Landslide hazard and loss‐of‐life risk assessment for Muriwai, New Zealand following Cyclone Gabrielle in February 2023

ABSTRACT Cyclone Gabrielle made landfall on 14 February 2023 and caused widespread damage in the North Island, New Zealand. At Muriwai, west of Auckland, many houses were destroyed, and two firefighters were killed, by debris avalanches originating from a 150 m‐long, up to 80 m high escarpment. In 1965 a similar event in Muriwai also caused the deaths of two people. The escarpment comprises extremely weak Awhitu Sand Formation sandstone that can form vertical outcrops, but weakens when saturated. Landsliding occurs in high magnitude, infrequent rainstorms, where increased porewater pressure causes failed blocks to disintegrate into downslope‐traveling, destructive, rapid debris flows, often entraining large trees. A qualitative ‘fahrböschung’ angle assessment partially informed emergency evacuation by identifying a broad zone of susceptible houses below the escarpment. For the longer term, a more precise loss of life Quantitative Risk Assessment was conducted to guide Auckland Council’s property buy‐out scheme. A key element was the RAMMS debris avalanche simulations, which were calibrated against some of the ∼35 recorded large landslides and provided robust expected extent of future landslide extent. Observations of building damage in Muriwai suggest that >0.5 m of debris could cause fatalities of occupants, a key observation for the risk assessment.