Rosser, B.J.; Wolter, A.; Boyes, A.F; Lin, S.-L.; Farr, J.; Chen, E.; Townsend, D.B., Jones, K.E. 2023 Phase II: Remote mapping of landslides triggered by the July 2021 and August 2022 Marlborough storms, and selected field investigations of landslide impact. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/20. 69 p.; doi: 10.21420/5RKR-1618
Abstract
Marlborough has experienced significant storms in the past few years that have triggered widespread and damaging landslides. This report presents an investigation of landslides occurring during the July 2021 and August 2022 storm events. For the July 2021 and August 2022 events, we mapped the distribution of landslides from remote imagery, analysed factors potentially controlling these distributions and sought to explain differences found between the two storms. For the August 2022 storm, we also undertook fieldwork to investigate nine building sites impacted by landslides. We assessed the building damage and related this to the landslide characteristics and also compared these relationships to similar data previously collected for buildings impacted by the July 2021 storm. Details of the 2021 rainfall event have been summarised in a previous companion report; here, we focus on describing the August 2022 storm. Over 1000 mm of rain fell in some areas during the 2022 storm event, which lasted from the 6th to the 19th of August, 2022. Landslides included mainly debris flows and slides, as well as incipient landsliding (evidenced by ground cracking) and re-activation of relict landslides. Compared to the July 2021 storm, landslides at the investigated building sites tended to be less mobile. Of the nine buildings investigated in detail, three were affected by slippage and six by debris inundation. Damage states (DS) of the buildings investigated ranged from 0 (no damage) to 5 (severe structural damage), with three classified as DS5.Landslide distributions in July 2021 and August 2022 do not seem to be controlled by the rainfall distribution alone. For both storms, the greatest number and density of landslides did not occur in the areas that received the highest rainfall. Most landslides were shallow and occurred on slopes in weathered schist or semi-schist. In 2021, the highest landslide densities were in harvested forests, and, in 2022, the highest landslide densities were in pasture and regenerating scrub communities. We recommend more detailed analysis of these datasets, and others, to determine possible landslide trigger thresholds and to conduct risk assessments for vulnerable areas, particularly in the Marlborough Sounds (auths)
