Hydraulics of Subglacial Cavities

AbstractA theoretical model is developed to describe the steady-state behavior of interconnected, water-filled cavities at the glacier bed. Physically plausible cavities should contain constrictions along the flow path, with flow in the wider sections being relatively sluggish. Mean flow rates in cavities may be at least one order of magnitude less than in channels incised into the basal ice (R channels). Melting due to viscous dissipation - the process that allows R channels to exist - probably plays a minor or negligible role, as compared to glacier sliding, in determining the size of cavities. Furthermore, a system of subglacial cavities should not show a tendency for localization of flow in a few main conduits, as does an R-channel system. If water pressure rises to within several bars of overburden pressure, the rate of cavity closure by creep falls below the rate of cavity opening by sliding and melting, with cavities then becoming unstable. Subsequent evolution of the drainage system should depend upon the total melt-water flux. Circumstances may arise in which cavities and channels act as conduits for melt water; such a configuration would probably show unusual transient behavior.

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Water Pressure in Intra- and Subglacial Channels

Journal of Glaciology ◽

10.1017/s0022143000022188 ◽

1972 ◽

Vol 11 (62) ◽

pp. 177-203 ◽

Cited By ~ 642

Author(s):

Hans Röthlisberger

Keyword(s):

Sediment Load ◽

Water Pressure ◽

Drainage System ◽

Water Layer ◽

Frictional Heat ◽

Overburden Pressure ◽

Bed Topography ◽

Water Head ◽

Local Topography ◽

Dependent Flow

AbstractWater flowing in tubular channels inside a glacier produces frictional heat, which causes melting of the ice walls. However the channels also have a tendency to close under the overburden pressure. Using the equilibrium equation that at every cross-section as much ice is melted as flows in, differential equations are given for steady flow in horizontal, inclined and vertical channels at variable depth and for variable discharge, ice properties and channel roughness. It is shown that the pressure decreases with increasing discharge, which proves that water must flow in main arteries. The same argument is used to show that certain glacier lakes above long flat valley glaciers must form in times of low discharge and empty when the discharge is high, i.e. when the water head in the subglacial drainage system drops below the lake level. Under the conditions of the model an ice mass of uniform thickness does not float, i.e. there is no water layer at the bottom, when the bed is inclined in the down-hill direction, but it can float on a horizontal bed if the exponentnof the law for the ice creep is small. It is further shown that basal streams (bottom conduits) and lateral streams at the hydraulic grade line (gradient conduits) can coexist. Time-dependent flow, local topography, ice motion, and sediment load are not accounted for in the theory, although they may strongly influence the actual course of the water. Computations have been carried out for the Gornergletscher where the bed topography is known and where some data are available on subglacial water pressure.

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Water Pressure in Intra- and Subglacial Channels

Journal of Glaciology ◽

10.3189/s0022143000022188 ◽

1972 ◽

Vol 11 (62) ◽

pp. 177-203 ◽

Cited By ~ 38

Author(s):

Hans Röthlisberger

Keyword(s):

Sediment Load ◽

Water Pressure ◽

Drainage System ◽

Water Layer ◽

Frictional Heat ◽

Overburden Pressure ◽

Bed Topography ◽

Water Head ◽

Local Topography ◽

Dependent Flow

AbstractWater flowing in tubular channels inside a glacier produces frictional heat, which causes melting of the ice walls. However the channels also have a tendency to close under the overburden pressure. Using the equilibrium equation that at every cross-section as much ice is melted as flows in, differential equations are given for steady flow in horizontal, inclined and vertical channels at variable depth and for variable discharge, ice properties and channel roughness. It is shown that the pressure decreases with increasing discharge, which proves that water must flow in main arteries. The same argument is used to show that certain glacier lakes above long flat valley glaciers must form in times of low discharge and empty when the discharge is high, i.e. when the water head in the subglacial drainage system drops below the lake level. Under the conditions of the model an ice mass of uniform thickness does not float, i.e. there is no water layer at the bottom, when the bed is inclined in the down-hill direction, but it can float on a horizontal bed if the exponent n of the law for the ice creep is small. It is further shown that basal streams (bottom conduits) and lateral streams at the hydraulic grade line (gradient conduits) can coexist. Time-dependent flow, local topography, ice motion, and sediment load are not accounted for in the theory, although they may strongly influence the actual course of the water. Computations have been carried out for the Gornergletscher where the bed topography is known and where some data are available on subglacial water pressure.

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Water pressure and basal sliding on Storglaciären, northern Sweden

Journal of Glaciology ◽

10.1017/s0022143000016130 ◽

1995 ◽

Vol 41 (138) ◽

pp. 232-240 ◽

Cited By ~ 69

Author(s):

Peter Jansson

Keyword(s):

Water Pressure ◽

Surface Velocity ◽

Northern Sweden ◽

Overburden Pressure ◽

Weakly Coupled ◽

Basal Sliding ◽

Order Of Magnitude ◽

Image Position ◽

Hydraulic Jacking ◽

Small Valley

AbstractThe subglacial hydrology of the ablation area of Storglaciären, a small valley glacier in northern Sweden, is dramatically affected by a subglacial ridge, or riegel. Water pressures above this riegel are relatively constant, while down-glacier from it they vary significantly. The lower part of the glacier accelerates in response to peaks in basal water pressure. The upper part may be weakly coupled to the lower part during these peaks.A power-law fit of observed basal water pressures and measured surface velocities yieldswhereusis the surface velocity andPEis the effective water pressure (ice overburden pressure minus subglacial water pressure). Data from Findelengletscher, reported by Iken and Bindschadler (1986), yield an identical exponent and a coefficient one order of magnitude larger. The similar exponent implies that the process producing the velocity variations on both glaciers is similar. The variations in velocity are inferred to be due to hydraulic jacking on both glaciers.

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Water pressure and basal sliding on Storglaciären, northern Sweden

Journal of Glaciology ◽

10.3189/s0022143000016130 ◽

1995 ◽

Vol 41 (138) ◽

pp. 232-240 ◽

Cited By ~ 2

Author(s):

Peter Jansson

Keyword(s):

Water Pressure ◽

Surface Velocity ◽

Northern Sweden ◽

Overburden Pressure ◽

Weakly Coupled ◽

Basal Sliding ◽

Order Of Magnitude ◽

Image Position ◽

Hydraulic Jacking ◽

Small Valley

AbstractThe subglacial hydrology of the ablation area of Storglaciären, a small valley glacier in northern Sweden, is dramatically affected by a subglacial ridge, or riegel. Water pressures above this riegel are relatively constant, while down-glacier from it they vary significantly. The lower part of the glacier accelerates in response to peaks in basal water pressure. The upper part may be weakly coupled to the lower part during these peaks.A power-law fit of observed basal water pressures and measured surface velocities yieldswhere us is the surface velocity and PE is the effective water pressure (ice overburden pressure minus subglacial water pressure). Data from Findelengletscher, reported by Iken and Bindschadler (1986), yield an identical exponent and a coefficient one order of magnitude larger. The similar exponent implies that the process producing the velocity variations on both glaciers is similar. The variations in velocity are inferred to be due to hydraulic jacking on both glaciers.

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Evolution of subglacial water pressure along a glacier’s length

Annals of Glaciology ◽

10.3189/172756405781813573 ◽

2005 ◽

Vol 40 ◽

pp. 31-36 ◽

Cited By ~ 33

Author(s):

Joel T. Harper ◽

Neil F. Humphrey ◽

W. Tad Pfeffer ◽

Tyler Fudge ◽

Shad O’Neel

Keyword(s):

Pressure Drop ◽

Pressure Field ◽

Water Pressure ◽

Drainage System ◽

Diurnal Variations ◽

Seasonal Development ◽

Glacier Surface ◽

Overburden Pressure ◽

Basal Sliding ◽

Speed Up

AbstractObservations from along the length of Bench Glacier, Alaska, USA, show that the subglacial water-pressure field undergoes a multiphase transition from a winter mode to a summer mode. Data were collected at the glacier surface, the outlet stream, and in a network of 47 boreholes spanning the length of the 7 km long glacier. The winter pressure field was near overburden, with low-magnitude (centimeter to meter scale) and long-period (days to weeks) variations. During a spring speed-up event, boreholes showed synchronous variations and a slight pressure drop from prior winter values. Diurnal pressure variations followed the speed-up, with their onset associated with a glacier-wide pressure drop and flood at the terminus stream. Diurnal variations with swings of up to 80% of overburden pressure were typical of mid-summer. Several characteristics of our observations contradict common conceptions about the seasonal development of the subglacial drainage system and the linkages between subglacial hydrology and basal sliding: (1) increased water pressure did not accompany high sliding rates; (2) the drainage system showed activity characteristic of the spring season long before abundant water was available on the glacier surface; (3) the onset of both spring activity and diurnal variations of the drainage system did not show a spatial progression along the length of the glacier.

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A test of common assumptions used to infer subglacial water flow through overdeepenings

Journal of Glaciology ◽

10.3189/2014jog14j027 ◽

2014 ◽

Vol 60 (222) ◽

pp. 725-734 ◽

Cited By ~ 5

Author(s):

Christine F. Dow ◽

Jeffrey L. Kavanaugh ◽

Johnny W. Sanders ◽

Kurt M. Cuffey

Keyword(s):

Melting Point ◽

Water Pressure ◽

Drainage System ◽

Variable Pressure ◽

Local Pressure ◽

Overburden Pressure ◽

Temperature Regimes ◽

Hydrological System ◽

Flow Through ◽

Pressure Melting

AbstractBorehole instrument records from a cirque glacier with an overdeepened bed are examined to assess the validity of widely held glacial hydrological assumptions. At this glacier, hydraulic-potential calculations suggest water below overburden pressure will flow into the overdeepening, where the steepness of the riegel causes water to pool in the basin and increase in pressure. Our subglacial water pressure data also show high consistent pressures in the overdeepening and the presence of an active, variable-pressure drainage system towards the margin of the cirque. Therefore, we find that although uniform hydraulic-potential calculations are not directly applicable, they can still be useful for interpretation of the subglacial hydrological system. We also examine supercooling assumptions under different pressure and temperature regimes for water flowing over a riegel, driven using our borehole records of subglacial water temperatures that are consistently above the pressure-melting point during the late melt season. Our results show that even a slight increase in basal temperatures relative to the local pressure-melting point is sufficient to prevent a reduction in basal hydraulic conductivity as a result of supercooling freeze-on.

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Interaction between water pressure in the basal drainage system and discharge from an Alpine glacier before and during a rainfall-induced subglacial hydrological event

Annals of Glaciology ◽

10.3189/s0260305500012325 ◽

1997 ◽

Vol 24 ◽

pp. 288-292 ◽

Cited By ~ 1

Author(s):

Andrew P. Barrett ◽

David N. Collins

Keyword(s):

Water Level ◽

Water Pressure ◽

Drainage System ◽

Water Levels ◽

Drainage Network ◽

Alpine Glacier ◽

Hydraulic Conditions ◽

Borehole Water ◽

Progressive Growth ◽

Resultant Increase

Combined measurements of meltwater discharge from the portal and of water level in a borehole drilled to the bed of Findelengletscher, Switzerland, were obtained during the later part of the 1993 ablation season. A severe storm, lasting from 22 through 24 September, produced at least 130 mm of precipitation over the glacier, largely as rain. The combined hydrological records indicate periods during which the basal drainage system became constricted and water storage in the glacier increased, as well as phases of channel growth. During the storm, water pressure generally increased as water backed up in the drainage network. Abrupt, temporary falls in borehole water level were accompanied by pulses in portal discharge. On 24 September, whilst borehole water level continued to rise, water started to escape under pressure with a resultant increase in discharge. As the drainage network expanded, a large amount of debris was flushed from a wide area of the bed. Progressive growth in channel capacity as discharge increased enabled stored water to drain and borehole water level to fall rapidly. Possible relationships between observed borehole water levels and water pressures in subglacial channels are influenced by hydraulic conditions at the base of the hole, distance between the hole and a channel, and the nature of the substrate.

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Hydraulic transmissivity inferred from ice-sheet relaxation following Greenland supraglacial lake drainages

Nature Communications ◽

10.1038/s41467-021-24186-6 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Ching-Yao Lai ◽

Laura A. Stevens ◽

Danielle L. Chase ◽

Timothy T. Creyts ◽

Mark D. Behn ◽

...

Keyword(s):

Laboratory Experiments ◽

Ice Sheet ◽

Drainage System ◽

Greenland Ice Sheet ◽

Field Observations ◽

Surface Uplift ◽

Drainage Networks ◽

Order Of Magnitude ◽

Lake Drainage ◽

Magnitude Increase

AbstractSurface meltwater reaching the base of the Greenland Ice Sheet transits through drainage networks, modulating the flow of the ice sheet. Dye and gas-tracing studies conducted in the western margin sector of the ice sheet have directly observed drainage efficiency to evolve seasonally along the drainage pathway. However, the local evolution of drainage systems further inland, where ice thicknesses exceed 1000 m, remains largely unknown. Here, we infer drainage system transmissivity based on surface uplift relaxation following rapid lake drainage events. Combining field observations of five lake drainage events with a mathematical model and laboratory experiments, we show that the surface uplift decreases exponentially with time, as the water in the blister formed beneath the drained lake permeates through the subglacial drainage system. This deflation obeys a universal relaxation law with a timescale that reveals hydraulic transmissivity and indicates a two-order-of-magnitude increase in subglacial transmissivity (from 0.8 ± 0.3 $${\rm{m}}{{\rm{m}}}^{3}$$ m m 3 to 215 ± 90.2 $${\rm{m}}{{\rm{m}}}^{3}$$ m m 3 ) as the melt season progresses, suggesting significant changes in basal hydrology beneath the lakes driven by seasonal meltwater input.

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Discharge of debris from ice at the margin of the Greenland ice sheet

Journal of Glaciology ◽

10.3189/172756502781831359 ◽

2002 ◽

Vol 48 (161) ◽

pp. 192-198 ◽

Cited By ~ 21

Author(s):

Peter G. Knight ◽

Richard I. Waller ◽

Carrie J. Patterson ◽

Alison P. Jones ◽

Zoe P. Robinson

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

High Rate ◽

Outlet Glacier ◽

Tidewater Glaciers ◽

Sediment Production ◽

Basal Ice ◽

Order Of Magnitude ◽

Sediment Transfer ◽

A Minor

AbstractSediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12–45 m3 m−1 a−1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

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