Assessing the impacts of tropical cyclone Tracy on residential building stock - 1974 and 2008

Abstract

Tropical cyclone Tracy (Tracy) remains one of the most destructive natural hazard events in Australia's history. Growth in the population and size of Darwin since 1974 makes it desirable to know what impact an event similar to Tracy would have on the present day built environment. To assess the impacts in 1974 and the present day, we apply the Tropical Cyclone Risk Model (TCRM) developed at Geoscience Australia.A parametric wind field generated by TCRM is applied to building damage models in an attempt to reproduce the widespread damage to residential structures associated with Tracy in 1974. Employing these models yields a mean damage estimate of 36 per cent of replacement cost across all residential building stock in Darwin -a figure lower than that determined by post-event damage assessments. The unaccounted impact of large windborne debris is one possible explanation for the discrepancy between the observed and simulated damage.Based on the satisfactory replication of the damage associated with the historical impacts of Tracy, the wind field is then applied to the current day residential building database in order to assess the impact of Tracy were it to strike Darwin in 2008. We find that the mean damage to Darwin for the same urban footprint as the 1974 analysis in the present day would be around five per cent. This represents an approximately 90 per cent reduction in the modelled damage, and a significant portion of this reduction can be attributed to building code improvements.

Local wind assessment in Australia: computation methodology for wind multipliers

“…It is well recognised from the damage surveys following tropical cyclone Tracy (Walker, 1975), Winifred (Walker et al, 1988), Iniki (Fujita, 1993), Hurricanes Hugo (National Research Council, 1994), Marilyn (Wernly, 1996) and Fabian (Mueller et al, 2006) that structures located on hillsides and hilltops are more susceptible to wind damage than those located at lower elevations (Powell and Houston, 1998). The phenomenon that flow accelerates near the crests of hills with a flow separation bubble extending downwind on the lee side is also evident in the studies investigating the influence of hills and escarpments on wind flow (Powell and Houston, 1998;Schofield et al, 2010). It is therefore necessary to include the topography multiplier to assess topographically induced wind gusts.…”

“…As important components of the assessment methodology, the terrain, shielding and topographic multipliers have been computed in a comprehensive way as prescribed by the Australian wind loading standard AS/NZS 1170.2. For example, Schofield et al (2010) applied Geoscience Australia's Tropical Cyclone Risk Model (TCRM) to simulate the Cyclone Tracy wind hazard, and further estimated building losses under current economic and societal conditions. After TCRM produced an estimate of the regional peak gust wind speed associated with tropical cyclone Tracy (on a grid of approximately 1 km resolution), wind multipliers were introduced and calculated at each specific location (25 m horizontal resolution) in order to incorporate the influence of terrain, topography and shielding, using the methodology developed at Geoscience Australia.…”

Low coherency of wind induced seismic noise: implications for gravitational wave detection

“…In order to provide reliable velocity resolution at this frequency [22], a seismic array will require an aperture of km scale for the spatial sampling of these large wavelengths. However, we do not need such a large aperture size to study the impact of wind on seismic coherency owing to the absence of regional roughness and topographically induced wind gusts known as topography multipliers [23]. HOPF is within a vast zone of remnant coastal belt sand dunes and associated shoreline deposits [24] with maximum 50 m variation in altitude over 10 km.…”

Low coherency of wind induced seismic noise: Implications for gravitational wave detection

“…This justifies apertures of Km scale for the spatial sampling of their large wavelengths to improve angular and velocity resolution [19]. However, we do not need such a large aperture size to study the impact of wind on seismic coherency owing to the absence of regional roughness and topographically induced wind gusts known as topography multipliers [20]. HOPF is within a vast zone of remnant coastal belt sand dunes and associated shoreline deposits [21] with maximum 50 meters variation in altitude over 10 km.…”