Direct Measurement of Basal Water Pressures: Progress and Problemss

Abstract

ABSTRACT. In 1975 and 1977,24 bore holes wer e drilled to the bed o f South Cascade G lacier, Wash ington, U.S.A., using both electrothermal an d hot-water drills. Only two h oles connected d irectly with the basal water syste m, a significant decrease from the four to five such con nections in 13 holes drilled in 1973 and 1974 (H odge, 1976) . Most of the bed, possibly as much as 90 % , appear s to be hyd ra ulically in acti ve an d isolated fr om a few active subgl acial co nduits. Bore h oles whi ch penetra te these inactive a reas initia lly shou ld connect eventually with the a cti ve basal water system due to bed p ress urization by th e water standing in the bore hole, provided there is a sufficient supply of water availa ble to form and maintain the connection passageway. These sealed-off a r eas probably consist of the sub-sole drift a nd permea bility barriers found recently a t the bed of Blue G lacier b y Engelhardt a nd o thers ( 1978) ; an increase in the a rea of bed covered by these fea tures probably caused the d ecrease in chance of bore-hole connection. T his apparently was not du e to any external cause but rather was the result of a real intern al change in the subglac ia l h yd raulic system which occurred between 1974 and 1975. If m ost of the a rea of a glacier bed is hydrauli cally isolated sub-sole drift, or something sim ila r, such features m a y well control large-scale gl ac i~r slid ing changes, since ch a nges in the amou n t of water having access to the glacier bed will ta ke considerable time to affect the inte rstitial water pressure in the more widespread sub-sole drift.Water pressures in the act ive part of the basa l w a te r system of South Cascad e Glacier a r e ge n era lly in the range of 50-75 % of the ice overbu rden pressure. \Vater levels in a co nn ected bore hole are probably representative over an area of the bed 100 m or more in extent. A correlation of bore-hole wate r levels with changes in surface motion supports the idea tha t the sliding of a tem p e r ate glacier is controlled largely by the basal water pressure. En 1975En et 1977 forages furent executes jusqu ' au fond du lit du South Cascade Glacier, Washington, en utilisa nt simulta nemen t les sondes eI ectriques et a eau chau de. Seulement deu x forages se trouveren t en communication directe avec le reseau hydra ulique du fond du glacier ; c'est une diminution significa ti ve e n compara iso n des 4 a 5 telles communications qui furent relevees sur Ies 13 fora ges realises en 1973 et 1974 (H odge, 1976) . L a plus grande partie du lit peut-etre jusqu 'a 90 % , a pparait comme h ydrauliquem e nt inacti ve et isolee des ch enaux sousglaciaires rela ti vement peu actifs. L es forages qui penetre nt dans ces zones in acti ves initialement se reliera ient eventuellement avec le reseau actif du fond , pourvu qu'il y ait un apport d'eau s.uffisant dans le forage pour former et mainteni r le passage d e I' ecoulement. C es zones closes sont probablemen t form ees d e sous-so l graveleux et d e ces ...

Greenland subglacial drainage evolution regulated by weakly connected regions of the bed

“…These regions are identified by boreholes that drain slowly when reaching the bed and exhibit diurnal water pressure variations that are out of phase with meltwater input and ice speed4203435363743. In many areas, these inactive regions cover large fractions of the bed; Hodge44 estimated 90% of the bed of South Cascade Glacier in Washington, USA, was hydraulically isolated. This behaviour was observed at all three of the FOXX boreholes, as well as three boreholes at an additional study site420, and boreholes only 30 m apart showed no obvious connection and large hydraulic gradients20, suggesting that weakly connected drainage likely covers most of the GrIS bed in this region.…”

Channelized, distributed, and disconnected: subglacial drainage under a valley glacier in the Yukon

“…Increased basal water pressure reduces N and provides partial support for the weight of the glacier, reducing the contact surface with the underlying bedrock, and thus enhancing basal sliding (Lliboutry, 1958;Hodge, 1979;Iken and Bindschadler, 1986;Fowler, 1987;Schoof, 2005;Gagliardini et al, 2007). A similar effect is observed on glaciers resting on a till layer, where a lower N reduces the yield stress of the till, and thus also enhances basal sliding (sliding is here intended to include motion at shallow depths within the till layer as well as at the ice-till interface) (Engelhardt et al, 1978;Iverson et al, 1999;Tulaczyk et al, 2000;Truffer et al, 2001).…”

“…Werder et al, 2013), still fail to reproduce direct borehole observations of subglacial conditions . These include the existence of disconnected areas that show no signs of flow-related changes in water pressure (Hodge, 1979;Engelhardt et al, 1978;Murray and Clarke, 1995;Hoffman et al, 2016), the development of widespread areas of high water pressure during winter Harper et al, 2005;Ryser et al, 2014a;Wright et al, 2016), large pressure gradients over short distances (Murray and Clarke, 1995;Iken and Truffer, 1997;Fudge et al, 2008;Andrews et al, 2014), sudden reorganizations of the drainage system (Gordon et al, 1998;Kavanaugh and Clarke, 2000), high spatial heterogeneity, boreholes exhibiting anti-correlated temporal pressure variations (Murray and Clarke, 1995;Gordon et al, 1998;Andrews et al, 2014;Lefeuvre et al, 2015;Ryser et al, 2014a), and englacial conduits (Fountain and Walder, 1998;Nienow et al, 1998b;Gordon et al, 1998;Fountain et al, 2005;Harper et al, 2010).…”

Using Vertical Aerial Photography to Estimate Mass Balance at a Point

“…This method uses mainly vertical aerial photography, which records the instantaneous geometry of a glacier, and, if time-sequential photography is available, also the pattern of surface displacement (Meier et al 1985). Bed topography was determined independently (Hodge 1979) from surface radar, direct measurements in boreholes, etc. This report describes a method of using photogrammetric results and compares the estimated mass balances with the independently measured 'true' values.…”