State Highway 60 (SH60) over Takaka Hill in Tasman District has a long history of damage and closure from rainfall-triggered slope failures. Extreme rainfall from Ex-tropical cyclone Gita on 20th February 2018 triggered numerous translational soil slides on the southern side of Takaka Hill resulting in a series of debris flows that damaged SH60 near Riwaka. Scour erosion of road seal, damage to culverts, and failure of embankment sections led to complete closure of the road for five days, preventing all road access to the Takaka- Collingwood region. The road continues to be reduced to a single lane with traffic control more than two years after the event as repairs are ongoing.

A programme of geomorphic mapping, geophysical profiling, and both in-situ and laboratory geotechnical testing was carried out to characterise the 2018 debris flows and investigate the hazard from rainfall-triggered slope failure and debris flows on Takaka Hill. Three stream catchments, on the lower slopes of the southern side of Takaka Hill, were the focus of this investigation. These are underlain by differing bedrock: granite, schist and a basic igneous suite.

Historical records indicate an approximately 30-year recurrence for major events resulting in road closure for ≥ 5 days. The main types of rainfall-triggered failures are culvert blockage, shallow soil slides and debris flows. Smaller failures resulting in temporary road closures occur more frequently.

Extreme rainfall events, often following periods of protracted rainfall trigger shallow translational soil slides in colluvial and residual soil profiles. Failure planes are generally shallower than 5 m. The debris from these failures enters the adjacent drainage courses and is then remobilised as channelised debris flows. The soil slide source colluvium is matrix supported with clasts up to boulder size. Pre-2018 debris flow deposits exposed in stream banks are 1 – 2 m in thickness. Geophysical profiling indicates up to 10 m of accumulated debris flow material in the debris fans on the lower slopes of Takaka Hill, indicating long-term debris flow activity in the area.

Geotechnical characterisation of the colluvial soils in the slide source areas, the debris flow materials from 2018 and prior debris flow events, as well as the weathered bedrock and residual soil profiles encountered in the study area were used to develop detailed engineering geological profiles for the Takaka Hill area. These will provide the basis for any future slope stability analysis.