The destruction wrought by Cyclone Marcia is likely to further spur the adoption of more resilient construction methods in coastal Queensland during the clean-up period and subsequently.
As the threat of Cyclone Marcia recedes experts and emergency workers are just beginning to catalogue the extent and cost of the damage it's inflicted on Queensland's coastal communities.
Marcia is one of the most extreme weather events to have ever hit Australian shores. According to experts it is only the sixth category five cyclone – the highest level on the Australian Cyclone Severity Scale, in Australia’s recorded history.
The cyclone tore through coastal parts of Central Queensland, with winds hitting speeds of 285 km/h in the town of Yeppoon situated just 42 kilometres to the north of Rockhampton.
By the time Marcia reached Rockhampton to the south it was still categorized as a category three cyclone, ripping apart roofing and trees, as well as destroying a significant number of low-set residences.
A total of 540 homes situated in the Yeppoon-Rockhampton area have suffered from structural damage or water inundation. According to Queensland Premier Annastacia Palaszczuk 56 schools have been damaged, and flooding levels in the town of Bundaberg are expected to reach 4.5 meters.
While the Insurance Council of Australia has refrained thus far from providing an estimate of the likely repair costs, Cyclone Yasi, another category 5 system which hit northern Queensland four years, caused approximately USD$3.6 billion in damage.
Marcia is the latest in a string of extreme weather events that have beset the Sunshine State since the start of the new decade, serving as a further potent reminder of the heightened risk of wind and flood damage in certain parts of the country due to global climate change.
The destruction wrought by Cyclone Marcia is likely to further spur the adoption of more resilient construction methods in coastal Queensland during the clean-up period and subsequently – measures that the state government has made efforts to encourage since the aftermath of Cyclone Yasi.
The Queensland Reconstruction Authority (QRA) outlined a raft of resilient construction measures in the wake of Yasi directed specifically at dealing with the possibility of similar contingencies in future such as Cyclone Marcia.
Chief amongst them is situating the floors of new residential dwellings at least above the storm tide height for extreme cyclones, by means of appropriate site selection or the construction of high-set houses.
Depending upon the height of the building floor, this can provide protection again the storm surges caused by extreme cyclones occuring between once every 50 to 500 years on average.
This measure is critical given the vast difference that flooding heights make to structural damage. Minimal structural damage is suffered if floor heights are above the level of water inundation, while damage to floor and plaster walls is only likely if the water level is 200mm above floor level.
Above the 1 metre threshold, however, walls and unreinforced masonry are highly likely to suffer significant damage, alongside housing interiors and fixtures.
According to QRA high-set housing provides the best safeguard against property damage during a storm tide, as long as it features unenclosed structures at ground level that do not transfer the heavy force of storm tides into the structural frame. QRA advocates the adoption of “flow-through” designs that facilitate the movement of water and waves through the ground level, while providing simultaneously support to the structure on top.
For low-set houses on stumps, floor levels should be raised as high as possible, while robust, impact resistant materials should be used, such as timber or thick plywood cladding in lieu of thin fibre-cement or metal sheeting.
Any ramps or stairs should be in alignment with the movement of incoming waves in order to minimize impact force, while cantilevered bracing columns or timber crossing sets should be used in lieu of subfloor walls or bracing panels for bracing purposes.
The wind resistance of new or rebuilt houses is another critical factor that must be considered in cyclone-prone parts of the Sunshine State.
The key to this is structural designs that provide load paths capable of resisting the force imposed by winds upon buildings and transmitting them to the ground, as well as footings that possess sufficient weight or embedment to resist uplift forces.
Australia already has two standards for the wind load requirements of residential dwellings - AS/NZS1170.2 and AS4055, with requirements varying depending upon wind site classifications.