Landslide preparation by forestry activity in a region of high erosion rates: Waingake Landslide, northeast New Zealand

ABSTRACT Northeastern New Zealand’s sedimentary rocks and soils are particularly vulnerable to landsliding, with some of the highest specific sediment yields, globally. In addition to earthquakes, rainfall is considered an important landslide triggering process. While land use change has also been proposed as an important landslide preparatory factor across the region, few case studies of landslides on slopes affected by forestry activities exist. We describe the circumstances surrounding the 2018 initiation of the Waingake Landslide, reactivation, and ongoing failure. The landslide is characterised by a headscarp zone of retrogressive slumping, transitioning downslope into a lobe that overwhelmed a forest road. Although in‐part related to precipitation, preparatory factors include the harvesting of Monterey Pines during an extremely wet year when rainfall had exceeded the 1990–2010 annual ‘normal’. Soft, clay‐rich residual soils became saturated above a less permeable weathered mudstone substrate, causing a decrease in effective stress. Cutting the forest road steepened the slope below its factor of safety, contributing to failure. Inadequate road drainage and blocked culverts then caused surface run‐off along the road, further saturating the landslide toe. Subsequently, excavation of the landslide toe during road re‐instatement reactivated the landslide.