Engineering geomorphological and InSAR investigation of an urban landslide, Gisborne, New Zealand

Abstract

The East Coast region of New Zealand has some of the highest erosion rates in the world, due to its proximity to an active plate boundary, susceptibility to high-intensity storms and steep terrain underlain by young soft sedimentary rock and soil. In the city of Gisborne, expansion of residential blocks into steeper terrain in peri-urban areas has required improved capacity for the characterisation and monitoring of slope stability. Landslides have affected several properties and have destroyed infrastructure. Slope failure commonly occurs during heavy rainfall events when slow-moving retrogressive slides transition into earthflows and mudflows. In this study, we used in situ sampling and testing methods combined with remote sensing techniques to provide an understanding of the pre-failure and post-failure behaviour of an urban landslide in Gisborne. High-resolution aerial imagery, unmanned aerial vehicle imagery and light detection and ranging data revealed slope morphology and contours of prehistoric failures in the area, and highlighted the more recent impacts of deforestation on slope stability. Furthermore, Sentinel-1 InSAR analysis determined that gradual deformation began in 2017, following two ex-tropical cyclone events. Deformation downslope continued until an initial failure in July 2020. Following that event, some parts of the slope proceeded to accelerate, leading to a further reactivation event in November 2021, following heavy rainfall. During this November 2021 event, average line of sight velocities ranged from −7.9 to −11.2 mm/year, and deformation rates in the vertical direction (related to rotational slumping) averaged −11.2 to −11.9 mm/year, consistent with field observations.

Interferometric Synthetic Aperture Radar (InSAR) and field‐based observations of rainfall‐triggered landslides from the November 2021 storm, Gisborne/Tairāwhiti, New Zealand

“…Some of these are new failures, while others are more likely to be reactivated existing landslides. Generally, the observed landslides reported herein accord with previous observations of shallow landslides in the region, that typically occur within the highly-weathered Pleistocene Mangatuna Formation soils (e.g., Cook, Brook, Hamling, et al, 2022;Mazengarb, 1997;McKelvey & Murton, 1991). In addition, however, several failures have been reported on 'fill' slopes, where slopes have been modified to accommodate construction activity, such as tennis courts or swimming pools.…”

“…Such fill failures are not uncommon across New Zealand, often due to poor compaction and drainage management in urban and suburban areas (e.g., Brook, 2018). Nevertheless, whether the landslide is within soil or fill, it is probably promoted by drainage and permeability contrasts that occur with the underlying Miocene Tunanui Formation mudstones (Cook, Brook, Hamling, et al, 2022). Such contrasts cause elevated groundwater tables during heavy, prolonged rainfall, as indicated by groundwater ‘daylighting’ out of exposed landslide headscarp exposures.…”

“…First, landsliding caused by vegetation removal in rural parts of the region due to forestry, has long been an issue in the region (Marden, 2004). However, removal of vegetation on slopes with the suburbs of Gisborne has also recently been identified as a key land stability issue (Cook, Brook, Hamling, et al, 2022), with tree cutting eliminating canopy cover, facilitating peak pore water pressure increases within soils (e.g., Keim et al, 2006). In addition, mechanical reinforcement of soils by roots is removed (e.g., Schwarz et al, 2013).…”

“…Nevertheless, residential house-holders probably prefer more resilient slopes within their property boundaries. However, slope cutting and modification on steep sites does occur (e.g., slopes above Richardson Avenue), including infilling of tension cracks and earthworks to flatten landforms developing due to active landslides (Cook, Brook, Hamling, et al, 2022). Indeed, modification of slope morphology at that site included in-filling of tension cracks, and excavation of the stepped morphology formed from shallow rotational movements (e.g., Hearn et al, 2011), yet the slope remains active.…”

Penerapan Metode GPR dan Geoteknik pada Kajian Jalan, Jembatan, dan Lereng : Review

“…Dalam hal ini Borehole (BH) atau data bor digunakan untuk membantu identifikasi lapisan tanah agar tepat. Dicetak ulang dari "Engineering geomorphological and InSAR investigation of an urban landslide, Gisborne, New Zealand" (Cook et al, 2022). Telah dilakukan penelitian serupa di daerah perkotaan Gisborne, Selandia Baru oleh Cook et al (2022) menggunakan teknik penginderaan jauh, GPR, dan menggunakan data bor.…”