Bushfire impact research – NSW South Coast

Steven George, James O’Brien, Salomé Hussein, Jonathan van Leeuwen, Risk Frontiers

Risk Frontiers deployed a team to the NSW South Coast region in late January, 2020 to undertake damage surveys following the bushfires. This research was supported by the Bushfire and Natural Hazards CRC (BNHCRC). The areas surveyed included Moruya, Mogo, Malua Bay, Rosedale, the Catalina area of Batemans Bay and Lake Conjola. The majority of damage occurred on December 31, 2019 as catastrophic weather conditions (extreme temperatures and strong winds) intensified existing fire fronts. The conditions transported large quantities of embers into vulnerable communities, destroying hundreds of residential and several commercial buildings. In total, the survey identified 426 bushfire affected properties, most of which were destroyed. Industries/infrastructure affected included: bowling/services club, a unit block (12 units), heritage park, industrial complex with numerous businesses and extensive damage to electricity infrastructure (power poles and wires along the Princes Highway). This report complements our report for northern NSW (Risk Frontiers, 2020).

Building age and resilience

As the 2019/2020 fire season progressed, the scale of damage and losses experienced across the country engendered a growing interest in evaluating the resilience of buildings to bushfires. Aspects of buildings such as age, performance of construction materials and a structure’s vulnerability due to its proximity to bushland were the key focus of the NSW South Coast survey. To evaluate the performance of building archetypes impacted by fire, the Insurance Council of Australia (ICA) charted the year of construction of over 25,000 residential buildings located within bushfire impacted areas across four states (Figure 1). Categories range from Old Colonial (pre-Victorian) to post-2009, when bushfire building standards began to be improved and were mandated in certain locations.

Building construction types impacted by 2019 bushfires in Australia
Figure 1: The period of construction for over 25,000 buildings located within the current bushfire impacted areas across four states. Source: Insurance Council of Australia, 2019.

The ICA data shows that only 9.5% of residences were constructed post-2009, when changes were made to Australian Standard 3959 after the Black Saturday fires of February, 2009, to ensure that new buildings in bushfire-prone areas were safer and more likely to survive a fire (BNHCRC, 2019). It was apparent that the scale of residential losses occurring this fire season presented a small window of opportunity to conduct further damage surveys, prior to recovery and debris removal, and would provide a considerable ‘post-2009’ cohort to assess building performance and inform future design. In the near future, further analysis will be undertaken by Risk Frontiers to establish the construction age of the South Coast properties, with a focus on any post-2009, to expand existing research.

Observations of destruction/damage – construction materials

The survey team recorded aspects of fire affected buildings such as construction materials and damage ratios (destroyed/partially destroyed). The field observations from the South Coast survey are compared to those in Rappville (2019) and Tathra (2018) in Figure 2.

Proportion of buildings destroyed  South Coast, Tathra and Rappville
Figure 2: The proportion and number (in column) of buildings categorised as destroyed/partially destroyed. The South Coast and Rappville (2019) damage surveys used a building footprint method where partially destroyed references the building, not the lot. The sampling method figures from the Tathra fire in 2018 assigned partially damaged on the proportion of the whole lot – that is, if a shed was destroyed but the house was undamaged, then a partially damaged rating was assigned. The data shows that once a building is alight, the likelihood of it being destroyed is very high. The total destruction rate across the three events ranged between two-thirds and 100%. The number of properties destroyed also indicates the difference in scale of the fire events between locations.

The South Coast findings reinforce those from the Rappville (2020) and Tathra (2018) surveys, in that, once a building catches fire, regardless of construction material, it will likely be totally destroyed. The official Tathra figures have 68% of all fire affected premises as being ultimately destroyed. Data collected from the South Coast and Rappville surveys provides much stronger indications of this trend, where 92% and 100% respectively, of the buildings observed were destroyed. (The Rappville and South Coast results represent only those properties located and observed, not all fire-affected properties).

In terms of building specifics, The South Coast survey provided numerous examples of fire-affected residences, primarily constructed of ‘non-flammable’ materials (brick and blockwork (piers and walls)). These structures demonstrated some resilience to the fire, at times remaining wholly or partially intact. However, the remaining material comprising the premises (structural roof/wall timbers, internal walls and house contents), once alight, ultimately rendered the entire building unsalvageable (destroyed). Timber beams supporting house roofs and carports were uniformly level on the ground (as though dropped). Metal framed buildings (e.g. sheds) and structural elements (e.g. lintels) did not perform well – failing due to extreme heat and leading to the building warping and impacting brick/masonry when collapsing. There were numerous examples of vehicles completely burnt out in front and rear yards and some isolated examples of aluminium boats that had undergone some degree of melting.

For partially destroyed properties, the building features most often impacted were constructed from timber such as external stairs and decking as well as external cladding. There were numerous examples of destroyed properties categorised as ‘asbestos contaminated’ though this was less common than during the Rappville survey where asbestos was present at over 50% of properties. A large number of asbestos contaminated assessments were speculative based on observations and erred on the side of caution with further assessment and testing usually noted as necessary. The possible exception to this would be Rosedale which experienced near total destruction and where homes predominantly appeared older, were often constructed using fibro or sheeting and surrounded by bushland.

Statistical dependence of bushfire risk on distance to bush and the influence of ember attack

Previous field research conducted by Risk Frontiers (Chen and McAneney, 2004 and other more recent) has established that proximity to bushland is the most important factor in determining a building’s vulnerability. Figure 3 depicts bushfire damage based on aggregated data from recent major bushfires and shows the cumulative distribution of destroyed buildings in relation to distance from bushland.

As with previous fires studied, Figure 3 confirms the significant role that ‘proximity to bushland’ played in the South Coast losses where approximately 38% of destroyed buildings were situated within 1 metre of surrounding bush. The average distance from bushland of all 426 properties surveyed was 55 metres (satellite imagery). However, a feature not obvious from the South Coast data in Figure 3, but apparent in the Rappville and Duffy examples, is the impact of extreme conditions and the capacity of embers to propagate fire over large distances. Witness accounts from fire fighters and locals have described embers being transported by extreme winds across Lake Conjola, over distances greater than 1km. The South Coast survey data would appear to confirm such reports, as two properties surveyed were >1.3km from bushland and 73 were located >100 metres from bush.

Buildings destroyed in relation to distance from bushland
Figure 3: Cumulative distribution of buildings destroyed in relation to distance from nearby bushland for recent major events.

References

BNHCRC 2019. Black Saturday ten years on – what did we discover?

Insurance Council of Australia 2020. Period of residential building construction – chart. Posted by Karl Sullivan.

Risk Frontiers’ Newsletter Vol. 19, Issue 1.

Risk Frontiers’ Newsletter Vol. 17, Issue 3.

Chen, K., and K. J. McAneney, 2004: Quantifying bushfire penetration into urban areas in Australia. Geophys. Res. Lett., 31, L12212.

Acknowledgement

This research was funded through the Bushfire and Natural Hazards CRC Quick Response Fund.