Landslide Dams Triggered by Extreme Rainfall Events: A Tairāwhiti Gisborne Case Study

Abstract

Landslide dams, formed when landslide debris blocks a river, occur across Aotearoa New Zealand. These events pose serious hazards to downstream communities and infrastructure, as sudden breaches can result in unexpected flooding and debris flows. They are also important components of complex cascading slope-river systems. Despite their contribution to sediment cascades and potential flood hazard, they remain under-researched. Understanding the interactions between hydrological, geological, and climatic factors governing landslide dam evolution is crucial for hazard management planning. The Tairāwhiti Gisborne region on the East Coast of Aotearoa New Zealand is prone to landslides and subsequent dams, due to its steep slopes, susceptible lithology, and exposure to tropical storms from the east. Despite this vulnerability, very little is known about the material properties and behaviour of landslide dams in this area. The primary aim of this thesis was to investigate the geomorphic and geotechnical aspects of dams in the Tairāwhiti Gisborne region, to understand their formation and breach processes. Two large landslide dams were chosen as case studies, the 1,485,500 m3 Waiorongomai Landslide Dam which formed in the north of the region in early January 2023, and the 98,500 m3 Tiniroto Landslide Dam which formed in the south of the region on the 3rd of October 2023. Three elements of analyses were used to investigate these landslide dams. First, historic landslides were mapped in the two catchments, to investigate the rate of landslide-river connectivity and landslide dam frequency. Each landslide source and deposit were mapped as polygons, using aerial imagery between 1988 and 2023. Second, the short-term evolution of the Waiorongomai and Tiniroto landslide dams was documented through a series of engineering geomorphic maps, using LiDAR and aerial imagery in ArcGIS Pro. Finally, the grain size distribution, liquid and plastic limits, and slake durability were analysed using sediment samples from each landslide dam. The historic landslide inventory showed that only 0.03% of fluvially coupled landslides formed landslide dams. The sites of the Waiorongomai and Tiniroto Landslide Dams both showed geomorphic precursors, including shallow and deep seated landslides, and tension cracks. While both dams were unstable and breached within a month of formation, they had varying geotechnical and geomorphic characteristics. The Tiniroto Landslide Dam material was finer grained with higher plasticity and was less durable than the Waiorongomai Landslide Dam. The Tiniroto Landslide Dam was manually breached within 12 days but retains a small lake. The Waiorongomai Landslide Dam initially breached during Cyclone Gabrielle, as the high rainfall raised the lake level above the dam, causing an overtopping breach. The lake fluctuated for three years before fully draining in early 2026, during another rainfall event. This research contributes to the wider body of literature by providing detailed assessments of the formation and breaches of two landslide dams in the Tairāwhiti Gisborne region, including the first insights into the plasticity of landslide dams in New Zealand and one of two slake durability assessments globally.