Reactivation of coastal landsliding at Sunkist Bay, Auckland, following ex-Tropical Cyclone Debbie, 5 April 2017

Abstract

The historically unstable Sunkist Bay cliff in Auckland, New Zealand, is formed of weak Pleistocene alluvial sediments, infilling a paleovalley eroded into Miocene sandstone. Weathered tephra layers dip at low angles out of the cliff, and part-control stability. The last major failure prior to 2017 occurred in 2011, immediately to the west of the 2017 failure. Despite the long-term history of instability, over the last two decades, residential property construction has continued to < 10 m of the cliff edge. On 5 April 2017, ex-tropical cyclone Debbie delivered 157 mm of rainfall in 24 h, the 3rd highest 24-h total in 40 years. This was preceded 1 month earlier by 210 mm of rainfall on 8 March (the ‘Tasman Tempest’ storm), locally a 1 in 100-year event. A seawall constructed at the base of the cliff may have acted to inhibit pore-water pressure reduction during March–April 2017, because horizontal drains were absent in the seawall directly in front of the 2017 slope failure. Irrespective of the exact process-mechanisms of the 2017 failure, this case study highlights the importance of careful land use planning and imposition of adequate setback distances in coastal zones in New Zealand, and globally.