GLACIOMARINE SEDIMENT FAILURE AT KHYEX RIVER, NORTHERN BRITISH COLUMBIA, CANADA

The Khyex River landslide, located 35 km east of Prince Rupert, northwestern British Columbia, occurred on November 28, 2003, at about 00:30 PST. The movement was an extremely rapid flow that involved liquefaction. The landslide consists of a steep head scarp measuring 45 m in height by 345 m in width. It was composed of glaciomarine sediments mantled by rubbly colluvium lying on, and against smooth bedrock of the valley wall. The failure displaced about 4.7 M m³ of sediment covering an area of 32 ha with a zone of depletion of 13 ha. The displaced material flowed up and down river over a distance of 1.7 km, blocked the river, and caused flooding upstream for a distance of 10 km. The angle of reach is about 4.5˚. We considered two possible triggers: 1. river-bank erosion or 2. undrained loading from an initial landslide in rubbly colluvium impacting glaciomarine sediment. The landslide severed a natural gas pipeline. As a result, Prince Rupert and surrounding area residents were without natural gas heat for 10 days. This landslide is one of many to have occurred over the last five decades in glaciomarine sediments in northwestern British Columbia. Glaciomarine sediments are common in the north coast valleys. Roads, railways, pipelines and utilities are located and constructed in these valleys on sensitive glaciomarine sediments. Thus, infrastructure is at continual risk from landslides in glaciomarine clays. Regional hazard mapping, geotechnical investigation and risk assessments are necessary to avoid catastrophic impacts to utilities and infrastructure in the region.