Direct Measurement of Basal Water Pressures: a Pilot Study

ABSTRACT. A mathematical model is presented for the stability of sheet flow of water beneath a temperate glacier. Enhanced viscous heat dissipation in thick parts of the sheet tends to make sheet flo w unstabl e. the instability increasing as sheet thickness and pressure gradient increase. However, incipient channels ma y be destroyed as the glacier slides over protuberances on its bed . Quasi-stable sheet flow ma y be possible for s heets up to several millimeters in thickness, especially beneath glaciers that have relatively gentle surface slopes and slide at moderate to high speeds. Such water sheets may somewhat reduce the effective roughness of glacier beds. but probably not enough to allow surge initiation . Furthermore, the presence of numerous water-filled cavities at th e glacier bed will tend to reduce the sheet thickness and lessen the degree of " lubri catio n" of the glacier bed by th e water sheet. RESUME. Stabilite d'un ecoulement en nappe d'eau sous des glaciers temperes et consequences su r les debacles glaciaires.On presente un modeJe mathematique pour la stabilite d' un eco ulement en nappe d'eau sou s un glacier tempere. La dissipation de chaleur par viscosite dan s les parties epaisses de la nappe tendent a rendre I'ecoulement en nappe instable, I'instabilite croissant lorsque I'epaisseur de la nappe et le grad ient de pression s'accroissent. Cependant les chenaux affiuents peuvent et re detruits par le gli ssement du glacier sur les protuberances de son lit. Un ecoulement en nappe quasi -stable est possibl e pour des nappes allant jusqu'a plusieurs millimetres d'epaisseur. specialement sous les glaciers qui ont une pente de s urface rel ati vement moderee et glissent a des vitesses moderees it fortes. De telles nappes d 'eau peuvent parfo is reduirc la rugosite efficace des fonds rocheux, mais probablement pas assez pour permettre le debut d'un e crue rapide. De plu s, la presence de nombreuses cavites pleines d'eau sur le lit glaciaire tendra.it reduire I'epaisseur de la nappe et de minimiser le degri: de "Iubrification" du lit glaciaire par la nappe d'eau . Z USAMMENFA SSUNG. Slabilitiit des Schichtflusses van Wasser unler temperiertel1 Glelschem II/Id Auswirkungen aul Gletscherausbriiche. Fur die Stabilitiit des Schichtflusses von Wasser unter tempe ri ertenGletschern wird ein mathematisches Modell vorgelegt. Erhiihter viskoser Wiirmea usta usc h in dicke re n Teil en der Eismasse verursacht Instabilitaten im Sc hichtfluss, die mit wachsender Eisdi cke und wac hsendem Druckgradienten zunehmen. Immerhin kiin nen neu gebildete Kanale zerstiirt werden, wenn der Gletsc her uber H iic ker auf seinem Bett gleitet. Ei n quasistabiler Schich tflu ss kann sich fUr Schichten bis zur Dicke vo n einigen Millimetern ausbilden , besonders under Gletschern mit relativ sa nfter Oberfl achennei gun g und mass ige r bis hoher Gleitgeschwindigkeit. Solche Wasserschichten kiinnen die effektive Rauhi gkeit d es Gletscherbettes urn eini ges verringern, vermuthlich jedoch nicht genug, urn einen Ausbruch zu ver...

Section: )-2 Or-----------------------~~--}lmentioning

confidence: 99%

“…1973; Hodge, 1976Hodge, , 1979Engelhardt, 1978;Engelhardt and others, 1978;Iken and others, 1979;Rothlisberger and others, 1979). Water-level soundings in bore holes drilled to glacier beds clearly indicate the existence of channels in which the water pressure may be substantially less than the overburden pressure.…”

mentioning

confidence: 99%

Stability of Sheet Flow of Water Beneath Temperate Glaciers and Implications for Glacier Surging

Walder

1982

“…Bore-hole water levels were measured with pressure transducers lowered into the holes, following the method of Hodge (1976); in 1978, a sounding float was used. The transducer signals were recorded continuously on strip-chart recorders; in 1980, the records included hourly time marks .…”

Section: • Theodolite Station 1980mentioning

confidence: 99%

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Kamb

Engelhardt

1987

ABSTRACT. Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-8 1 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These "mini-surges" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-1 1 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal out flow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The mini-surge wave propagation is controlled by the propaga tion of the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longi tudinal gradient (up-glacier increase) in hydraulic conductivity of the basal water system in response to the up-glacier increase of the basal water pressure in the mini surge wave. The mini-surge waves are initiated in a succession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial *Contribution No.3734,

“…Such a solution might also be helpful in understanding the sometimes confusing results of experiments in which bore holes have been drilled to the base and the water level in the bore is measured. The water level in such a bore hole is not necessarily a simple expression of some sort of averaged water pressure at the base of a glacier (Hodge, 1976). The presence of the bore hole must modify the hydrological regime in its vicinity and must alter the dynamic water-flow pattern there.…”

Section: Water-flow Problemmentioning

confidence: 99%

The Unsolved General Glacier Sliding Problem

Weertman¹

1979

ABSTRACT. The status of the theo ry of glacier sliding is rev iewed in this paper. The theory of Nye and K am b esse nti a lly solves the problem of glac ier sliding wh en the basa l ice is in intimate con tact with a nond eformabl e bed. But exper imen ta l work is needed o n cycli c creep and the regelation p rocess in o rder to put better numbe rs in to the theory . The outstanding pro blem that remains to b e solved , called in this paper the genera l glac ie r sliding problem, is the case of sliding when basal ice is no t in intimate contac t with a nond eforma ble bed because of the presence of caviti es and a water film of finit e thickness . This problem in volves solving an intri catel y cou pled basal water-flow proble m. A new field of research is the study of the motion of glaciers over d eforma bl e beds.REsu ME. L e problhne general non resolu du glisseme1lt des glaciers. On revo it d a ns cet articl e I'etat d e la theori e du glissem e nt des glaciers. La theorie de Nye et de K am b resout essen ti e ll e m e nt le probl em e du glissement d es glaciers ou la g lace de fond est e n contact intime avec un lit non deformable. Ma is il fa ut d es recherches experimenta les sur la fluage cycl iqu e et sur le process us de regel pour introduire de m e ill e ures donnees numeriques dans la theori e. Le problemc qui reste a resoudre, qu'on a ppel le d ans cet articl e le probleme general du glisse m ent des glaciers, est le cas du glisse m e nt lorsque la glace d e fond n'es l pas en con tac t intime avec un lit n o n deformable, e n raiso n d e la prese nce d e cav ites et d ' un film d 'eau d 'epaisse ur finie . Ce probleme implique la solution du probleme qui lui est etro it ement me le d e l' ecoulem ent d e I'ea u du fond du glacier. Un nouveau cha mp de reche rches est I'etude du mouvement d e g lac ier sur des lits defo rma bl es.ZUSAMMENFASSUNG. Das wlgeloste Problem des aligemeinell Glelschergleitell.L I n dicsc m Beitrag wird der Sta nd der Theo ri e des Gletsch ergl e ite ns betrachtet. Die Th co rie vo n Ny e und K a mb lost das Problem d es G letschergleitens, wenn das Eis am Untergrund in e ngem Konta kt mit e ine m ni cht deformi e rba ren Bett steht. Doch mlissen Versuche libe l' das zyklische Kri ech en uacl den R egelations"organg anges te llt werclen, urn besseres Zahlenmateria l in di e TheOl'ie einfuh ren z u konn en. Das w ese ntliche Problem , das noch zu losen bl eibt und d as hier das Proble m d es a llgemeine n G lelschergleitens genan nt wircl, li egt vor, wenn das Eis am Untergrund infolge von H o hlra umen und d e m Vorh a nclcnse in e in es VVasserfilmes begre nzter Dicke ni cht in enge m K ontakt mit e inem ni cht deformi e rbaren Gleitbett ste ht. Di cses Problem e rfold ert die Losung eines kom pliziert mit ihm verbunclencn Pro blems der Stromung im W asser a m Unte rgrund . Ein neues Forschungsfeld ist clas Studium d er Gletscherbe w egung uber defo rmi e rba ren Ben en.