Landslide Damage to Road Networks in the 2021 Marlborough Storm

A large storm occurred in Marlborough in July 2021, causing widespread damage and severe disruption to the Marlborough Sounds road network from landslides, flooding, scour, and failure of retaining walls. Comparison of rainfall gauge data and historical records indicate that the average recurrence interval (ARI) of the storm was between a 15 and 40-year event, though hourly rainfall intensities may have had higher ARIs. Poor drainage and early road construction practices contributed to the damage, and the impacts were exacerbated by the narrow road corridors and lack of alternative routes. Initial rapid route assessment was carried out in the week after the storm to triage landslide damage and over 1500 network faults were recorded. Of these, 165 significant geohazard sites were added to a monitoring regime across three key roads in the Marlborough Sounds. The observed landslide mechanisms included debris flows, deep rotational slides, shallow planar slides, and rock falls. Evacuation of historic side-cast fill deposits was frequently observed, and scour and backfill loss was the primary cause of retaining wall failure. Subsequent weekly and post-rainfall inspections of the worst-affected roads have been carried out from October 2021 to March 2022. This paper presents a categorisation of the observed failures and observations on the ongoing behaviour of slip-damaged sites.