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

Citation
Ja. Matthews et al., Alpine debris-flows in Leirdalen, Jotunheimen, Norway, with particular reference to distal fans, intermediate-type deposits, and flow types, ARCT ANTARC, 31(4), 1999, pp. 421-435
Citations number
54
Categorie Soggetti
Multidisciplinary
Journal title
ARCTIC ANTARCTIC AND ALPINE RESEARCH
ISSN journal
15230430 → ACNP
Volume
31
Issue
4
Year of publication
1999
Pages
421 - 435
Database
ISI
SICI code
1523-0430(199911)31:4<421:ADILJN>2.0.ZU;2-A
Abstract
The landforms and deposits associated with AD 1996 debris-flows at three si tes 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 foo tslope zone and in the valley bottom. Five facies are recognized in the fan s: (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 thi ck; (4) massive silty sand or sandy silt (intermediate-type deposits) of th ickness 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 lan dform-sediment assemblage and proximal-distal trends are explained by a fou r-stage model of an integrated debris-flow event in which (1) slope failure and sediment disaggregation are followed sequentially by (2) debris flow s ensu stricto (cohesive debris flow), (3) wet mudflow or hyperconcentrated f low, and (4) water flow. Debris flow sensu stricto accounts for an estimate d 48 to 51%, wet mudflow/hyperconcentrated flow 21 to 26%, and water flow 2 4 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 interm ediate-type deposits with little or no structure, which are attributed here to wet mudflow and/or hyperconcentrated flow. Water-lain deposits, also in tegral to the debris-flow event, tend to be thinner and finer, better sorte d, and distinctly laminated.