Jökulhlaup (ice-dammed lake outburst flood) impact within a valley-confined sandur subject to backwater conditions, Kangerlussuaq, West Greenland

Abstract

This paper examines the controls on the morphology and sedimentology of a valley-confined proglacial outwash plain or ‘sandur’ in West Greenland subject to glacial outburst floods (jökulhlaups). Jökulhlaup flow conditions observed during a recent (1987) flood are linked to sandur sedimentology and morphological change. Backwater effects over the distal sandur resulted in a reduction of stream power despite increasing discharge. Bar units were typically large. Bar front advance of 60 m during 1987 jökulhlaup occurred prior to waning stage bar dissection by chute channels. Channel change was confined to the distal sandur, due to the greater erodibility of the finer-grained bars. A temporary zone of sediment storage prevailed at the transition between free flowing and ponded flood waters until flood powers increased during waning flood stage as backwater effects were removed. Extension of pre-existing high-relief amplitude bars within the distal sandur records sedimentation related to at least eight jökulhlaups, whilst lower-relief amplitude, proximal sandur sediments provide a composite record of the last high stage jökulhlaup flows. Backwater conditions during jökulhlaups have increased the rapidity of spatial variation of grain sizes and fluvial facies types. Complex, within-event, sediment reworking and routing is generated by the onset and break up of backwater effects. The role of local topography is therefore crucial when interpreting the sedimentary record of former proglacial channels potentially subject to jökulhlaups. This study may prove useful in identifying the presence of flood cycles within palaeo-fluvial systems in both formerly glaciated and non glaciated areas.