Beach Nourishment to Mitigate the Impact of Sea Level Rise in Southeast Australia

MASTER OF SCIENCE THESIS Beach Nourishment to Mitigate the Impact of Sea Level Rise in Southeast Australia J.M.P.A. Langedijk November 2008 Disclaimer: This is a preliminary report. The final report will be uploaded to this website when all comments have been received and incorporated. Cover pictures: The Gold Coast in southeast Queensland, Australia (left; Australian education information portal for overseas students, 2008), and a trailing suction hopper dredger rainbowing sand in the nearshore zone (right; Hollemans, 2005). Beach Nourishment to Mitigate the Impact of Sea Level Rise in Southeast Australia MASTER OF SCIENCE THESIS in partial fulfilment of the requirements for the degree of Master of Science in Civil Engineering at Delft University of Technology by J.M.P.A. Langedijk November 2008 Examination Committee: dr. P.J. Cowell The University of Sydney, Institute of Marine Science, Coastal Studies Unit prof. dr. ir. M.J.F. Stive Delft University of Technology, Faculty of Civil Engineering, Hydraulic Engineering Section dr. ir. J. van de Graaff Delft University of Technology, Faculty of Civil Engineering, Hydraulic Engineering Section dr. ir. B. Enserink Delft University of Technology, Faculty of Technology, Policy and Management, Policy Analysis Section Abstract The global average sea level is rising at an increasing rate. It is projected that by the end of the 21 st century, the sea level in southeast Australia will be 18 to 91 cm higher than the present level. According to the Bruun Rule, the most widely applied model for predicting the impact of sea level rise on a coastline, the rising sea level will cause sand from the nearshore zone to move offshore, resulting in beach erosion. Hence, sea level rise poses a significant risk to the beaches of southeast Australia, which are of great value to tourism and the safety of beachfront properties. This risk can be mitigated by means of beach nourishment, which is an effective way of combating beach erosion. In Australia, beach nourishment is currently only applied on the Gold Coast in southeast Queensland, but the political and community support to nourish many other beaches in southeast Australia seem to be on the rise. The costs of nourishing the beaches in southeast Australia to mitigate the impact of the projected sea level rise in the 21 st century have been estimated in the present study. The considerable uncertainty associated with sea level rise and its impact on the coast has been managed by adopting a probabilistic approach. This means that the results of the present study have been expressed in terms of probabilities rather than a single outcome. In order to determine the future beach nourishment costs, the expected offshore sand losses due to sea level rise in the 21 st century have been estimated first. For this purpose, the “raising the profile method” has been applied, which is a derivative of the Bruun Rule. The resulting offshore sand losses are between 1.3 and 2.9 billion m 3 in New South Wales and between 1.5 and 3.3 billion m 3 in southeast Queensland, where the actual value depends on the selected risk level (between 1 and 50%). The offshore sand resources in southeast Australia contain enough good quality sand to replenish these sand losses and can be extracted with a trailing suction hopper dredger in water depths of at least 20 m (on parts of the coast that will not be nourished), without causing significant impacts on the coastline and the marine environment. Next, the amount of sand that needs to be replenished has been determined. The required amount of sand is a fraction of the offshore sand losses and depends on the number of beaches that will be nourished. Because the economic viability of applying beach nourishment depends strongly on the degree of beach development, it has been assumed that only the presently developed beaches and, in due time, the future developed beaches (the greenfield beaches) will be nourished. At present, about 20% of the sandy coastline of southeast Australia has been developed and this number may increase up to 60% within the 21 st century. Finally, the beach nourishment costs in the 21 st century have been estimated by multiplying the required amounts of sand with the sand costs. The sand costs are the lowest in case of using offshore sand resources and a medium-sized dredger. In that case, the costs of nourishing the presently developed beaches are between 1.1 and 2.6 billion Australian dollars in New South Wales and between 1.5 and 3.7 billion Australian dollars in southeast Queensland, where the actual amount of money depends on the selected risk level (between 1 and 50%). In addition, when both the presently developed beaches and the greenfield beaches are nourished, these amounts of money increase to iv Abstract between 3.7 and 8.9 billion Australian dollars in New South Wales and between 3.6 and 8.5 billion Australian dollars in southeast Queensland. Accordingly, the annual beach nourishment costs in case of offshore sand extraction are between 11 and 89 million Australian dollars in New South Wales and between 15 and 85 million Australian dollars in southeast Queensland. These costs can be decreased if the deployed dredger extracts sand for the building industry as well. However, offshore sand extraction is currently prohibited in New South Wales. In case the beaches of New South Wales are nourished with sand that has been sourced onshore, the costs of nourishing the presently developed beaches are between 7.6 and 20 billion Australian dollars and the costs of nourishing the presently developed and greenfield beaches are between 26 and 69 billion Australian dollars, where the actual amount of money depends on the selected risk level (between 1 and 50%). The corresponding annual beach nourishment costs are 76 to 690 million Australian dollars, which is about seven times higher than in case offshore sand is used. The results of the present study can be used to develop an economically optimum beach nourishment programme in southeast Australia. Starting to nourish the beaches that are already heavily eroding will be of great value to the development of such a programme. Due to the flexibility and scalability of beach nourishment, the high values of beach properties and the high tourism revenues from many beaches, it can be concluded that a beach nourishment programme will be an effective and economically viable method of mitigating the impact of sea level rise on the coast of southeast Australia. Preface This thesis is the result of my graduation project, which was carried out for the main part at the Coastal Studies Unit of the University of Sydney, Australia, between December 2005 and May 2006. The remainder of the project, including the writing of this report, was carried out at Delft University of Technology, the Netherlands, between June 2008 and November 2008. At the University of Sydney I was supervised by dr. Peter Cowell. I am very grateful to Peter for his support, encouragement and advice throughout the course of my graduation project. My stay in Australia has been a very valuable and pleasant experience and I owe many thanks to Peter for the opportunity to come to Sydney. During the writing of this report I was supervised by my cousin dr. ir. Akke Suiker at the Faculty of Aerospace Engineering of Delft University of Technology. I would like to deeply thank Akke for helping me and encouraging me to finish my thesis. Thanks to his support, trust and advice I regained the confidence and motivation required to complete my studies. In addition, I would like to thank the members of my examination committee at Delft University of Technology, prof. dr. ir. M.J.F. Stive, dr. ir. J. van de Graaff and dr. ir. B. Enserink, for their support and supervision. Furthermore, I would like to express my gratitude to all other people who have assisted me with my project. In this regard, I particularly would like to thank: Marc Daley, John Hudson, Genny Pezzimenti, Peter Roy, Andrew Short and Bruce Thom (University of Sydney), Darren Skene (Sydney Marine Sand), Sally Kirkpatrick and Greg Stuart (Griffith Centre for Coastal Management, Queensland), David Beharrell and Daylan Cameron (Warringah Council, Sydney), Geoff Withycombe (Sydney Coastal Councils Group), Bart Hollemans (Van Oord Australia), ir. G.L.M van der Schrieck and dr. ir. P.H.A.J.M. van Gelder (Delft University of Technology), Matthijs Boon, Frank Claessen and Richard de Jager. I would also like to thank my friends for the great times in Delft, Sydney and elsewhere. Finally, I would like to thank my family, and in particular my parents, for their support throughout my studies. Jeroen Langedijk Delft, November 2008 vi Preface Contents Abstract.................................................................................................................................................iii Preface....................................................................................................................................................v List of Acronyms..................................................................................................................................xi List of Symbols ...................................................................................................................................xiii 1 Introduction ................................................................................................................................1 1.1 Background ........................................................................................................................1

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