REVIEW OF NT CYCLONE RISKS Cyclone Monica at 2 PM on 24th April 2006 Image Source https://www.yoko.npmoc.navy.mil A REPORT Prepared by Mike Nicholls for: Community Group for the Review of NT Cyclone Risks Inc. March 2007 Review of NT Cyclone Risks ACKNOWLEDGEMENTS Thanks are given to the many people who have assisted with the project. Particular mention is made of the assistance provided by Mr. Geoff Garden and Mr. Ian Shepherd of the Bureau of Meteorology, Darwin; by Prof. Robert Wasson at the Charles Darwin University and Dr Patrick Baker at the Monash University. Much credit must also be given to the members of the Community Group of the Review of NT Cyclone Risks Inc, who have attended meetings, assisted with administration and provided invaluable assistance in reviewing and editing the final report. COPYRIGHT Intellectual Property Rights reside with the Community Group for the Review of NT Cyclone Risks Inc. (the Group). 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The Australian Government had no role in the development of this report other than funding it through Australian Government Local Grant Scheme (2005 – 2006) Review of NT Cyclone Risks CONTENTS Page 1 INTRODUCTION 1.1 Project Background ……………………………………………………... 1 1.2 Community Group for the Review of NT Cyclone Risks Inc ………... 2 1.3 The writer’s background ……………………………………………….. 3 1.4 Intended readership and method of presentation ……………………. 3 1.5 What is meant by Risk? …………………………………………….….. 4 1.6 Initiatives by governments to mitigate natural disaster risks ……..… 6 1.7 Past and current studies by NT government on cyclone risk ………. 9 2 PROBABILITY OF AN INTENSE CYCLONE OCCURRING 2.1 Engineering Estimates (a) Summary……………………………………………..………………… 10 (b) From written records – 1824 to 2006, within 50 km of a settlement…… 12 (c) From satellite era records – 1974 to 2006, within 500 km of Darwin…. 16 (d) From the Wind Code used for building design………………………… 20 (e) From two recent reports to NT Government…………………………… 26 (f) Conclusions and Recommendations…………………………………… 27 2.2 Indications from Meteorology, Climatology and Wind Damage & Storm Tide (a) Summary ……………………………………………………………… 29 (b) Tracks and swathes …………………………………………………… 29 (c) Maximum Potential Intensity…………………………………………. 33 (d) Other regional differences – NT, WA and Qld .……………………… 35 (e) How accurate are the Bureau’s wind speed estimates for the NT? …… 40 (f) How accurate are estimates of wind speed from wind damage? …….. 46 (g) How accurate are estimates of wind speed from storm tide levels ?.... 49 (h) Conclusions and Recommendations………………………………….. 50 2.3 Estimating from Nature’s Long-term Records (a) Summary ……………………………………………………………… 52 (b) Tree-Rings……………………………………………………………. 53 (c) Beach Ridges and Lagoon Sediments…….………………………….. 57 (d) Conclusions and Recommendations for future research…………….. 63 2.4 Possible Effects of Climate Change (a) Summary …………………………………………………………….. 67 (b) Statement by world authorities ……………………………………… 67 (b) Thelma, Ingrid and Monica …………………………………………. 68 3 COMPILATION OF RECOMMENDATIONS……………………. 70 4 REFERENCES ………………………………………………………… 76 i Review of NT Cyclone Risks APPENDICES Appendix A Tables of Properties & Unit Conversions Appendix B Glossary Appendix C Risk: The Hazard and the Consequences Appendix D Wind Code Speeds – Derivations and Variations Appendix E Cyclones within 50 km of a Settlement, 1824 to 2006 Appendix F Review of Darwin’s 1937 cyclone Appendix G Cyclones within 500 km of Darwin, 1974 to 2006 Appendix H Gust Speed Weakening after Landfall Appendix I Two Recent Risk Studies for Darwin Appendix J Research on Tree-Rings (Baker) Appendix K Beach Ridges and Lagoons (NT locations and images) Appendix L Research on Coral Rubble Ridges, NW Vernon I. (Wasson) Appendix M Research on Lagoons (preliminary investigation on a tidal flat) Appendix N Submission for a Review of the Bureau of Meteorology Appendix O The ‘TIM’ Cyclones – Just a Chance Cluster? ii Review of NT Cyclone Risks FOREWORD This report deals with the likelihood of an intense cyclone occurring but not the physical consequences of or measures required to mitigate the effects of such events. It questions the currently accepted methods used to determine the risk for the NT coast from intense cyclones. As a result, it concludes that the risk of an intense cyclone impacting Darwin is seriously underestimated. It recommends that the risk values currently used be urgently reviewed in the light of the above findings. If this conclusion is confirmed then current building standards for dwellings and shelters and disaster response plans will require substantial review. The main report is a presentation of the conclusions and includes supporting arguments. The appendices provide more detailed technical discussions in support of the arguments in the main report. The results presented in this report are ‘ball park’ estimates using raw data and then common sense is used to arrive at the conclusions. Engineers routinely use such ‘back of the envelope’ calculations to check that results arrived at using more complex methods do in fact make sense. (I recall that when electronic calculators came into common use, rough manual calculations were often utilised to make sure that the electronic results were realistic). This approach can be quite useful to identify potential flaws in the methods and assumptions used in more complex methods - as this report amply demonstrates. The report suggests there may be flaws in: Elements of the records used; The interpretation of the record; and, The current understanding of cyclone behaviour and characteristics in the Darwin Region. Also, the period of record commonly used for determining the risks is very limited. It is suggested that natural and cultural records (tree rings, sediment analysis, contemporary written & anecdotal evidence, etc.) be used to extend this record. This approach may identify a more realistic pattern of intense cyclone frequency and severity along the NT coast. It is further suggested that current government funding for studies and mitigation measures is woefully small compared to the potential costs associated with an intense cyclone strike on the NT coast in general and Darwin in particular. It recommends a significant increase in funding. I am convinced that the evidence presented in this report supports the conclusions reached. Sufficient doubts have been raised to support the call for a comprehensive review of risks and the allocation of adequate funding to conduct the recommended investigations and research programs. Herman Mouthaan 16th March 2007 iii Review of NT Cyclone Risks 1. INTRODUCTION 1.1 Project Background This report was prepared by the Community Group for the Review of NT Cyclone Risks Inc. The object is to review cyclone risk for Top End coastal communities in terms of the likelihood of an intense cyclone impacting any particular locality. The detailed consequences of such an impact are not examined in this report. KEY FINDING The review has found that that the risk of a Category 5 cyclone impacting Darwin, or another NT coastal community, may be much higher than is currently provided for in government building codes and regulations. Measures taken by governments to minimize or mitigate the immediate impact of a cyclone are obviously important. However the only aspect of mitigation examined here will relate to the further research that is necessary to make realistic estimates of the likelihood of a Category 5 impact. Furthermore, only the risks resulting from high winds or storm surge are reviewed. Flooding of major rivers in the Top End is also normally caused by cyclones but it is not examined here. There are thirty-two coastal towns and communities and many more small out-stations along the Top End coastline.