Predicting Auckland’s Exposure to Coastal Instability and Erosion December 2020 Technical Report 2020/021 Cover picture: Stanmore Bay PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION Predicting Auckland’s exposure to coastal instability and erosion, TR2020/021 December 2020 Auckland Council ISSN 2230-4525 (Print) ISSN 2230-4533 (Online) ISBN 978-1-99-002266-1 (Print) ISBN 978-1-99-002267-8 (PDF) Reviewed and recommended for publication by: Name: Branko Veljanovski Position: Head of Engineering Design & Asset Management Approved for publication by: Name: Sarah Sinclair Position: Chief Engineer Recommended citation: Roberts, R., N Carpenter and P Klinac (2020). Predicting Auckland’s exposure to coastal instability and erosion, Auckland Council, technical report, TR2020/021 © 2020 Auckland Council Auckland Council disclaims any liability whatsoever in connection with any action taken in reliance of this document for any error, deficiency, flaw or omission contained in it. This document is licensed for re-use under the Creative Commons Attribution 4.0 International licence. In summary, you are free to copy, distribute and adapt the material, as long as you attribute it to Auckland Council and abide by the other licence terms. PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION Acknowledgements This document was prepared with technical input from Auckland Council and industry experts including, but not limited to, the following individuals: Authors (Part 1) Natasha Carpenter, Ross Roberts, Paul Klinac (Auckland Council) Authors (Part 2) Patrick Knook, Ben Westgate, Rebekah Haughey, Tom Shand, Richard Reinen-Hamill (Tonkin + Taylor). Editor Ross Roberts (Auckland Council) Peer reviewers Mark Dixon (University of Auckland) Murray Ford (University of Auckland) Martin Brook (University of Auckland) PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION Part 1: Table of Contents Part 1 Non-Technical Study Executive Summary .......................................................................................................................................... i List of abbreviations ......................................................................................................................................... iv 1.0 Introduction ......................................................................................................................................... 1 1.1 Background ........................................................................................................................................ 1 1.2 Purpose .............................................................................................................................................. 1 1.3 National and local policy context ....................................................................................................... 2 2.0 Method ................................................................................................................................................. 4 2.1 Scale of assessment .......................................................................................................................... 4 2.2 Areas susceptible to coastal instability and/or erosion (ASCIE) ........................................................ 4 2.3 Division of the coastline into beaches, cliffs, and reclamation .......................................................... 4 2.4 Effects of climate change ................................................................................................................... 6 2.5 Selection of timeframes ..................................................................................................................... 8 2.6 Modelled scenarios ............................................................................................................................ 8 2.7 Calculation of area susceptible to coastal instability and erosion ..................................................... 8 3.0 Results ............................................................................................................................................... 10 3.1 Cliffs ................................................................................................................................................. 10 3.2 Beaches ........................................................................................................................................... 12 4.0 Next steps .......................................................................................................................................... 14 Part 2 Regional Assessment of Areas Susceptible to Coastal Instability and Erosion Part 1: Non-technical summary PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION i Executive Summary The Auckland region has over 3,200 km of coastline including three major harbours and a range of sandy beaches and dunes, rocky shores and cliffs, estuaries and offshore islands. As well as a long and diverse coastline, Auckland has the largest population density to coastline ratio in New Zealand. As a result, the city has a high exposure to coastal hazards including coastal instability and erosion. These hazards can present a safety risk, adversely affect property and infrastructure, and damage or destroy cultural and environmental sites. A study undertaken in 2006 is the basis for the current coastal erosion rules within the Auckland Unitary Plan. No more up-to-date information related to the potential exposure of Auckland’s coastline to erosion was available at the time of the plan’s development. To fill this knowledge gap, a programme of research has been undertaken to identify, at a regional level, the Area Susceptible to Coastal Instability and/or Erosion (ASCIE). ASCIE is the area landward of the current coastline that is at risk because of coastal erosion or instability caused by coastal erosion. Titled “Regional Assessment of Areas Susceptible to Coastal Erosion and Instability”, this study was undertaken for Auckland Council by Tonkin + Taylor Ltd., with a peer review by the University of Auckland. The study forecasts the areas of Auckland’s coastline that could be affected by coastal erosion and instability under a range of climate change (sea-level rise) scenarios and timeframes. A non-technical overview of the study is given in Part 1 of this Technical Report while the detailed study as reported by Tonkin + Taylor is presented in Part 2. The intent of this separation is to provide a Council synopsis and interpretation for non-technical readers. To assess the ASCIE, the coastline of Auckland was divided into 568 coastline ‘cells’, where the controlling factors are broadly consistent. The impact of sea level rise was modelled for each cell at 30, 50, and 100+ year timescales, and two climate change scenarios were used. A high emission scenario with little mitigation controls (RCP 8.5M), which matches the current track for global emissions, was used across all three timeframes. To get a full understanding of the worst-case scenario and check the sensitivity of the models, an extreme emissions scenario with no mitigation (RCP 8.5H+) was also modelled at the 100+ year timeframe. The ASCIE has been calculated for each of these combinations, and the results for each cell presented as a distance in metres landward of the current coastline. The ASCIE of cliffs and beaches are controlled by differing processes and were calculated separately using current best practice. The results are presented in tabular format and on a regional ‘heat map’. While the regional ‘heat map’ provides a high-level understanding of the potential erosion rates across Auckland’s coastline, further work needs to be done to refine the model and provide more PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION ii detailed mapping showing areas susceptible to coastal instability and erosion at a sub-regional level. ASCIE was not calculated for reclaimed shorelines as it was assumed that structures reinforcing these shorelines will be maintained, supporting the planning definition in the Auckland Unitary Plan of reclaimed land as permanent. PREDICTING AUCKLAND’S EXPOSURE TO COASTAL INSTABILITY AND EROSION iii Part 1: Table of Contents Executive Summary .......................................................................................................................................... i List of abbreviations ......................................................................................................................................... iv 1.0 Introduction ......................................................................................................................................... 1 1.1 Background ........................................................................................................................................ 1 1.2 Purpose .............................................................................................................................................. 1 1.3 National and local policy context ....................................................................................................... 2 2.0 Method ................................................................................................................................................. 4 2.1 Scale of assessment .......................................................................................................................... 4 2.2