The 23rd January 2019 Cape Kidnappers coastal cliff collapse, Hawke's Bay, New Zealand

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de Vilder SJ, Dellow GD, Archibald GC, Morgenstern R. 2019. The 23rd January 2019 Cape Kidnappers coastal cliff collapse, Hawke's Bay, New Zealand. Lower Hutt (NZ): GNS Science. 11 p. (GNS Science report; 2019/26). doi:10.21420/CT0N-7284.

An approximately 25,500 m³ debris avalanche occurred on the 23rd January 2019 from the coastal cliffs of Cape Kidnappers, Hawke’s Bay. The debris avalanche was observed by multiple bystanders, and seriously injured two tourists. The tourists were walking along the beach, which forms the public accessway to the Gannet Colony at Cape Kidnappers – a popular tourist attraction in the Hawke’s Bay. The cliff collapse occurred within a conglomerate unit located in the upper 50 m of the cliff. The site has been the location of previous failures with a significant debris avalanche occurring from the lower half of the cliff in the 2015. The remnants of the 2015 debris deposit were still present at the base of the cliff on the 23rd January 2019, with the 2015 deposit acting as a ramp, allowing the 23rd January 2019 debris avalanche debris to travel further. ‘Small’ precursory rockfalls were observed sporadically through the week prior to the 23rd January 2019 event. There was no discernible trigger for the debris avalanche, with no seismicity and limited rainfall recorded in the week prior. Marine erosion at the toe of the slope may have been a contributing factor in the 2015 failure. The most likely cause of 23rd January debris avalanche is upward propagation of cliff failure through time. The final cause of failure would be the culmination of cliff material weakening through time (due to weathering processes such as saltwater wetting and drying and failure surface development in response to ongoing stress relief). As the strength of the material decreases and the failure propagates, ‘low’ apparently benign environmental stresses can act as a trigger for final failure. Several smaller rockfalls have occurred after the 23rd January 2019 debris avalanche, including a 10,000 m³ debris avalanche located to the east of the 23rd January debris avalanche. This subsequent cliff failure narrowly missed two tourists who were walking along the beach below (the beach and accessway were closed at the time). Anecdotal evidence, field observations, and aerial LiDAR analysis all indicate that landslides of a similar size, or smaller, occur regularly along this 7 km long section of coastal cliffs between Clifton and Black Reef. However, the baseline risk users of the beach accessway are exposed to is unknown. As such, the logical next step would be to undertake a quantitative risk assessment to quantify this baseline risk. (auth)