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Combined measurements of Subglacial Water Pressure and Surface Velocity of Findelengletscher, Switzerland: Conclusions about Drainage System and Sliding Mechanism

  • Almut Iken (a1) and Robert A. Bindschadler (a1)

Abstract

During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.

The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.

The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.

Résumé

Pendant la période de fusion de la neige de 1982 la pression d’eau a été enregistrée dans 11 forages communiquant avec le réseau de drainage sous glaciaire. Dans la plupart de ces forages le niveaux d’eau étaient à peu près à la même profondeur, à environ 70 m de la surface. Les grandes variations de pressions, comme les variations diurnes, étaient généralement d’amplitude comparable et en phase aux différents points de mesure. Néanmoins à deux reprises, à l’occasion de pressions exceptionnellement élevées, des différences de phase ont été observées, traduisant la propagation vers l’aval d’une onde de haute pression à la vitesse d’environ 100 m/h.

Les vitesses superficielles étaient mesurées plusieurs fois par jour sur 4 lignes de balises. Les variations de vitesse sont corrélées avec les variations de pression. La relation entre pressions et vitesses suggère que les fluctuations de vitesses sont causées par des fluctuations du décollement. La relation empirique est comparée à celle du glissement sur un lit sinusoidal parfaitement lubréfié. Sur la base des relations pression-vitesse observées ce modèle prévoit une valeur raisonnable de la rugosité mais des vitesses de glissement trop importantes et un glissement instable pour une valeur de pression trop faible. La principale raison de ce désaccord provient sans doute du fait que l’on néglige le frottement dû aux débris rocheux dans le modèle de glissement.

Les pressions d’eau mesurées sont beaucoup plus élevées que celles prévues par la théorie de l’écoulement permanent dans des chenaux cylindriques rectilignes près du lit. Les raisons en sont examinées, le très grand désaccord entre pressions observées et mesurées conduit à penser qu’il n’existe jamais de chenaux cylindriques rectilignes.

Zusammenfassung

Während der Schneeschmelze wurde 1982 in 11 Bohrlöchern, die mit dem subglazialen Abflusssystem in Verbindung standen, der Wasserdruck am Gletscherbett registriert. In den meisten Bohrlöchern waren die Wasserspiegel etwa gleich tief (um 70 m unter der Oberfläche). Im allgemeinen waren die grösseren Schwankungen der Wasserdrucke an verschiedenen Orten ähnlich und in Phase. In zwei Fällen mit ungewöhnlich hohem Wasserdruck wurden jedoch systematische Phasenverschiebungen beobachtet: Eine Welle hohen Wasserdruckes wanderte mit einer Geschwindigkeit von etwa 100 m/h gletscherabwärts.

Die Oberflächengeschwindigkeit des Gletschers wurde an 4 Stangenprofilen mehrmals täglich gemessen. Die Schwankungen der Geschwindigkeit des Gletschers korrelierten mit den Wasserdruckschwankungen. Die funktionale Beziehung von Wasserdruck und Geschwindigkeit weist darauf hin, dass die Geschwindigkeitsschwankungen durch Veränderungen der Ablösung des Gletschers von seinem Bett verursacht wurden. Die empirische funktionale Beziehung wird mit derjenigen verglichen, die für reibungsfreies Gleiten über ein sinusförmig gewelltes Bett gilt. Aufgrund der empirischen Beziehung wird mit diesem Modell ein vernünftiger Wert der Rauhigkeit des Gletscherbettes ermittelt, hingegen wird eine zu hohe Gleitgeschwindigkeit berechnet und instabiles Gleiten bei einem zu niedrigen Wasserdruck vorhergesagt. Der Hauptgrund für diese Diskrepanz ist wahrscheinlich die Vernachlässigung der Reibung durch Schutt im Modell.

Der gemessene Wasserdruck war erheblich höher als derjenige, der theoretisch für stationären Durchfluss durch gerade, zylindrische Kanäle in der Nähe des Gletscherbettes zu erwarten wäre. Mögliche Ursachen werden betrachtet. Der sehr grosse Unterschied zwischen gemessenem und berechnetem Druck lässt vermuten, dass keine geraden, zylindrischen Kanäle existierten.

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References

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