Alpine debris-flows in Leirdalen, Jotunheimen, Norway, with particular reference to distal fans, intermediate-type deposits, and flow types

Abstract

The landforms and deposits associated with AD 1996 debris-flows at three sites in the low-alpine zone, Jotunheimen, southern Norway, are described and analyzed. Parallel levees, composed of diamicton, occur on the valley-side slopes but distinct frontal lobes are absent: instead, low-angle fans, up to about 50 m wide and ca. 500 m long have overridden vegetation in the footslope zone and in the valley bottom. Five facies are recognized in the fans: (1) cobble-rich diamicton, up to 50 cm thick; (2) pebble-rich diamicton, typically up to 30 cm thick; (3) pebbly sand lenses, up to about 15 cm thick; (4) massive silty sand or sandy silt (intermediate-type deposits) of thickness 5-15 cm; and (5) laminated fine sands and silts (typically a few cm thick). These succeed one another in a vertical and lateral sense. The landform-sediment assemblage and proximal-distal trends are explained by a four-stage model of an integrated debris-flow event in which (1) slope failure and sediment disaggregation are followed sequentially by (2) debris flow sensu stricto (cohesive debris flow), (3) wet mudflow or hyperconcentrated flow, and (4) water flow. Debris flow sensu stricto accounts for an estimated 48 to 51%, wet mudflow/hyperconcentrated flow 21 to 26%, and water flow 24 to 31%, by volume of material mobilized during each debris-flow event at two of the sites. Results highlight the potential complexity of debris-flow events and the importance of the associated relatively fine-grained intermediate-type deposits with little or no structure, which are attributed here to wet mudflow and/or hyperconcentrated flow. Water-lain deposits, also integral to the debris-flow event, tend to be thinner and finer, better sorted, and distinctly laminated.