Engineering geological Investigation of Kopua Coastal Landslide at Wharerata, East Coast, New Zealand

Abstract

Landslide activity is widespread in the Gisborne region of New Zealand’s North Island, and several investigations have occurred in recent years focused on the spatial pattern of landsliding, along with triggering factors. High-intensity rainfall, earthquakes, and land use change (e.g., slope cutting, deforestation) are all triggering factors, with the weak soils and sedimentary rocks of the region particularly susceptible to slope failure. Across the region, recent rainfall events in November 2021 and early 2022 have shown that there is a requirement for slope stability investigations in particularly vulnerable areas. Such investigations require evaluation of in-situ soil and rock material properties, and an understanding of process-mechanics of failure and the rate of deformation, to mitigate future failure at the sites. The focus here is the Kopua coastal landslide at Wharerata, 21 km southwest of Gisborne. This is a landslide ‘complex’, which was triggered by rainfall, is ongoing, and is approximately 0.5 km long from the headscarp to the toe, which is in the ocean. Sediment from cutting of the toe by wave action is disturbing marine life and shellfish (kai moana) for the local iwi. The methods employed here include (1) in-situ testing of materials; (2) unpiloted aerial vehicle (UAVs) surveys for engineering geological mapping and digital elevation model (DEM) differencing; and (3) laboratory testing of soil index properties and mineralogy. The landslide can be categorized as a clay/silt rotational coastal landslide caused by combined effects of drying-fissuring-wetting failure continuum. Recommendations for mitigation include physical engineering measures such as retaining walls, revetments, slope coverings and bio-engineering techniques like branch packing and vegetated gabion walls. Monitoring using global positioning system (GPS), tiltmeters and extensometers (ExT), inclinometers, shape acceleration arrays (SAA), Acoustic Emission (AE), piezometers as well as regular UAV surveys would be appropriate.