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

Abstract

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. Marren et al., 2002 ;Marren et Schuh, 2009 fig. 3). 3,5 (Lunt et al., 2004 ;Rubin et al, 2006). A partir de l'analyse de données de terrain et photo-interprétées, Sambrook Smith et al. (2005) (Lunt et al., 2004). Lunt et al. (2004) et Rubin et al. (2006 (Sambrook Smith et al., 2005 ;Kelly, 2006 (Marren, 2002b(Marren, , 2005 Généralement, la surface d'un banc de jökulhlaup est clairement identifiable par la présence de nombreuses marques laissées par les blocs de glace transportés et déposés lors de la crue. La phase de décrue du jökulhlaup de 1996 et les remaniements ont incisé le banc et l'ont divisé en sous-bancs qui ont constitué le point de départ du remaniement postérieur. En 2001, les dépôts du jökulhlaup de 1996 dans le chenal de la Skeiðará étaient 2,5 à 3 m plus hauts que la surface des bancs remaniés adjacents (Marren et al., 2002). La remobilisation des dépôts du jökulhlaup de 1996 a commencé au cours de la saison d'ablation de 1997. Les photographies aériennes de 1997, 2003 et 2012 témoignent de l'ampleur des changements du chenal au cours de la période de réajustement post-jökulhlaup. A l'intérieur du chenal principal de la Skeiðará, toutes les traces de la crue de 1996 ont rapidement été effacées. Le remaniement latéral des bancs a progressivement réduit leur superficie même si aucune crue n'a été en mesure de détruire la totalité des traces à la surface des bancs du jökulhlaup de 1996 (fig. 4). Dans le secteur oriental ...

Reconstructing glacial outburst floods (jökulhlaups) from geomorphology: Challenges, solutions, and an enhanced interpretive framework

“…It also applies at a landscape scale, with different sequences of events generating similar suites of landforms or sediments (Schumm, 1991). Sandurs offer an excellent example, as glaciers, proglacial rivers, and jökulhlaups repeatedly erode, deposit, and rework material to produce a similar set of landforms and depositional sequences (Maizels, 1997;Marren, 2005Marren, , 2016Marren and Schuh, 2009). For example, hummocky terrain on the sandur at Breiðamerkurjökull outlet glacier, south Iceland, can be formed either by the melting of glacier ice covered by glaciofluvial outwash sediment or by jökulhlaup-deposited ice blocks melting beneath flood sedimentation (Evans and Twigg, 2002;Russell et al, 2005).…”

“…Alternatively, jökulhlaups may contain low sediment concentrations if the drainage route is sediment-starved-for example, if previous floods have flushed material from the system. Thus, larger floods do not necessarily deposit greater quantities of material (Dunning et al, 2013;Maizels, 1997;Marren, 2005Marren, , 2016Westoby et al, 2014).…”

“…Each jökulhlaup landform or depositional sequence is the product of a unique set of hydraulic and environmental conditions that are controlled by a complex array of interdependent factors, including flood stream power, triggering mechanism, hydrograph shape, dam characteristics, sediment availability, drainage route topography and lithology, base level, and channel geometry. These parameters can vary significantly over the course of a single flood and between different flood events (Carrivick, 2011; Carrivick et al, 2013a; Cenderelli and Wohl, 2003; Clague and Evans, 2000; Kershaw et al, 2005; Korup and Tweed, 2007; Maizels, 1997; Marren, 2005, 2016; Roberts, 2005; Rushmer, 2006; Russell et al, 2005; Tomczyk et al, 2020; Tweed and Russell, 1999). Thus, jökulhlaups can generate some, all, or none of the same landforms, even if floods have similar hydraulic conditions or drain across the same landscape (Clague et al, 2021; Herget, 2005).…”

“…Variability in landform or sediment preservation also poses specific challenges in interpreting jökulhlaup geomorphologic evidence. Landscapes are palimpsests, sculpted by a complex sequence of natural (and sometimes anthropogenic) processes, and jökulhlaup landforms and sediments may be removed, modified, or obscured to leave an incomplete geomorphologic record of the flood event (Carrivick and Tweed, 2019; Marren, 2016; Schumm, 1991). Landform and sediment preservation depends on a complex array of factors (Carling, 2013; Maizels, 1997; Marren, 2005).…”

“…One interpretive strategy is to group landforms into assemblages that develop under similar environmental conditions, as has been defined for glacial landsystems, for example (Ballantyne, 2002; Benn and Evans, 2006; Evans et al, 2006; Möller and Dowling, 2018; Shaw, 2010; Storrar et al, 2020). Yet, while researchers have promoted this approach for jökulhlaup landscapes, studies have been largely limited to megaflood sedimentary deposits (Carling, 2013; Carling et al, 2009a), sediments on sandurs (glacial outwash plains) (Maizels, 1997; Marren, 2002, 2005, 2016; Marren and Schuh, 2009), and individual case studies (Supplemental Table 1—supplementary information). No previous publication has addressed jökulhlaup geomorphologic interpretation or developed jökulhlaup landform assemblages at a comprehensive, global scale.…”