Reactivation of coastal landsliding at Sunkist Bay, Auckland, following ex-Tropical Cyclone Debbie, 5 April 2017
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Alex Palma I Rosie Garrill I Martin S. Brook I Nicholas Richards I Jon Tunnicliffe
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 longterm 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 porewater 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. Keywords Coastal landslide . Rainfall triggering . Shear strength . New Zealand . Setback Introduction Many regions of New Zealand are susceptible to rainfall-induced slope failure (Rosser et al. 2017). This is due to a blend of the geotechnical character of particular soils, topography, and precipitation patterns, and land use changes over the last 150 years (Brooks et al. 2002). This paper is concerned with the failure of the soft coastal cliffs in the eastern part of Auckland on New Zealand’s North Island (Fig. 1). At 1.6 million inhabitants, Auckland is New Zealand’s largest city, and the Auckland region encompasses c. 3300 km of coastline, with a highly variable wave climate and cliff geology (Brookes and Green 2001). Significant coastal cliff instability occurs along the eastern coasts, which are heavily populated, with houses often constructed close to the cliff edge. This is a lee coast, protected by islands in the Hauraki Gulf and Coromandel Peninsula, which contrasts with Auckland’s sparsely inhabited, high energy west coast, adjacent to the Tasman Sea. The focus here is the April 5 2017 failure of the cliff seaward of 40 First View Avenue (Fig. 1), in the Sunkist Bay area of the Beachlands coast. This followed 24-h rainfall totals of 210 mm and 157 mm, falling on the 8 March and 5 April, respectively. Development of the land on the seaward side of First View Avenue continued over the last century despite various slope failures, with building footprints often encroaching within 10 m of the ~ 18 m high cliff. These typically are high-value properties, with a present rateable value, for example, of NZ$2.53m (US$1
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