The results of systematic movement studies carried out by means of an automatic camera on the Unteraargletscher since 1969 (Flotron, 1973) are discussed together with more recent findings from theodolite measurements made at shorter intervals and over a longer section of the glacier. In addition to the typical spring/early-summer maximum of velocity known from other glaciers, an upward movement of up to 0.6 m has been recorded at the beginning of the melt season. It was followed, after various fluctuations of the vertical velocity, by a similar but slower downward movement which continued at an almost constant rate for about three months. The uplift was not confined to the section covered by the camera but occurred nearly simultaneously in profiles located 1 km below and 2 km above. The times of maximum upward velocity (increases of up to 140 mm/d) coincided approximately with periods of large horizontal velocity and occurred after increases of melt-rate. The following explanations for the variations of vertical velocity are considered: (1) Changes of longitudinal strain-rate. (2) Changes of the sliding velocity in a channel of variable width and with a bed slope deviating from horizontal. (3) Changes of volume due to opening or closing of crevasses. (4) Swelling or contraction of veins at the grain edges. (5) Growth (and closure) of cavities in the interior of the glacier. (6) Changes of large-scale water storage at the bed. Although all of the mechanisms (1)–(5) have some effect on the vertical ice movement, they cannot account for the observed variations of vertical velocity. We therefore conclude that large-scale water storage at the bed is the main cause of the uplift. Apparently the storage system is efficiently connected with the main subglacial drainage channels only during times of very high water pressure in the channels. The findings are of some interest to the concepts of glacier sliding: As mentioned above the maxima of horizontal velocity—and thus of the sliding velocity—have not been measured at the time when the storage had attained a maximum, but at the time of maximum vertical velocity, which we assume to be the time of most rapid growth of cavities at the bed. This behaviour of the sliding velocity agrees with that predicted by a simple finite-element model of the basal ice on a wavy bed with water-filled cavities. In particular, the model shows that the sliding velocity is larger during the process of cavity growth than at the final stage when the cavities have grown to the size which is stable for the applied water pressure.

A warning by the glaciologist is hardly taken seriously unless it includes a forecast of the time of final rupture. At present the most promising approach for such a prediction is the almost perfect regularity by which certain large ice masses accelerate for a very long time prior to the instant when the avalanche starts to fall (see the Weisshorn case) . The limitation of the forecasts lies as much in short-term irregularities as in the extreme difficulties of obtaining sufficiently accurate data without interruptions, and further in the lack of experience on the critical velocity that is reached immediately before final breakage. The physical explanation of the observed law is another question. Using a finite-element computational model for the analysis of stress and flow in a somewhat different case, Iken (1977) has shown that a stepwise crack extension alternating with phases of flow leads to the observed form of velocity-time relationship.There are various possibilities in the way of preventive measures; none of them is completely satisfactory, however. The most certain consists in avoiding the danger zone altogether, a solution usually not acceptable to those involved. In view of the requirements necessary for withstanding an impact of exceptional magnitude, protective structures are generally not practical either. A stepwise elimination of the unstable ice mass by blasting could be considered, though the realization would be difficult and not without danger-not to speak of the legal problem encountered if the blasting should trigger the full-size avalanche. Consequently a certain calculated risk will occasionally have to be accepted whereby the dangers during an operation on the glacier and the extreme costs have to be weighed against the severity and small probability of a catastrophe. Viewed in prop ortion to other risks in everyday life (especially in road and air traffic, but also in relation to earthquakes), the dangers of ice avalanches should not be exaggerated.Glaciological research can obviously contribute in a variety of ways to the solution of practical problems with ice avalanches, and there are ample opportunities for future studies. Of urgent need are more and better observations on almost every aspect of the problem, extending to phenomena hitherto not observed such as frequ ency and intensity of seismic signals . The inherent difficulty with the more interesting large-scale events is their scarcity, precluding proper experimentation. Advances in the theoretical treatment are also to be hoped for. There is also room for new ideas, for instance on how to prevent a particular ice mass from forming. ABSTRACT. In 1972 the state ofa hanging glacier on the Weisshorn gave cause for alarm , as part of it seemed to be accelerating and a repetition of an earlier avalanche of ice seemed possible (see Rothlisberger, previous abstract). For this reason movement surveys were undertaken. The various surveying methods applied on the Weisshorn are outlined and the JOURNAL OF GLACIOLOGY REFERENCES

No abstract

ABSTRACT. Resu lts of systematic mo vement studies carried out by means of an auto mati c ca mera on Unteraargletscher si nce 1969 are discussed toget her with suppl ementary theodolite measurement s made at shorter intervals and over a longer section of the glacier. In addition to the typical sprin g/ earl y summer m ax imum of velocity known fro m other glaciers, an upward movement of up to 0.6 m has been reco rded at the beginning of the mel t season. It was followed , after a few fluctu ations of the verti cal velocity, by an eq ual but slowe r downwaru mo vement which continued at an almost consta nt rate for about three month s. Poss ibl e ex pl anation s of th e uplift a re di scussed, the most satisfactory explanation being water storage at the bed. The observations then suggest that this storage system is efficiently connected with the mai n subgla cial drainage channels on ly durin g times of very high water pressure in th e channels. Detailed measurements showed that the times of maxi mum horizontal velocity coin cided with the times of maximum upward velocity rather than wi th the times when the ele va ti on of the surveyed po les had reached a maximum . On the basis of the hypothesis of water storage at th e bed thi s findi ng means that the sliding velocity is influenced mainl y by the subglaci al water press ure and the act ual. tran sicnt sta ge of cavity development, while the amount of stored water is of lesser influence. R ESUME. Le mouvement vertica l de /'Un teraargletscher enregistre au debut de la fame des neiges est-il la consequence d'une accumulation d'eau au niveau du lit glaciaire? Z USAMMENFAS SUNG. Die Hebung des Unteraargletschers zu Beginn der Schneeschmelze-eine Folge van Wasserspeicherung alii Gletscherbelt ?Am Unteraargletscher werden seit 1969 system atische Bewegungsmess un gen mit einer a uto mati sc~e n Kamera durchgefiihrt. Ergebnisse dieser Mess un gen werden zusa mmen mit erganzenden Theodoli tmessungen, welche in kiirzeren Zeitabstand en und in einem grosseren Gebiet des Gletsc hers durchgefii hrt wurden, analysiert. Wahrend der Schneeschmelze wurde neben dcm typi sc hen Geschwindigkeitsmaximum, das auch von anderen Gletschern bekannt ist, ei ne Aufwart sbewegung bis zu 0.6 m beobachtet. Dieser Aufwartsbewegun g folgte-nach einigen Auf-und Abbewegungen--eine gleichgrosse. aber langsame kontinuierliche Abwartsbewegung, die etwa drei Monate andauerte. Mogliche Erk liir ungen der Heb ung werden di skutiert ; die befriedigendste ist Wasserspeicherung am Gletscherbett. Di e Beobachtun gen lassen vermuten, dass das Speichersystem nur wahrend Zeiten sehr hohen Wasserdru ckes eine gut funktionierende Verbindun g zu den grosseren Kanalen des subglazial en Abflusssystems besitzt. Detaillierte Mess un ge n zeigten. dass der Zeitpunkt der maxim alen Horizontalgesc hwindigkeit des Gletschers mit dem der max im alen Vertikalgesc hwindigkeit iiberein stimmte und nicht mit dem Zeitpunkt, zu welchem die Messmarken im Gletsc her

A warning by the glaciologist is hardly taken seriously unless it includes a forecast of the time of final rupture. At present the most promising approach for such a prediction is the almost perfect regularity by which certain large ice masses accelerate for a very long time prior to the instant when the avalanche starts to fall (see the Weisshorn case) . The limitation of the forecasts lies as much in short-term irregularities as in the extreme difficulties of obtaining sufficiently accurate data without interruptions, and further in the lack of experience on the critical velocity that is reached immediately before final breakage. The physical explanation of the observed law is another question. Using a finite-element computational model for the analysis of stress and flow in a somewhat different case, Iken (1977) has shown that a stepwise crack extension alternating with phases of flow leads to the observed form of velocity-time relationship.There are various possibilities in the way of preventive measures; none of them is completely satisfactory, however. The most certain consists in avoiding the danger zone altogether, a solution usually not acceptable to those involved. In view of the requirements necessary for withstanding an impact of exceptional magnitude, protective structures are generally not practical either. A stepwise elimination of the unstable ice mass by blasting could be considered, though the realization would be difficult and not without danger-not to speak of the legal problem encountered if the blasting should trigger the full-size avalanche. Consequently a certain calculated risk will occasionally have to be accepted whereby the dangers during an operation on the glacier and the extreme costs have to be weighed against the severity and small probability of a catastrophe. Viewed in prop ortion to other risks in everyday life (especially in road and air traffic, but also in relation to earthquakes), the dangers of ice avalanches should not be exaggerated.Glaciological research can obviously contribute in a variety of ways to the solution of practical problems with ice avalanches, and there are ample opportunities for future studies. Of urgent need are more and better observations on almost every aspect of the problem, extending to phenomena hitherto not observed such as frequ ency and intensity of seismic signals . The inherent difficulty with the more interesting large-scale events is their scarcity, precluding proper experimentation. Advances in the theoretical treatment are also to be hoped for. There is also room for new ideas, for instance on how to prevent a particular ice mass from forming. ABSTRACT. In 1972 the state ofa hanging glacier on the Weisshorn gave cause for alarm , as part of it seemed to be accelerating and a repetition of an earlier avalanche of ice seemed possible (see Rothlisberger, previous abstract). For this reason movement surveys were undertaken. The various surveying methods applied on the Weisshorn are outlined and the JOURNAL OF GLACIOLOGY REFERENCES