Fluvial Response to Large RockSlope Failures: Examples from the Himalayas, the Tien Shan, and the So

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Fluvial Response to Large Rock-Slope Failures
Examples from the Himalayas, the Tien Shan, and
the Southern Alps in New Zealand
by Oliver Korup, Alexander L. Strom, Johannes
T. Weidinger

• Geomorphology, Volume 78, Issues 1-2, August 15,
  2006

Dave Eddy
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Landslide Dams

• Common in tectonically active mountain belts
• Largely ephemeral, 80 of historically
  documented dams failed in 1yr
• Of interest due to hazard of potential
  catastrophic outburst flows resulting from dam
  failure

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Landslide Dams

• Fluvial response to pulsed supply of hillslope
  sediment theoretically involve adjustment of
• Sediment yield
• Channel planform, cross-section, and gradient
• Bed configuration

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Landslide Dams

• Authors focused on channel gradient
• Hypothesize that formation of a sufficiently high
  rockslide dam would cause a fluvial response
  marked enough to be recognized in the river long
  profile
• Used rockslide dam height and channel steepness
  to quantitatively test

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Methods

• Analyzed river long profiles (from DEMs) to
  detect geomorphic imprints of rockslide dams on
  gradient
• Collected long-term fluvial incision rates to
  characterize fluvial response

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Kalopani rockslide, Nepal
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Beshkiol rockslide, Tien Shan
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Braga rockslide, Nepal
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Djashilkul rockslide, Tien Shan
Polnoon Burn, New Zealand
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RiverLong Profilesof PrehistoricRockslides
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Gohna Tal rockslide, India
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Findings

• Highest steepness index (ks) frequently where
  channels were blocked by large rockslides and
  rivers have re-incised into the debris
• Not all, but most, major knickpoints coincide
  with rockslide-dam breach or bypass channels

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River Long Profiles
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Discussion

• Landslide dam blockages can last from 1 day
  10,000 years
• Factors contributing to long-lived lakes include
  very large dam size and arid to semi-arid climate
  in the headwaters
• Dams produce significant volumetric changes to
  sediment budget

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Discussion

• In active mountain belts, create disequilibrium
  in the balance between uplift and erosion due to
  blockage and backwater aggradation
• No robust correlation between peak values of
  steepness index and and rockslide size
• High rates of fluvial incision promote
  preservation of rockslide debris

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Conclusions

• Deposits control fluvial response by
• Retaining sediment and forcing alluviation
• Relocating river channels through diversion or
  seepage
• Delaying fluvial bedrock incision through
  aggradation behind dams
• Forming high-energy breach channels with
  knickpoints and knickslopes
• Shaping valley-floor morphology on 101-104
  timescales