Debbie. Tree and vegetation damage appeared to be much worse across the island than at Airlie Beach and Proserpine. 

“Structural issues with window fixings and frames were observed in some of the buildings on Hamilton Island,” said Dr Henderson.

Issues ranged from window reveals not being secured to the building frame to minimal-capacity lintels unable to resist lateral wind pressures, and sliding doors bowing out of tracks. Examples of wind-driven rain entering buildings were also seen.

“Several modern buildings at Hamilton Island and on the mainland had roof overhangs or awnings that had failed at the structural supports and then peeled back over the building, causing more damage,” Dr Henderson noted. 

Further damage to larger buildings was reported, including the airport, multistorey apartments, apartments under construction and newer houses in  locations exposed to the south-east.

The team has completed a formal report, available at www.jcu.edu.au/cyclone-testing-station.

The Cyclone Testing Station has a series of videos for both homeowners and builders that provide information about repairing damaged roofs from cyclones. The videos can be found at www.youtube.com/user/cyclonesandbuildings/videos.