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Sediment Concentration in Melt Waters as an Indicator of Erosion Processes Beneath an Alpine Glacier

  • David N. Collins (a1)

Abstract

Suspended-sediment concentrations in melt waters from the Gornera, Gornergletscher, Switzerland, were determined at hourly intervals for periods during the ablation seasons of 1974 and 1975. Rapid erratic fluctuations of suspended-sediment concentration produced peaks which occurred both before and after highest daily flows. Clockwise daily hysteresis rating loops between sediment concentration and discharge included many involutions. Suspended-sediment-concentration-discharge rating curves were different for rising and falling limbs of individual diurnal hydrographs and varied from day to day. Close-interval measurements of sediment concentration and discharge records allow interpretation of the nature of ice–water–sediment interactions at the bed of an Alpine glacier. At Gornergletscher, subglacial sediment is delivered to melt waters flowing in the smaller basal conduits, which often change course suddenly, entraining unworked sediment stored at the bed. During diurnal discharge maxima, sediment concentration in the Gornera is reduced because the rate of increase of water volume outstrips the rate of supply of sediment. The drainage of the ice-dammed lake Gornersee, producing exceptionally high flows, extended the drainage network over large areas of the glacier bed, and evacuated much sediment.

Résumé

On a déterminé les concentrations en matériaux en suspension dans les eaux de fusion provenant du Gornera sur le Gornergletscher, en Suisse, à des intervalles horaires pour des périodes des saisons d’ablation de 1974 et 1975. Des fluctuations brusques et capricieuses de la teneur en matériaux en suspension ont produit des pointes soit avant soit aprés les crues journaliéres. Des phénoménes d’hystérésis au cours de la journée provoquant des chevauchements entre la concentration en sédiments et les débits ont produit de nombreux cas de figure. Les courbes de teneur en matériaux en suspension selon les débits sont différentes pour les branches ascendantes et descendantes des hydrogrammes quotidiens individuels et variables d’un jour à l’autre. Les mesures fréquentes de concentration en sédiments et les enregistrements de débits permettent une interprétation quant à la nature des interactions glace–eau–sédiments le long du lit d’un glacier alpin. Au Gornergletscher, les matériaux sous-glaciaires sont livrés aux eaux de fusion coulant dans les plus petits chenaux du fond, qui changent souvent brusquement de cours, entrainant des matériaux non travaillés stockés sur le lit. Pendant la période de débit journalier maximum, la teneur en matière solide du Gornera est réduite parce que la vitesse d’accroissement du volume de l’eau excède la vitesse de fourniture du matériel. La vidange du lac derrière le barrage de glace du Gornersee, produisant des eaux exceptionnellement hautes étend le réseau de drainage à de vastes surfaces du lit glaciaire et évacuent beaucoup de sédiments.

Zusammenfassung

In Schmelzwassern der Gornera, Gornergletscher, Schweiz, wurde für Perioden während der Ablationszeit von 1974 und 1975 stündlich die Konzentration suspendierter Sedimente bestimmt. Schnelle, regellose Schwankungen der Sediment-Konzentration führten zu Spitzen sowohl vor wie nach den täglichen Höchstständen. Tägliche Hystereseschleifen im Uhrzeigersinn zwischen Sediment-Konzentration und Abfluss zeigten viele Involutionen. Die Kurven der Abflussraten und der Konzentration von suspendierten Sedimenten waren unterschiedlich für die steigenden und fallenden Äste einzelner Tageswasserstandsgänge und änderten sich von Tag zu Tag. Kurzfristige Messungen der Sediment-Konzentration und Abflussaufzeichnungen gestatten eine Interpretation der Wechselwirkungen zwischen Eis, Wasser und Sediment am Grunde eines Alpengletschers. Am Gornergletscher gelangt suglaziales Sediment in Schmelzwässer, die in den kleineren Wasserführungen am Untergrund fliessen, oft ihren Lauf unvermittelt ändern und dabei unbearbeitetes Sediment vom Untergrund aufnehmen. Während der täglichen Abflussmaxima ist die Sediment-Konzentration in der Gornera reduziert, weil die Zunahme der Wassermenge den Sedimentennachschub überwiegt. Der Abfluss des vom Eis abgedämmten Gornersees verursachte ungewöhnlich hohe Wasserstände, erweiterte das Abflusssystem über grosse Gebiete des Gletscherbetts und führte viel Sediment ab.

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References

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Sediment Concentration in Melt Waters as an Indicator of Erosion Processes Beneath an Alpine Glacier

  • David N. Collins (a1)

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