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Glacier Movement ISSN 2047-0371 Glacier Movement C. Scott Watson1 and Duncan Quincey1 1 School of Geography and water@leeds, University of Leeds ([email protected]) ABSTRACT: Quantification of glacier movement can supplement measurements of surface elevation change to allow an integrated assessment of glacier mass balance. Glacier velocity is also closely linked to the surface morphology of both clean-ice and debris-covered glaciers. Velocity applications include distinguishing active from inactive ice on debris-covered glaciers, identifying glacier surge events, or inferring basal conditions using seasonal observations. Surface displacements can be surveyed manually in the field using trigonometric principles and a total station or theodolite for example, or dGPS measurements, which allow horizontal and vertical movement to be quantified for accessible areas. Semi-automated remote sensing techniques such as feature tracking (using optical or radar imagery) and interferometric synthetic aperture radar (InSAR) (using radar imagery), can provide spatially distributed and multi-temporal velocity fields of horizontal glacier surface displacement. Remote-sensing techniques are more practical and can provide a greater distribution of measurements over larger spatial scales. Time-lapse imagery can also be exploited to track surface displacements, providing fine temporal and spatial resolution, although the latter is dependent upon the range between camera and glacier surface. This chapter outlines the costs, benefits, and methodological considerations when using field-based and remote sensing techniques for deriving glacier velocity, and example workflows are presented. KEYWORDS: glacier velocity, feature tracking, SAR interferometry, time-lapse, surge Introduction Glacier movement can be attributed to basal sliding, internal ice deformation, and Glacier movement is spatially and temporally subglacial deformation of the glacier bed; variable, both within glacierised catchments, hence there is an interaction between the and between glacierised regions, determined gravitational driving force of downward ice by topographic and climatic conditions. movement, and the resistance encountered Quantifying movement rates in conjunction at the bed and margins (Benn and Evans, with mass balance estimates, helps reveal 2010). Difficulties in accessing the ice-bed the response of glacial environments to interface of a glacier makes quantifying basal climate change. Glacier movement can be movement difficult, though not impossible decoupled from climatic warming, such as (e.g. Hubbard, 2002; Kavanaugh and Clarke, during sudden surge events where rapid 2006) hence glacier surface movement is displacement can occur over a short period, commonly used as a proxy to represent or enhanced glacial retreat that can be overall glacier motion. Therefore, methods of initiated at lacustrine- or marine-terminating quantifying glacier surface movement form glaciers. Assessing the future evolution and the focus of this chapter. mass balance of glaciers is essential to determine their contribution to sea level change, seasonal meltwater availability Geomorphological interest including supra-, en-, sub- and pro-glacial As an agent of significant landscape change, water storage, and glacial lake outburst flood glacier movement and stagnation, both past (GLOF) hazard development. and present, is of notable interest to geomorphologists at local to regional scales. British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 4.5 (2015) Glacier Movement 2 Examples include reconstructing past glacial a long quiescent phase (decades to years) extents (e.g. Benn and Owen, 2002; Nesje, and a short active phase (years to decades) 2009), assessing the proglacial implications of rapid ice movement, where velocities can of GLOF events following glacial recession be ca. 100 times greater (Meier and Post, and subsequent lake development (e.g. 1969). These events can bias snap-shot Westoby et al., 2014), and examining glacial mass balance and glacial extent observations morphology and dynamics such as during if not accounted for. However, over longer surge events (e.g. Clarke et al., 1984; Kamb periods this effect may be diminished: et al., 1985; Quincey et al., 2011), or on Gardelle et al. (2013) observed similar mass debris-covered glaciers exhibiting stagnation budgets for surge-type and non-surging and surface lowering (e.g. Immerzeel et al., glaciers in the Pamir-Karakorum Himalaya 2014). The contemporary response of ice over a 10 year period. The spatial and sheets to climate change is also receiving temporal change in glacier velocity during a significant attention owing to the implications surge can be observed using remote sensing for sea level rise and the potential for techniques (e.g. Quincey et al., 2011) (Figure resource exploitation. Here, ice movement 1), which can help describe the distribution, reflects the export and subsequent loss of ice duration, magnitude, and return period of mass into the ocean, which is coupled to glacier surges, for which the controlling surface meltwater fluxes (Zwally et al., 2002). mechanisms are not fully understood. The likely response of ice sheets during Evidence of past surge-type behaviour can deglaciation can be inferred from be derived from characteristic deformation interpretations of deglaciated landscapes landforms such as thrust moraines, (e.g. the BRITICE mapping project) (Evans et concertina eskers (Evans and Rea, 1999), or al., 2005), and used in conjunction with deformed medial moraines on the numerical modelling to assess contemporary contemporary glacier surface (Meier and ice sheet change (Bingham et al., 2010). Post, 1969). Debris-covered glaciers Assessments of glacier movement are important when studying debris-covered glaciers owing to a complex response to climate change. Debris-covered glaciers that are stagnating and exhibiting surface lowering are associated with the development of supraglacial ponds (Bolch et al., 2012). The thermal energy stored in ponds and transmitted to the underlying glacier ice can locally increase ablation, and under favourable conditions ponds can also coalesce into larger glacial lakes with Figure 1: Illustrative propagation of a glacier subsequent water storage and potentially surge front derived from centre line surface hazardous implications (Benn et al., 2012). velocities from Quincey et al. (2011). Note Exposed ice faces often surround that areas of cloud cover or poor image supraglacial ponds and can represent areas contrast can lead to data voids. of enhanced ablation in a melt regime that is largely suppressed by thick debris cover Observations of glacier movement (Sakai et al., 2002; Reid and Brock, 2014). Here, the persistence and evolution of both Glacier movement is associated with supraglacial ponds and ice cliffs are closely erosional and depositional landforms, and the linked to a low glacier velocity (Quincey et al., transport of sediment down glaciated 2007). catchments. This can be important for contemporary glacier dynamics, such as a terminal moraine-dammed lake promoting Surge glaciers calving retreat (e.g. Imja Tsho, Nepal), and Surge glaciers are characterised by an can also be used to interpret and reconstruct oscillatory behaviour which switches between past glacial landscapes. Water-occupied British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 4.5 (2015) 3 C. Scott Watson and Duncan Quincey over-deepened basins epitomise glacial increasing channelization of the drainage erosion and subsequent glacial retreat. system subsequently reduces subglacial water pressure. However, horizontal Seasonal velocity observations can indicate velocities may remain raised compared to basal thermal conditions. For example, winter in response to continued meltwater contrasting summer and winter velocities input, despite a more effective drainage suggest meltwater driven basal sliding is system (Bingham et al., 2006). likely, which is linked to bed erosion and ultimately landscape evolution. Studies investigating glacier movement over shorter Summary of techniques temporal timescales (e.g. hours, days, and Early observations of glacier movement weeks) than is common for remotely sensed utilised transects of stakes inserted into the observations are able to identify the links glacier surface, which can be tracked from between glacier-hydrology and glacier surrounding moraines using a theodolite and velocity. Seasonal meltwater input into a triangulation techniques (e.g. Kodama and glacier system can act to reduce basal drag Mae, 1976). Alternatively, increasingly and subsequently increase horizontal and utilised remote sensing techniques can vertical glacier velocity as the sub-glacial enable a fully distributed analysis of glacier drainage system evolves (e.g. Mair et al., surface movement both horizontally and 2001; Copland et al., 2003). Hence the water vertically (Quincey et al., 2005), which pressure in this drainage system is closely complements geomorphological observations linked to glacier sliding (Iken et al., 1983). (e.g. Hambrey et al., 2008), and allows an This increase in velocity can be short-lived integrated assessment of glacier dynamics where a subglacial meltwater reservoir alongside mass balance observations (e.g. outbursts (e.g. Bingham et al., 2006). Benn et al., 2012). Depleted surface meltwater inputs and Table 1. A summary of techniques for assessing glacier surface motion and practical considerations. Technique
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