Abstract
Glaciological and hydraulic factors that control the timing and mechanisms of glacier lake outburst floods (GLOFs) remain poorly understood. This study used measurements of lake level at 15 min intervals and known lake bathymetry to calculate lake outflow during two GLOF events from the northern margin of Russell Glacier, west Greenland. We used measured ice surface elevation, interpolated subglacial topography and likely conduit geometry to inform a melt enlargement model of the outburst evolution. The model was tuned to best-fit the hydrograph rising limb and timing of peak discharge in both events; it achieved Mean Absolute Errors of <5%. About one third of the way through the rising limb, conduit melt enlargement became the dominant drainage mechanism. Lake water temperature, which strongly governed the enlargement rate, preconditioned the high peak discharge and short duration of these floods. We hypothesize that both GLOFs were triggered by ice dam flotation, and localized hydraulic jacking sustained most of their early-stage outflow, explaining the particularly rapid water egress in comparison to that recorded at other ice-marginal lakes. As ice overburden pressure relative to lake water hydraulic head diminished, flow became confined to a subglacial conduit. This study has emphasized the inter-play between ice dam thickness and lake level, drainage timing, lake water temperature and consequently rising stage lake outflow and flood evolution.
Highlights
Understanding of sudden and rapid glacier outburst floods, or “jökulhlaups” is important because glacier lakes are increasing in number and size worldwide in mountain regions (Carrivick and Tweed, 2013) and at ice sheet margins, in south-west Greenland (Carrivick and Quincey, 2014)
Since the equations are autonomous and the lake highstand too loosely located in time for us to put the “t = 0” precisely on the observational time line, in each comparison we always first align the hydrographs in time by sliding the simulated one against the observed one until the Mean Absolute Error (MAE) in Q between them is minimized for their overlap period
We suggest that the two floods were triggered with the lake level lower in 2012 compared to 2010 because the icedam had thinned, for which there is evidence from oblique field photographs, eye-witness reports from the Greenlandic mountain guide Adam Lyberth [pers. comm.], and comparison in this study of ice surface elevation made via ground-based dGPS surveys in 2015 and from a digital elevation model constructed from stereo images acquired in 2011
Summary
Understanding of sudden and rapid glacier outburst floods, or “jökulhlaups” is important because glacier lakes are increasing in number and size worldwide in mountain regions (Carrivick and Tweed, 2013) and at ice sheet margins, in south-west Greenland (Carrivick and Quincey, 2014). Sudden drainage of ice-marginal lakes can affect local ice dynamics (e.g., Anderson et al, 2005; Walder et al, 2006; Sugiyama et al, 2007; Riesen et al, 2010). Understanding how glacier lakes suddenly drain is challenging, not least due to the different triggers and drainage mechanisms that can act and interact, and due to a paucity of direct measurements. There are a number of theoretical models of ice-dammed lake drainage (e.g., Nye, 1976; Spring and Hutter, 1981; Clarke, 1982, 2003; Fowler, 1999, 2009; Flowers et al, 2004; Kessler and Anderson, 2004; Kingslake and Ng, 2013; Kingslake, 2015) and where models follow the approach of Nye (1976) they often ignore flood initiation, i.e., the flood trigger, by assuming the pre-existence of a conduit whose evolution controls the simulated flood. Ng and Björnsson (2003) concluded their analysis of ice-dammed lake drainage by stressing: (i) the importance of identifying the flood trigger, (ii) a need for more monitoring of lake levels during floods and (iii) reliable measurements of ice dam thickness and lake geometry
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