Engineering Geological Investigation of Shallow Landslides from the 2017 Storms and Material Properties of Kawakawa Bay, New Zealand

Abstract

Landslides are triggered by various processes such as earthquakes, snow melt, volcanic eruptions and land use change, however, rainfall remains the most common trigger in New Zealand. This study examines the shallow mass wasting induced by high intensity rainfall from the 2017 Tasman Tempest and Ex-Tropical Cyclone Debbie storms in the hillslopes of Kawakawa Bay in the North Island of New Zealand. These storms had 24-hour rainfall totals of 210 mm and 157 mm, respectively, and some of the resultant landslides damaged residential properties and infrastructure in the town of Kawakawa Bay near Auckland, New Zealand. This study utilises GIS to quantify and classify landslides and explores relationships between landslide occurrence and topography. Examination of rainfall and hydrological data is used to investigate the role of rainfall intensity and antecedent soil moisture on landslide triggering. Geotechnical investigations and laboratory methods were used to characterise the slope materials and their engineering properties. Landslide geomorphology was characterised from site visits as well as Uncrewed Aerial Vehicle (UAV) surveys and LiDAR data, to produce Digital Surface Models (DSMs), Digital Terrain Models (DTMs), and orthophoto-mosaics of Kawakawa Bay. This allowed simple temporal topographic differencing to determine change detection. Limit Equilibrium Method (LEM) modelling was also attempted at a selected slope to examine Factor of Safety (FoS). An observable 102 landslides were mapped, with a mean density of 210.6 landslides/km2. These identified landslides comprised 67.7% shallow earthflows, 20.5% shallow translational landslides and 10.8% slumps. Length and width dimensions correlate positively, but are highly variable, and landslide length and width are negatively correlated with slope angle. This variability results from the undulating topography, relict landslides, and anthropogenic disturbance and land-use changes. Areas of high Topographic Wetness Index (TWI), larger shallow earthflow source areas and sparse vegetation, correlate with longer earthflow tracks (i.e., runout). The majority of these landslides occurred on steep slopes (35-40°), with shallow earthflow source zones distributed on slopes with high concavity and TWI. Soil properties are predominantly highly plastic silty-clays, with clays exhibiting characteristic clay-like behaviour, which are susceptible to flow. Evidence of soil piping, tension cracks, and FoS (Factor of Safety) ranges of 0.862-1.311 indicates the slopes are close to failure and sensitive to large and/or intense rainfall events. Hence, prudent, cost-effective mitigation strategies should be explored.