Characteristics of slope failures induced by Tropical Cyclone Winston (20-21 Feb, 2016) along King’s Road, northeast Viti Levu, Fiji Islands

Abstract

Slope failures and soil slips are caused by a number of natural and anthropogenic factors, but the main factors predisposing the stability of a slope are its geology, geometry (aspect and slope), (pore)-water conditions and drainage properties, vegetation cover along with any human modification. Many Pacific region countries in humid tropical environments are highly susceptible to slope failures due to their young geological age, steep terrain, high earthquake and volcanic potential. These factors are compounded by high rainfall magnitudes and intensities, especially during tropical cyclones, often in areas of continual and often unregulated urban and rural development. Although tectonically and volcanically relatively stable compared to many of its neighbours, Fiji is particularly susceptible to numerous and damaging landslides exacerbated by loss of forest vegetation and rapid rates of road and other urban development. Slope failures resulting from tropical cyclones have caused a high number of fatalities and great damage on or near Fiji’s roads in the past, particularly for the King’s Road, one of the two major highway’s spanning Fiji’s largest island, Viti Levu. Tropical Cyclone (TC) Winston was one of the strongest cyclones to ever cross the southern hemisphere, battering the Fiji group from the early hours of 20 February until 3 am, 21 February, 2016, with wind gusts of up to 306 km/hr. Fiji’s National Disaster Management Office estimated damage at $476.3M (a preliminary estimate), with the highest individual figure of $13M reported by the Fiji Roads Authority. Of the road-related costs approximately 30% relates to restoration related to slope failures. A rapid field reconnaissance study examined the characteristics and possible failure mechanisms of selected slope failures induced by TC Winston along a 30 km stretch of the King’s Road between Barotu and Wailevu villages in northeast Viti Levu. Approximately 30 distinctive shallow, small-scale failures were identified most of which were debris-slides (both rotational and translational). Some of these were deep enough to entrain bedrock, while other failures included a minor flow component. Evidence of recurring slips along Fiji’s King’s Road indicates a need for new engineering solutions to resolve slope failure issues close to infrastructure. It is hoped that further research developing from this preliminary study will be able aid Fiji authorities to identify areas of greatest vulnerability for slope failure and recommend long-term solutions managing slope stability hazard.