Earthflows in the Interior Plateau, southwest British Columbia

Abstract

Examples of large, slow-moving earthflows from the western part of the Interior Plateau zone of British Columbia are discussed, the larger examples exceeding 107 m3. The study provides a regional framework for previous, site-specific work on the geotechnical properties of earthflows in this region. Factors responsible for earthflow development are (1) low shear strength claystone, shale, and siltstone units within, principally, middle Cretaceous to Eocene sediments and volcanics; (2) steep topographic gradients at the plateau margins, plus availability of tributary valley incisions to provide lateral confinement of earthflow debris; (3) sufficient moisture surplus at the plateau level to cause seasonal fluctuations in piezometric head, both within and beneath earthflow debris. So far, work on the Holocene record of movement suggests (1) relatively modest earthflow activity during the warm, dry period of the first 3000 years or so of postglacial time; (2) a post-Mazama maximum of movement, probably correlated with the onset of cooler, moister conditions; (3) following this, a progressive decrease in the area of actively moving debris on most flows. Key words: earthflow, Holocene mass movement, earthflow morphology, slope movement.