Alluvial cover dynamics in response to floods of various magnitudes: The effect of the release of glaciogenic material in a Swiss Alpine catchment

作者：Van den Berg Fabien^*; Schlunegger Fritz

来源：Geomorphology, 2012, 141: 121-133.

DOI：10.1016/j.geomorph.2011.12.030

• Swiss Alps
• Glaciogenic sediments, fluvial processes and channel morphology
• Flood modeling
• Photogrammetry
• Alluvial cover

Fluvial incision into formerly glaciated landscapes is commonly affected by the release of glaciogenic sediments from the hillslopes, via landsliding, hillslope failure and debris flow activity. These deposits typically comprise abundant block-size clasts. Here, we determine the ability of the fluvial processes to evacuate glacially derived sediments in an Alpine hanging valley blanketed by %26gt; 100-m-thick glacial till. We assess along-stream profiles of basal shear stress during annual, decadal and centennial floods, by means of the hydrological model HEC-RAS. We use bed surface grain size data to: (i) evaluate the Shields stress profiles for these floods, (ii) determine at which timescale the bed undergoes significant changes, and to (iii) predict transport capacity patterns for each flood. In addition, we present digital photogrammetry-based estimates of bed elevation changes over two selected time intervals that experienced low- and high- magnitude floods, respectively. The results indicate that floods with return periods of %26lt;= 10 years are capable of mobilizing the coarse fraction of the bed material, thereby substantially altering the channel geometry. The predicted near-critical Shields stress values for these floods in the area where the glacial deposits are thickest suggest a strong hillslope-channel coupling, which may have resulted in the formation of the knickzone observed in this sector. However, decadal to centennial floods are required to transport the material over long distances and scour the bed in the upper part of the inner gorge. In addition, the observed downstream trend from bed scouring to deposition in the inner gorge for these floods and the opposite trend for low-magnitude floods suggest that these patterns are not directly controlled by downstream variations of transport capacity. We propose that the supply of sediment through hillslope failure and bank collapse significantly contributes to the sediment flux for high-magnitude floods. We conclude that the morphology and the timescale of sediment transport in this glacially conditioned landscape have been substantially controlled by the hillslopederived supply of coarse-grained material.

• 出版日期2012-3-1