Preliminary investigation of emerging suburban landsliding in Gisborne, New Zealand

Abstract

Recently uplifted, soft Pleistocene sediments in northern New Zealand are particularly vulnerable to landsliding because they are often underlain by less permeable, clay-rich Neogene mudstone/siltstone rocks. Typically, instability is rainfall-induced, often due to a high intensity rainfall event from extra-tropical cyclones, following wetter months when antecedent soil moisture has increased. Using remote sensing, field surveys and laboratory testing, we report on some emerging slope instability hazards in the eastern suburbs of the coastal city of Gisborne, on the North Island. Retrogressive failure of the main landslide (at Wallis Road) is ongoing and has already led to the abandonment of one home, while an adjacent landslide (at Titirangi Drive) appears to be in an incipient phase of failure. The Wallis Road landslide has been particularly active from mid-2017, with slumping of the headscarp area transitioning to a constrained mudflow downslope, which then descends a cliff before terminating on the beach. In contrast, the incipient Titirangi Drive landslide at present displays much more subtle effects of deformation. While activity at both landslides appears to be linked to rainfall-induced increases in soil moisture, this is due to the effects of prolonged periods of rainfall rather than the passage of high intensity cyclonic storms.

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

“…Identifiable features such as preferential alignment are present and share a similar structure to the smectite-rich soils reported at the Wallis Road landslide in Gisborne (e.g. Cook et al 2022). Indeed, smectite clay minerals are characterised by having very small particles, even among the clay minerals, and well-developed sheet-like structures (Fityus and Buzzi 2009;Meunier 2006).…”

“…Previous work on soils in Gisborne (e.g. Cook et al 2022) reported smectite was the most common clay mineral, followed by illite on soil-mantled slopes within the Mangatuna Formation and Tolaga Group. Expanding clay minerals would induce The soil at BH2 is a good representation of how soils on the landslide behave once saturated, with high sensitivity (quick clay) and loss of strength upon remoulding, causing high mobility and runout potential.…”

“…Indeed, negative ENSO (i.e. El Niño conditions) leads to drier conditions (Cook et al 2022), with the 2019-2020 El Niño event likely increasing the shrinking and fissuring of the clayey surface soils. The fissuring allows meteoric water to infiltrate deeper and faster than when soils are moist, and any fissures are narrowed (e.g.…”

Investigating slow-moving shallow soil landslides using Sentinel-1 InSAR data in Gisborne, New Zealand

“…The formation comprises alluvial gravels, tephra-rich sands and high plasticity montmorillonite and bentonite (Mazengarb 1997). Previously identified landslides include rotational slumps and earthflows (Cook et al 2022;Davies and Cave 2017), and some failures have been known to occur on slopes with angles as low as 5° (Mazengarb 1997). Shallow soil failures are also common within the regolith overlying the Miocene and Pliocene bedrock in the east of the study area in Wainui, Okitu and Makorori (Fig.…”

“…Indeed, the extent and ongoing surface deformation of landslides in the Gisborne area are poorly known, and areas at risk from slope instability are often unknown until a slope failure occurs. For example, slope failures have been active at several residential properties in the city (Davies and Cave 2017;Cook et al 2022), where buildings and infrastructure are still at risk from further deformation caused by slow-moving, complex landslides. Also, limited space on the lowlying flood plain means that there is growing incentive to build residential housing on increasingly precarious terrain on slopes surrounding the city.…”