• Changing glaciers in the Brooks Range and western Chugach Mountains, Alaska: mass loss, runoff increase, and supraglacial volcanic tephra coverage

      Geck, Jason; Hock, Regine; Coakley, Bernard; Dial, Roman; Loso, Michael (2020-12)
      Glaciers in Alaska cover over ~87,000 km² (~ 6 % of the state) with most glaciers thinning and retreating at an increasing rate. The thinning and retreating of glaciers worldwide can have an immediate socio-economic implication in addition to the longer-term glacier meltwater contribution to sea level rise. This dissertation investigated Alaskan glaciers in the Brooks Range for mass loss and area reductions over the period 1970-2001 (Chapter 2), historic mass balance and runoff for Eklutna Glacier, located in western Chugach Mountains, using a temperature index model over 1984-2019 period (Chapter 3), and the persistence of tephra from a volcanic eruption of Mt. Spurr in 1992 on seven western Chugach Mountain glaciers (Chapter 4). Glaciers in the Brooks Range in Arctic Alaska (> 68° N) are important indicators of climate change and provide information on long-term climate variations in an area that has few high elevation meteorological stations. Digital elevation models (DEMs) reconstructed from topographic maps were differenced from an interferometric synthetic aperture radar DEM to calculate the volume and mass changes of 107 glaciers (42 km²). Over the period 1970-2001, total ice volume loss was 0.69 ± 0.06 km³ corresponding to a mean (area-weighted) specific mass balance rate of -0.54 ± 0.05 m w.e. a⁻¹ (± uncertainty). The arithmetic mean of all glaciers' specific mass balance rates was -0.47 ± 0.27 m w.e. a⁻¹ (± 1 std. dev.). A subsample of 36 glaciers found a 26 ± 16 % mean area reduction over ~35 years. Alaska's largest city, Anchorage, is critically dependent upon the melt water of Eklutna Glacier (29 km²) for both drinking water and hydropower generation; however, the glacier is rapidly retreating. We used a temperature index model to reconstruct the glacier's mass balance for the period 1985-2019 and quantify the impacts of glacier change on runoff. Eklutna Glacier experienced a significant annual mean surface mass balance negative trend (-0.38 m w.e. Decade⁻¹). Mean annual cumulative melt increased by 24 % between the 1985-93 and 2011-19 period. Additionally, the day of the year when 95% of annual melt has occurred was eight days later in the later time period than in the earlier period, demonstrating a prolongation of the melt season. The modeled mean annual discharge increased at a rate of 0.2 m decade⁻¹. This indicates that peak water, i.e. the year when annual discharge starts decreasing as the glacier becomes smaller, has not been reached. The past increases in runoff quantity and melt season length provide opportunities for water resource managers that must be balanced against future decreased runoff as the glacier continues to shrink. Volcanic eruptions deposit volcanic tephra on glaciers in Alaska, modifying surface albedo and glacier melt. We mapped the distribution of tephra originating from the eruption of Mt. Spurr in 1992 using aerial photos and satellite imagery on seven glaciers located approximately 180 km east of the volcano in western Chugach Mountains in southcentral Alaska. The glaciers were completely covered with ≥ 500 g m⁻² tephra immediately after the event. Tephra deposits are still visible on all glaciers 26 years after the eruption. Using LandSat 8 surface reflectance bands, we quantified percentages of tephra glacier coverage. Results suggest an increasing tephra extent on five of the seven investigated glaciers over 2013-2018 period explained by firn line retreat. The mean percent increase for all glaciers was 4% with Troublesome Glacier showing greatest increase (~ 7 %) and Finch Glacier showing a slight decrease (~ 1 %). This long- term tephra persistence on glacier surfaces most likely enhanced melt although the precise effect remains unknown.
    • Exact and numerical solutions for stokes flow in glaciers

      Mitchell, William H. (2012-08)
      We begin with an overview of the fluid mechanics governing ice flow. We review a 1985 result due to Balise and Raymond giving exact solutions for a glaciologically-relevant Stokes problem. We extend this result by giving exact formulas for the pressure and for the basal stress. This leads to a theorem giving a necessary condition on the basal velocity of a gravity-induced flow in a rectangular geometry. We describe the finite element method for solving the same problem numerically. We present a concise implementation using FEniCS, a freely-available software package, and discuss the convergence of the numerical method to the exact solution. We describe how to fix an error in a recent published model.
    • Modeling supraglacial lake drainage and its effects on the seasonal evolution of the subglacial drainage system in a tributary glacier setting

      Franco, Nevil Arley; Truffer, Martin; Wackerbauer, Renate; Delamere, Peter (2021-08)
      This work aims to gain a better understanding of the relationship between glacier motion and water distributed through subglacial drainage systems. A numerical scheme (GlaDS) is used to model both inefficient and efficient drainage systems to see which dominates after the draining of a supraglacial lake on a synthetic glacier that is made up of an outline that features a main branch and a tributary. The geometry is based on the surgetype Black Rapids Glacier (Ahtna Athabascan name: Da lu'itsaa'den) in Alaska, where a lake develops in the higher ablation zone, and drains rapidly early in the melt season. It has also been observed that this lake drainage causes a twofold or threefold speed-up of the main branch, with some acceleration of the lower-lying Loket tributary. This speed-up can be considered a surrogate for a surge, which also initiates in the main branch, while, during times of quiescence, the ice flow on the tributary is dominant. We investigate the effects of varying timing and volume inputs of lake drainage with a focus on its effects beneath the tributary. We find that the response of the glacier depends on the seasonal timing, the amount of water from the draining lake, and its location on or near the margins of the glacier. Results show that an inefficient drainage system is the cause of the glacier speed-up, both when the lake drains rapidly or when there is an extended time in drainage, at any time of the season. The speed signals vary throughout the glacier depending on the location of the lake relative to that of an evolved efficient drainage system.
    • Strategies for applying active seismic subglacial till characterization methods to valley glaciers

      Zechman, Jenna M.; Truffer, Martin; Larsen, Christopher F.; Coakley, Bernard J.; Amundson, Jason M. (2017-05)
      Subglacial materials play an important role in glacier dynamics. High pore-pressure, high porosity (dilatant) tills can contribute to high basal motion rates by deforming. Amplitude Variation with Angle (AVA) analysis of seismic reflection data uses the relationship between basal reflectivity and reflection incidence angle to characterize the subglacial material. This technique can distinguish between dilatant tills and less-porous, non-deforming (dewatered) tills due to their distinctive reflectivity curves. However, noise from crevasses and glacier geometry effects can complicate reflectivity calculations, which require a source amplitude derived from the bed reflection multiple. We use a forward model to produce synthetic seismic records, including datasets with and without visible bed reflection multiples. The synthetic data are used to test source amplitude inversion and crossing angle analysis, which are amplitude analysis techniques that do not require absolute reflectivity calculations. We and that these alternative methods can distinguish subglacial till types, as long as reflections from crevasses do not obscure the bed reflection. The forward model can be used as a planning tool for seismic surveys on glaciers, as it can predict AVA success or failure based on crevasse geometries from remote sensing data and glacier bed geometry from radar or from a worst-case-scenario assumption of glacier bed shape. Applying lessons from the forward model, we perform AVA on a seismic dataset collected from Taku Glacier in Southeast Alaska in March 2016. Taku Glacier is a valley glacier thought to overlay thick sediment deposits. It has been the subject of numerous studies focusing on its ice-sediment interactions. Our analysis indicates that Taku Glacier overlies unconsolidated tills with porosity values greater than 33 %, though because of uncertainties due to the lack of a bed reflection multiple, it is possible that the tills are not dilatant.