Mechanisms linking active rock glaciers and impounded surface water formation in high-mountain areas (original) (raw)
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Near-surface hydraulic conductivity of northern hemisphere glaciers
Hydrological Processes, 2018
The hydrology of near-surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this nearof a bespoke capacitance-based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high-resolution record of near-surface water level fluctuations. Piezometers were employed at ten northern hemisphere glaciers, and through the application of standard bail-recharge techniques, we derive hydraulic conductivity (K) values from 0.003 to 3.519 m d-1 , with a mean of 0.185 ± 0.019 m d-1. These results are comparable to those obtained in other discrete studies of glacier near-surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative shortwave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and This article is protected by copyright. All rights reserved. differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near-surface ice hydrology than hitherto appreciated, and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. This has implications not only for the accuracy of predictive meltwater runoff models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, contaminants, microbes and nutrients to downstream ecosystems. Since continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions and, consequently, detailing weathering crust hydraulics represents a research priority since the flow-path it represents remains poorly constrained.
Hydrogeology Journal, 2016
Relict rock glaciers are complex hydrogeological systems that might act as relevant groundwater storages; therefore, the discharge behavior of these alpine landforms needs to be better understood. Hydrogeological and geophysical investigations at a relict rock glacier in the Niedere Tauern Range (Austria) reveal a slow and fast flow component that appear to be related to the heterogeneous structure of the aquifer. A numerical groundwater flow model was used to indicate the influence of important internal structures such as layering, preferential flow paths and aquifer-base topography. Discharge dynamics can be reproduced reasonably by both introducing layers of strongly different hydraulic conductivities or by a network of highly conductive channels within a low-conductivity zone. Moreover, the topography of the aquifer base influences the discharge dynamics, which can be observed particularly in simply structured aquifers. Hydraulic conductivity differences of three orders of magnitude are required to account for the observed discharge behavior: a highly conductive layer and/or channel network controlling the fast and flashy spring responses to recharge events, as opposed to less conductive sediment accumulations sustaining the long-term base flow. The results show that the hydraulic behavior of this relict rock glacier and likely that of others can be adequately represented by two aquifer components. However, the attempt to characterize the two components by inverse modeling results in ambiguity of internal structures when solely discharge data are available.
Active rock glaciers as shallow groundwater reservoirs, Austrian Alps
Grundwasser
Rock glaciers are the most prominent landforms of alpine permafrost and comprise complex shallow aquifer systems in (high) alpine catchments. Recession analyses of groundwater discharge of four active rock glaciers that contain permafrost ground ice show that they have a base flow component of the order of a few liters per second, similar to that of a relict rock glacier in which permafrost ground ice is absent. This is related to an unfrozen (fine-grained) base layer with a thickness of about 10 m. Based on a threshold analysis of precipitation events and event water discharge, depressions atop the bedrock or the permafrost table seem to play only a minor role in storing groundwater. This important finding has rarely been documented, but is highly relevant for optimal groundwater resources management in sensitive (high) alpine catchments and ecosystems. All the rock glaciers analyzed here are located in the Austrian Alps and represent the nationwide sites where suitable discharge d...
Geomorphology, 2009
Rock glaciers occur as lobate or tongue-shaped landforms composed of mixtures of poorly sorted, angular to blocky rock debris and ice. These landforms serve as primary sinks for ice and water storage in mountainous areas and represent transitional forms in the debris transport system, accounting for~60% of all mass transport in some alpine regions. Observations of active (flowing) alpine rock glaciers indicate a common association between the debris that originates from cirque headwalls and the depositional lobes that comprise them. The delivery of this debris to the rock glacier is regulated primarily by the rate of headwall erosion and the point of origin of debris along the headwall. These factors control the relative movement of individual depositional lobes as well as the overall rate of propagation of a rock glacier. In recent geophysical studies, a number of alpine rock glaciers on Prins Karls Forland and Nordenskiöldland, Svalbard, Norway, and the San Juan Mountains of southwest Colorado, USA, have been imaged using ground penetrating radar (GPR) to determine if a relationship exists between the internal structure and surface morphology. Results indicate that the overall morphologic expression of alpine rock glaciers is related to lobate deposition during catastrophic episodes of rockfall that originated from associated cirque headwalls. Longitudinal GPR profiles from alpine rock glaciers examined in this study suggests that the difference in gross morphology between the lobate and tongueshaped rock glaciers can be attributed primarily (but not exclusively) to cirque geometry, frequency and locations of debris discharge within the cirque, and the trend and magnitude of valley gradient in relation to cirque orientation. Collectively, these factors determine the manner in which high magnitude debris discharges, which seem to be the primary mechanism of formation, accumulate to form these rock glaciers.
Quaternary Science …, 2010
Ground ice of permafrost origin and sedimentary ice of glacial origin can coexist in locations where rock glaciers and glaciers interacted, as well as in glacigenic sediments abandoned by a retreating glacier and subsequently exposed again to atmospheric cooling. Some of these geomorphological settings in the Central (Foscagno rock glacier) and Western Alps (Marinet and Schiantala rock glaciers, Schiantala debris-covered glacier, Maledia glacier) were explored by means of geoelectrical tomographies. The aim was that of inferring the presence of ice and cryologically interpreting electrical stratigraphies in order to test whether or not the internal structure of these landforms can be used for the reconstruction of recent permafrost and glacier evolution. Geomorphological data assisted these reconstructions and available borehole stratigraphies were used to calibrate the resistivities.
Engineering geomorphology of rock glaciers
Geomorphology, 1999
A partnership between geomorphology and engineering is facilitating human development in this harsh environment. Rock glaciers provide locations for urban water sources, construction borrow sources, drill sites, shaft and tunnel portals, ski tower supports, and dam abutments. Rock glaciers, as dynamic landforms, necessitate proper identification in the field. Placing structures on, in, or adjacent to rock glaciers requires an appreciation and understanding of their temporal stability. Internal and surface characteristics provide important clues to the development and deformation of rock glaciers. Rock glaciers play a significant role in the alpine debris transport system. Active movement and mass wasting are perhaps the most obvious geologic hazards affecting engineered works. The structure of the rock glacier is conducive to the production of a steady, continuous supply of meltwater during summer months. Thus, rock glaciers serve as alpine aquifers. Consideration of rock glaciers as potential aquifer sources requires caution because of the long-term impact of climate change on the temporal nature of the landform. From the rock glaciers that we have monitored for water quality characteristics, it appears that they provide quality potable water. This paper provides a foundation for appreciation and understanding of rock glaciers from an engineering geomorphologic point of view. The approach taken in this paper provides practical, important information to aid the engineer and engineering geologist in prudent evaluations of rock glaciers as potential sites for human development and uses. The bottom line of our paper is: rock glaciers must be aÕoided for essentially all structures.
Internal structure of the Green Lake 5 rock glacier, Colorado Front Range, USA
Permafrost and Periglacial Processes, 2011
Information about the internal structure of rock glaciers is needed to understand their reaction to ongoing climate warming. Three different geophysical techniques-shallow seismic refraction, ground-penetrating radar (GPR) and electrical resistivity tomography-were used to develop a detailed subsurface model of the Green Lake 5 rock glacier in the Colorado Front Range, USA. Below a thin zone of fine sediments and soils (0.7-1-m thickness; 0-20 kVm and 320-370 m s À1), a 1-3-m thick zone with low p-wave velocities (790-820 m s À1) and high electrical resistivity (20-100 kVm) is interpreted as the ice-free, blocky active layer with large void spaces. The data corroborate strong reflections of the GPR signals which travel at this depth at 0.11 m ns À1. A third layer that extends from depths of 1-3 m to about 5 m is characterised by lower electric resistivities (5-20 kVm) and has lower electromagnetic wave velocities (0.65 m ns À1), representing unfrozen, finer and wetter sediments. At around 5-m depth, the measured physical parameters change drastically (vp ¼ 3200-3300 m s À1 , 50-150 kVm, vGPR ¼ 0.15 m ns À1), showing an ice-rich permafrost zone above the bedrock. This model of the internal structure was used to evaluate an existing hydrological flowpath model based on the hydrochemical properties of water outflow from the rock glacier. Copyright
Moraine-dammed lakes at debris-covered glaciers are becoming increasingly common and pose significant outburst flood hazards if the dam is breached. While moraine subsurface structure and internal processes are likely to influence dam stability, only few sites have so far been investigated. We conducted electrical resistivity tomography (ERT) surveys at two sites on the terminal moraine complex of the Ngozumpa Glacier, Nepal, to aid assessment of future terminus stability. The resistivity signature of glacier ice at the site (100-15 kΩ m) is more consistent with values measured from cold glacier ice and while this may be feasible, uncertainties in the data inversion introduce ambiguity to this thermal interpretation. However, the ERT data does provide a significant improvement to our knowledge of the subsurface characteristics at these sites, clearly showing the presence (or absence) of glacier ice. Our interpretation is that of a highly complex latero-terminal moraine, resulting from interaction between previous glacier advance, recession and outburst flooding. If the base-level Spillway Lake continues to expand to a fully formed moraine-dammed glacial lake, the degradation of the ice core could have implications for glacial lake outburst risk.
Journal of Glaciology, 2000
We have obtained common offset, common midpoint (CMP) and borehole vertical (VRP) ground-penetrating radar profiles close to the margin of Falljökull, a small, steep temperate valley glacier situated in southeast Iceland. Velocity analysis of CMP and VRP surveys provided a four-layered velocity model. This model was verified by comparison between the depths of englacial reflectors and water channels seen in borehole video, and from the depths of boreholes drilled to the bed. In the absence of sediment within the glacier ice, radar velocity is inversely proportional to water content. Using mixture models developed by Paren and Looyenga, the variation of water content with depth was determined from the radar velocity profile. At the glacier surface the calculated water content is 0.23–0.34% (velocity 0.166 m ns−1), which rises sharply to 3.0–4.1% (velocity 0.149 m ns−1) at 28 m depth, interpreted to be the level of the piezometric surface. Below the piezometric surface the water conte...