The history of colluvial events over the last 8500 years is reconstructed in three Norwegian alpine slope-foot mires fed by three independent debris-flow systems. Chronologies for each site are constructed based on 155 radiocarbon-dated peat samples. At the multimillennial scale, debris-flow activity was greatest during the transition from the Holocene Thermal Maximum to the late Holocene ( c. 4300—2800 cal. BP) when debris-flow events occurred with a frequency of 1 in 33 years: two peaks in activity, characterized by 1 event in 14 years and 1 event in 25 years, were reached at the beginning and end of this interval, respectively. Least activity occurred during the Holocene Thermal Maximum from c. 8000 to 7100 cal. BP, with one event in 900 years. Eight distinct century- to millennial-scale phases of high debris-flow frequency (>3 events per 100 years) are identified at c. 8300—8000, 7100—7000, 4300—3700, 3200—2800, 2200—1900, 1500—1200, 800—700 and 300—0 cal. BP. Similarities in the records from the three sites suggest underlying climatic variations linked to the frequency of intense summer and autumn rainfall events (the primary meteorological trigger of slope failure in the source areas). Differences between the records reflect local site sensitivity to the initiation of debris-flow activity and the triggering of each subsequent debris-flow event. It is also inferred that each debris-flow system passes through multimillennial stages of at first increasing and later decreasing sensitivity as its source area expands. The climatic signals in the debris-flow record appear to differ in various respects from those derived from other precipitation-sensitive proxies from southern Norway (river floods, snow avalanches and glacier variations). Debris-flow records provide, therefore, complementary information relating to extreme climatic events and demonstrate instabilities in the Holocene landscape but provide little or no support for the concept of an increasing landscape impact of debris flows in response to global warming.
Landform-sediment-process assemblages associated with four gorges and their corresponding downstream boulder fans in the alpine periglaciofluvial system of the Storutla river, Jotunheimen, southern Norway, are described. The potential volume of frost-weathered sediment excavated from the gorges is compared using a sediment-budget approach to calculate the volume of angular sediment within the fans accumulated during the Holocene. Fan volumes represent an estimated 18 to 53% of the total gorge volume. Allowing also for the volume of relatively small caliber material flushed through the system, 24 to 97% of the gorge volume is accounted for by an estimated minimum long-term Holocene rate of gorge excavation of 0.002 to 0.010 m3 m–1 yr–1 (minimum long-term Holocene gorge incision rate of 0.15–0.39 mm yr–1) Most of the remaining gorge volume is attributed to substantial pre-Holocene subglacial gorge incision by meltwater action. These rates of Holocene periglaciofluvial erosion of bedrock appear to exceed those characteristic of temperate fluvial systems unaffected by tectonic uplift. The implied rates of frost weathering (macrogelivation) are less than those under optimum conditions in arctic-alpine environments but support the efficacy of frost weathering in locations susceptible to the annual freeze-thaw cycle.
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 levées, 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.
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 levées, 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.
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