Freak storm

A large number of properties in the Huntingwood, Bungarribee, and Eastern Creek areas were damaged when a storm front swept through the western suburbs of Sydney on 25 April 2015 at about 3.30pm. Questions were raised as to why the storm had caused the collapse of at least eight warehouse structures. Many properties suffered significant damage associated with roof leakage and water penetration. Costin Roe Consulting was commissioned to evaluate the storm event and report on the likelihood of re-occurrence in the context of design standards for Australian buildings.

“Discovering the true nature of this Anzac Day 2015 storm in Western Sydney was a very interesting exercise in forensic engineering,” said Grant Roe, director of Costin Roe Consulting. “We were presented with numerous localised examples of warehouses which had collapsed. Similar types of buildings surrounding the collapses appeared to have been able to withstand the same storm event. It could have been assumed from preliminary circumstantial evidence that there may have been some major differences in the design or construction between affected and non-affected buildings. The role of forensic engineering is to determine, analyse, interpret, and explain all relevant facts so that the truth can be revealed.”

Forensic investigation by Costin Roe Consulting

Mark Wilson, BEng(Civil) BSurv ME CPEng, Associate Director (Civil Engineering), Costin Roe Consulting

The report ‘Extreme Storm Event – Huntingwood NSW 25 April 2015’ prepared by Mark Wilson, BEng(Civil) BSurv ME CPEng, Associate Director (Civil Engineering) of Costin Roe Consulting

“When any structural failure is examined, one of the more obvious lines of enquiry would be to determine if the structure had been compliant with relevant standards in the first place,” Grant Roe said. “Where both affected and non-affected buildings met or exceeded regulatory requirements, there had to be some deeper explanation for the seemingly randomised failure versus survival of similar structures in a localised area during the same storm. For this deeper line of enquiry we assigned Mark Wilson, BEng(Civil) BSurv ME CPEng, Associate Director of Costin Roe Consulting, to evaluate Bureau of Meteorology (BOM) data, as well as eyewitness reports, on-site observations and a range of imagery, to determine what had happened and if it could be expected to happen again.”

“The total repair bill arising from this storm would have been astronomical. A forensic engineering report is vital for building owners and insurers when it comes to understanding and managing risk to property. There was also a human imperative to determine the cause of damage on this occasion because if it had not been Anzac Day with most workers away on leave, there could have been much greater risk to safety as a result of storm activity at some of the affected sites,” said Grant.

Storm was a “freak”

The report by Costin Roe Consulting titled ‘Extreme Storm Event – Huntingwood NSW 25 April 2015’ detailed the astonishing peculiarities about the nature of the storm which moved eastwards across Sydney in the middle of last Anzac Day afternoon. “Our findings could have surprised anyone,” said Grant Roe. “This storm was a freak, in that it was a unique and extremely unusual event.”

“Mark Wilson’s research and analysis indicated that this particular type of extreme storm event was known to occur in parts of the northern hemisphere but not normally within Australia,” Grant said. “Some areas within this storm’s reach experienced only light to moderate rainfall. In other areas there were distinct zones of intense storm activity where the resulting structural damage could have been attributed initially to an unusually intense hail fall; or wind, rain and hail in combination.”

Primary cause of damage not wind or rain

Great Western Highway closed by excessive hail

Sections of the Great Western Highway were closed by an excessive accumulation of icy snow-like material (photograph © 2015 Taylor Constructions)

BOM data showed that wind pressure was not a key factor in the structural damage caused by the Anzac Day storm. According to report author Mark Wilson, the 30-40km per hour local wind gusts represented a small fraction of the wind pressure tolerance provided by modern building design. Rain was also dismissed as a primary contributor to structural damage. “At nearby automatic weather stations the rainfall was recorded at no more than 20-30mm per hour over about 60-90 minutes of localised storm activity. Just as we had discovered with our evaluation of local wind data, the rainfall component of this storm was relatively light. The heavier episode of rainfall occurred after the hail fall which in itself lasted for about 10-15 minutes. Automatic weather stations nearest to the affected areas were not equipped to measure hail or snow, however,” said Grant Roe.

“It was widely known from news bulletins and social media posts that the Great Western Highway, M4, and other roadways in the area were partly blocked by hail which was 300-500mm deep at some locations. Yet within a short distance either side of these locations, we found that the hail accumulation had been negligible,” Grant said. “This radicalised pattern of intense hail fall was unlike anything commonly seen in Australia. Further forensic investigation was warranted.”

BOM data shows intense storm activity around Huntingwood in Western Sydney

BOM radar images show the heaviest rating for precipitation intensity (darkest red) around Huntingwood in Western Sydney at 3.24pm on 25 April 2015 (image © 2015 Australian Bureau of Meterology)

Satellite imagery suggested that some of the warehouse buildings in the Huntingwood and Eastern Creek areas were subjected to hail precipitation which could have equated to about 300mm per hour in rainfall had local automatic weather stations been fitted with heating devices to allow accurate gauging of icy precipitation. “To contextualise the equivalent of 300mm per hour in rain precipitation, based on all recorded meteorological data, the worst-case prediction for this area was an extreme rainfall event of 220mm per hour to occur no more often than once in any 100 year period,” said Grant. “The 300mm equivalent precipitation surmised to have occurred on 25th April 2015 was an event expected to occur in this area only once in every 10,000 or more years. As Mark Wilson was bringing to light more detailed meteorological evidence and evaluating the implications, gradually the phenomenal nature of this particular storm event was being defined and confirmed.”

“…an event expected to occur in this area only once in every 10,000 or more years.” – Grant Roe

Load tolerances exceeded by snow

“Another very unusual aspect of the Anzac Day storm was that the activity described as an intense hail fall predominantly consisted of an icy firm snow,” Grant Roe said. “The accumulated material would have been denser in composition than the hailstone formations typically seen in storms around Sydney. Some buildings examined during our investigations would have had roof areas covered by heavy snow-like material to a depth of anything up to half a metre. The weight of such thick, dense coverage on some of the roofs would have been up to 7-10 times greater than the maximum load-bearing capacity required by current standards. Any typical Australian building if subjected to such extreme stress would almost certainly sustain significant structural damage.”

Hail swaths of destruction

Grant Roe, BE (Hons) MEngSc MBA MIEAust CPEng NPER, director, Costin Roe Consulting

“…on Anzac Day 2015, whether a warehouse had collapsed… came down to luck or the lack of it…” Grant Roe, BE (Hons) MEngSc MBA MIEAust CPEng, NER Director of Costin Roe Consulting

“The rare nature of this type of storm event meant that the icy firm snow material fell very heavily within narrow bands of extreme weather intensity called hail swaths,” Grant Roe said. “Between the hail swaths, storm activity would have been milder. This meteorological phenomenon explained how very dense debris had accumulated up to half a metre deep on the surface of one collapsed section of warehouse roofing, for example, or one collapsed warehouse roof, while at the same time other sections of roofing and roofs on nearby buildings had received much more moderate loads.”

“If this highly unusual hail swath activity had climaxed over a predominantly residential area in Sydney, the pattern of destruction could have appeared more widespread because the storm would have impacted a greater number of individual properties,” said Grant. “We identified two distinct hail swaths passing over the Huntingwood and Eastern Creek districts during the Anzac Day storm event. Given the more catastrophic damage to at least eight warehouses in these industrial areas where the roof of one major warehouse could hypothetically cover a small neighbourhood, any of the buildings in the affected zone could have received an excessive load of dense material dumped by one of the hail swaths, or been spared from the most seriously damaging effects,” Grant said.

“In this particular instance on Anzac Day 2015, whether a warehouse had collapsed or remained fundamentally intact after the storm, it really came down to luck or the lack of it,” Grant said.

Future-proofing against extreme weather

“The extreme weather occurrence at Huntingwood and Eastern Creek on 25th April 2015 was almost entirely unprecedented in Australia. Statistically speaking, it would be reasonable to predict that such an event may never happen in this same area again,” said Grant Roe. “However, considering that the forces of nature can evolve over the ages, there can be no infinite guarantee.”

“Australian Standards allow ample protection against extremes of weather and other impacts that we could reasonably expect a building to withstand over its lifetime, in the context of its specific location and approved purpose,” Grant said.

“…freak weather… can occasionally wreak havoc upon built environments.” – Grant Roe

“All over the world, it has always been a fact of life that freak weather and other unpredictable and unmanageable events can occasionally wreak havoc upon built environments. We can only continue to learn about such events, and improve our responses.”

Video of the Anzac Day 2015 storm made at various locations around Western Sydney by ‘storm chasers’ (amateur weather-watchers) as published on Official data sources, videos, photographs, news reports, and eye-witness accounts were included in the broad range of holistic evidence analysed by Costin Roe Consulting.