Knickpoint evolution in a supraglacial stream

Jayne Elizabeth Kamintzis, Tristram Irvine-Fynn, Thomas Holt, John Paul Pryderi Jones, Stephen Tooth, Hywel Griffiths, Bryn Hubbard

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Abstract

Despite numerous studies of knickpoints in bedrock and alluvial channels, no detailed description of knickpoint change on ice has been reported to date. This paper presents the first investigation of knickpoint evolution within a supraglacial stream. Repeat longitudinal profile surveys of a knickpoint on Vadrec del Forno, Switzerland reveal a step height increase of 115 mm and upstream migration of 0.26 m over three days during the 2017 ablation season. Rates and magnitudes of erosion vary spatially across the knickpoint in relation to differing discharge regimes. At high discharges (∼0.013 m3 s−1), erosion is focused at the step base; at low discharges (∼0.003 m3 s−1), erosion is focused on the reach upstream of the knickpoint, at the step lip and the step-riser face. This results in replacement of knickpoint morphology, driven by frictional thermal erosion and hydraulic action. Pool formation further influences step morphology, inducing secondary circulation and increased melt at the base of the step-riser, causing steepening. Results highlight the complexities of water flow over knickpoints, demonstrating that the stream power law does not accurately characterise changing knickpoint morphology or predict retreat rates. Although morphological similarities have been reported between supraglacial and bedrock/alluvial channels, knickpoints in non-ice-walled channels will not necessarily respond to discharge similarly to those on ice due to the different erosional processes involved.
Original languageEnglish
Pages (from-to)118-135
Number of pages18
JournalGeografiska Annaler: Series A, Physical Geography
Volume101
Issue number2
Early online date29 Nov 2018
DOIs
Publication statusPublished - 03 Apr 2019

Keywords

  • knickpoint
  • step
  • supraglacial
  • evolution
  • hydrodynamics
  • discharge
  • Knickpoint

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