Tropical Cyclone Yasi, Briefing 1

Tropical Cyclone Yasi, Briefing 1 (February 7, 2011). Matthew Mason, Risk Frontiers Severe tropical cyclone Yasi made landfall early on the morning of February 3, 2011 near Mission Beach on the Queensland north coast. At landfall the Australian Bureau of Meteorology (BoM) reported Yasi as a category 5 storm on the Australian Cyclone Scale, i.e. surface 3 second wind gusts > 279 km/h. Yasi was a large cyclone with damaging wind gusts forecast over hundreds of kilometres along the coastline. Despite the Category 5 rating, Bureau of Meteorology stations didnt record wind gusts approaching the 279 km/h threshold leading to speculation the event was somewhat conservatively assigned the upper most rating. Figure 1 shows the geographical distribution of maximum gust wind speeds recorded by BoM stations. Maximum gusts of 185 km/h were recorded at Willis Island and Lucinda Point. 93 km/h was recorded at Cairns, 130 km/h at South Johnstone (near Innisfail) and 135 km/h in Townsville. As expected, winds were greatest on the southern side of the eye with gusts greater than 100 km/h persisting for several hundred kilometres inland. The large physical size of the storm causes it to decayemore slowly than typically occurs with landfalling cyclones on Queenslands east coast. The central pressure at landfall was around 930 hPa, measured at the Clump Point gauge in close proximity to the eye. Fortunately, Yasi came ashore as low tide approached, minimising the impact of storm surge. Nonetheless a peak storm tide residual (surge depth) of approximately 5.35 m was recorded at Cardwell, located to the south of the eye, and is reported to have caused major damage. Cairns and Townsville reported minimal, if any, impact from surge. The surge tides recorded by the Queensland Department of Environment and Resource Management gauges were 1.15 m in Cairns with 2.35 m in Townsville.

Initial Observations

Cairns Cairns survived Yasi with only minimal damage, with damage mostly restricted to downed trees or branches. This is not surprising given that the peak recorded gust at the citys airport was below 100 km/h. In the city centre there was, less than two days after landfall, little sign that anything major had occurred. Moving to the outer, more tree lined residential suburbs, the impact was more obvious with damage to trees and branches on most streets. A few instances of trees fallen onto and damaging homes were seen, Figure 2, but the amount of housing material in the street side debris piles was negligible.

Tropical Cyclone Yasi, Briefing 1

Despite the relatively benign wind speeds (as far as cyclones go), there was some evidence of failures to partially engineered structures. The first observed case was damage to the fabric roofing of the Redlynch Sports Stadium, Figure 3, and the second was failure of two roofs at the neighbouring shopping centre, Figure 4. All windicators suggest winds came over the mountains to the south-west, possibly causing some localised gustiness in the wind field. The failure of the car shade roofs looks to be an intriguing failure driven by a complex flow interaction between the wake behind the first upwind roof (no damage) and the second and third roofs (both failed). Similar types of wake driven failure have been an issue for wind engineers designing stadium roofs.

Innisfail Inspecting the town of Innisfail, minimal structural damage was evident, but tree failure was more prevalent than in Cairns. Given the peak gust of 130 km/h recorded at the South Johnstone weather station (about 10 km south-west of Innisfail) this was expected. In addition, through inspection of street side debris piles and conversations with local residents, failure of garden sheds and shade awnings was also an issue, Figure 5. Travelling with a team of investigators from the James Cook University Cyclone Testing Station we visited a number of residential and commercial buildings damaged during Cyclone Larry in 2006 to determine how they performed this time. In all cases buildings performed well with no major failures evident. Winds were, however, weaker in Innisfail during Yasi (about 200 km/h during Larry) and were from a more westerly direction as compared with north-south during Larry. The topography of Innisfail meant that some of the buildings damaged during Larry were well shielded during Yasi and will have experienced much lower wind speeds. However Innisfail is a much more resilient town today than it was in 2006 with the large number of damaged homes significantly strengthened when rebuilt or renovated using up-to-date building standards.

KurramineBeach Travelling further south with the CTS team, stark evidence of increasing wind speeds was clear as we moved down the Bruce Highway. More damage to crops, sheds and an increase in stripping of foliage was evident. Moving into Kurramine Beach, which was hard hit by Larry (gusts around 200 km/h), damage to homes was also more evident. Well known issues with internal pressurisation, Figure 6, and the constant problem of failure of garage doors, Figure 7, seemed to be driving this observation. Failures were more pronounced on older buildings, Figure 8, with many of the homes rebuilt or subsequently constructed since Cyclone Larry performing extremely well. Evidence of storm surge was seen in Kurrimine Beach with sediment up to about 40 m inland, Figure 9. From our (limited) observations it did not appear as though this had affected any permanent structures (some caravans looked to be affected).

Tropical Cyclone Yasi, Briefing 1

Figure 1. Severe Tropical Cyclone Yasi track and peak gust wind speed observations.

Tropical Cyclone Yasi, Briefing 1

Figure 2. Damaged home in Manunda, Cairns.

Figure 3. Damage to shade sails at the Redlynch sports stadium.

Tropical Cyclone Yasi, Briefing 1

Figure 4. Failure of roofing at Redlynch shopping complex.

Figure 5. Undamaged home in Innisfail with remnants of the shed in a debris pile in the front yard.

Tropical Cyclone Yasi, Briefing 1

Figure 6. Internal pressurisation of the lower storey popping the window frame out.

Tropical Cyclone Yasi, Briefing 1

Figure 7. Failure of garage door and awnings on a home in Kurrimine Beach

Figure 8. Failure of an older home on the Bruce Highway north of Kurrimine Beach.

Tropical Cyclone Yasi, Briefing 1

Figure 9. Evidence of storm surge in Kurrimine Beach.