• Bar deposition in glacial outburst floods: scaling, post-flood reworking, and implications for the geomorphological and sedimentary record

      Marren, Philip M.; University of Chester (GFG, 2016-03-31)
      The appearance of a flood deposit in the geomorphological and sedimentary record is a product of the both the processes operating during the flood, and those that occur afterwards and overprinting the deposit with a record of ‘normal’ processes. Nearly half of the total discharge of the November 1996 jökulhlaup on Skeiðarársandur was discharged through the Skeiðará river. The flood deposits have been extensively reworked since, up until 2009 when the channel was abandoned, effectively leaving the Skeiðará as a terrace, when retreat of Skeiðarárjökull directed meltwater to the adjacent Gígjukvísl river system. This paper describes the creation and modification of jökulhlaup barforms in the Skeiðará river, relating the changes to post-flood fluvial processes and glacier retreat. Large compound bars formed from the amalgamation of unit bars up to 1.5 km long. The location of the compound bars was governed by the macro-scale topography of the flood channel, and their size by upstream channel width in accordance with bar-scaling theory. Jökulhlaup bars are therefore scale invariant and formed in a similar fashion to braid bars in non-jökulhlaup braided rivers. Post-flood fragmentation and reworking of the bars consistently increased the length-width ratio of preserved bar fragments from approximately two and one half to over five. These observations increase our understanding of the preservation potential and final form of jökulhlaup deposits and provide the basis for an improved model for the recognition of jökulhlaup deposits in the geomorphological and sedimentary record.
    • Iceberg jam floods in Icelandic proglacial rivers: testing the self-organized criticality hypothesis

      Roussel, Erwan; Toumazet, Jean-Pierre; Marren, Philip M.; Cossart, Etienne; University Clermont Auvergne; University Blaise Pascal; CNRS ; University of Chester (GFG, 2016-03-31)
      In this paper, we describe a fluvial marginal process associated with the formation of iceberg jams in Icelandic proglacial lakes. The floods triggered by the release of these iceberg jams have implications for the geomorphic evolution of the proglacial fluvial system. The process of iceberg jam floods share some conceptual characteristics with Self-Organized Criticality (SOC) approach of complex systems. Using a simple numerical model and field observations, we test the hypothesis that iceberg jam floods exhibit SOC. Field observations and aerial photo-interpretations in southeastern Iceland demonstrate the occurrence of icebergs jam in ice-contact lakes. The mapping of the south Vatnajökull margins between 2003 and 2012 reveals an increase of the calving potentiality and a rise in the likelihood of iceberg jam flood occurrence. Based on the results of the numerical model and field observations, we suggest that iceberg jam floods should be recognized as a SOC phenomenon. Analysis of the simulated time-series show that the iceberg jam floods become less frequent and more similar in magnitude over time. This global trend is related to the gradual enlargement of the lake outlet channel.