PDF | 2 MB - Australian Building Codes Board

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PDF | 2 MB - Australian Building Codes Board

5. Inland weakening and penetration of tropical cyclones

5.1 Introduction

As the high convection regions of a tropical cyclone cross a coastline from a warm

ocean, the storm loses its source of energy and starts to weaken. This results in

maximum wind speeds falling progressively with distance from the coastline. This is

reflected in AS/NZS1170.2 by 50 kilometre wide strips that designate the wind

regions in the Standard. At present, the width of these strips is the same on the

Western Australian coastline as on the more topographically complex east

(Queensland) coast. The validity of this has been questioned and will be addressed

in this report.

This Chapter reviews recent studies of the inland penetration of tropical cyclones and

hurricanes, with consideration of the relevance to the current regional zoning system.

The current Standard defines the regional strips with respect to the ‘smoothed’

coastline without defining it. Chapter 10 of this report addresses this point and

attempts to establish a workable definition.

5.2 Observations in Cyclone ‘George’ (2007)

Boughton and Falck (2007) have surveyed the available information on wind gusts,

for up to 120 kilometres inland, from Tropical Cyclone ‘George’ which crossed the

Pilbara coast of Western Australia east of Port Hedland in March 2007.

Unfortunately for this event, there was a lack of anemometer data, and few simple

road signs, that have been used in past cyclones as an indicator of upper and lower

limits of wind gust speeds, were available. Hence Boughton and Falck used tree

damage and damage to vehicles, buildings and masts at isolated mining camps to

establish wind speeds at various distances from the coastline. The authors claim

accuracy of +/- 10% for estimates off gust wind speeds made from tree damage and of

+/-5% for predictions made from structures and vehicles.

Maximum gust speeds at Port Hedland on the coast were estimated at 55m/s from the

failure of road signs. However the township was located 60 to 70km west of the

storm track and outside of the eye wall of the cyclone. Tree damage in Port Hedland

was subsequently used to calibrate observed damage at other parts of the track.

Estimates of 200 to 270 km/hour (55 to 75 m/s) at locations along the storm track

about 50 kilometres from the coast were made, but these are quite wide limits, and it

is difficult to judge the weakening of the storm without also having wind speeds at

landfall.

Calculations from the failure of the top of a radio mast at Strelley about 50 km inland

along the track gave a peak gust of 64 to 78 m/s. However, the authors apparently did

not consider possible resonant vibrations of this slender structure which would have

amplified the stresses. Ignoring this possibility may have resulted in overestimates of

the peak gusts.

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