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Dams dangerous in tectonically unstable areas (for eample on the italian side of the Alps)
517
DAMS DANGEROUS IN TECTONICALLY UNSTABLE AMPLE ON THE ITALIAN SIDE OF THE ALPS)
AREAS (FOR EX-
F.A. VENING MEINESZ Amersfoort
(Received
(The Netherlands)’
March 18, 1965)
SUMMARY The Alpine area is subject to tectonic movements and to stresses of a general as well as of a local character. These stresses will occur as a consequence of the loading and unloading of the crust in specific areas. Changes in ice loads, effects of erosion, sedimentation and the water load behind the dam play an important role in this matter. It will be necessary to reveal these movements by repeated geodetic measurements.
For some time the author has had in mind a paper on the above subject. The appearance of a communication in a recent issue of “De Ingenieur” (Anonymous, 1965) dealing with the causes of dams failing, is a stimulus for taking up this subject, especially as the cause here mentioned was not included in the above paper. It is clear that major movements of crustal blocks in dam areas must constitute a serious menace to these structures. It is also clear that the Alpine area may be subject to such movements. The Alps are generally known to have risen to their great elevation in recent times. Thus they confirm the geophysical views about the history of high folded mountain ranges. This history begins with the down-buckling of a crustal belt, caused by strong horizontal compression in the crust, which leads to the formation of a geosyncline. In this belt the compression brings about strong folding and overthrusting. The belt is accompanied on both sides by upward crustal waves, and the whole structure of geosynclinal downward waves and the associated upward waves is in isostatic equilibrium. When the crustal compression gradually disappears, the three crustal sectors become sufficiently loosened from each other, for each part to regain its own isostatic equilibrium independently of the adjoining sectors. The central part rises to a great height and the adjoinin, = sectors subside. This is the stage in which the Alps are now. The crustal compression was caused by the dra’g on the crust exerted by a great system of convection currents in the mantle, brought about by the cooling at the earth’s surface. These currents make only a half-turn, thus bringing the cooled upper mantle matter down and the high-temperature lower - Postal address: Potgieterlaan Tectonophysics,
1 (6) (19G)
5, Amersfoort
517-519
(The Netherlands).
1’.11.VE:NIN(; MEINb,Si:
518
1na11tle matter up. At present the half-turn current has nearly cumpleted i\> half-turn, but it appears probable that it has not yet completely stopped. ;!lthough the Alps were not prevented from rising. Besides causing the main geosyncline and mountain formation, the mantle current has had an effect on the down-buckled root which became more ductile under the high temperature which it reached by being pushed down. A considerable part of the root was carried along by the current towards the foreland, and this matter lifted up the foreland crust. This created the “Mittelgebirge” in a broad belt on the outside of the Alpine curve. These Mittelgebirge reach as far as the southeastern part of the Netherlands. During and after its rise the Alpine area - and also the uplifted foreland - was subjected to erosion and the eroded material was deposited elsewhere. Probably only a relatively small amount of the detritus reached the oceans. Evidently this process is still continuing. It may also be supposed that the rise of the Alps is still going on. Summarizing, we may assume that the Alpine area is subject to movements and stresses of a general as well as of a local character. This supposition is confirmed by geophysical and geodetic data. Long ago Niethammer (1921) determined the gravity field in Switzerland and found that after isostatical reduction the anomalies showed a negative belt below the main folded range, south of the Rh6ne and Upped Rhine, and positive values on both sides. This indicates that isostatic balance of this main range has not yet been completely attained, and this points towards the continuing of the elevation of the range. The same conclusion may be drawn from the results of the gravimetrical survey of the Eastern Alps, (Heiskanen and Vening Meinesz, 1958, pp.194-195) where the cross-section of the isostatic gravity anomalies (fig.7: 1) shows the same picture of negative values below the main range and positive values on both sides. A more direct indication of the continued rise of the Alps is given by repeated precise levelling. Not much has yet been done in this direction, but some data have been obtained. Through the good offices of Dr. G. Bakker of The Netherlands Geodetic Survey at Delft the following data were provided by Prof. Dr. F. Gassmann of the Technical University at Ziirich. He mentions that in 40 years the area of Sargans in the St. Galler Rhine-valley has risen 3 cm with respect to Altststten; the latter station is about 40 km farther north than the first. A second example is shown in the Rh6ne valley, where during about the same time interval the area of Martigny has risen about 2 cm relative to Aigle and the area of Sion about 3 cm relative to Aigle. The distance between Aigle and the valley axis through Martigny and Sion is about 15 km. In 1964 plans were made for making a more complete investigation of recent crustal movements in Switzerland. So we may hope that in the future more will be known in this connection. In addition to these large size movements we may expect that movoments will occur as a consequence of the loading or unloading of the crust in specific areas. Three kinds of masses must play a part in this way. Firstly, there are changes of ice loads, as are clearly indicated, for example. by the advance or retreat of glaciers. It is well-known that recently the glaciers have withdrawn markedly. Secondly, we can point to the effects of erosion and sedimentation. which continue to affect the whole area of the Alps and their foreland. Tectonophysics,
1 (6) (1965) 517-519
DAMS DANGEROUS IN TECTONICALLY
UNSTABLE
AREAS
519
In the third place the presence of the dam gives rise to a considerable water load. All these factors render it probable that the Alpine area, as well as the foreland on the outside of the Alpine arc, and the dam area itself, are subject to stresses. Obviously, this means danger for the dam structure, and so it seems important to investigate whether these stresses lead to movements. This could be done by repeated geodetic measurements, which could reveal such movements. The following possibilities might be considered: (1) A regularly repeated general survey of the elevations of the Alpine summits and other high geodetic stations, to check whether rise is still going on. (2) A more local repeated survey of triangulation points in dam areas. (3) A more local repeated precise levelling in dam areas. The results of these observations may be expected to show whether the dams and their associated water reservoirs can be retained.
REFERENCES
Anonymous, 1965. De oorzaken van het bezwijken van stuwdammen. Ingenieur (Utrecht),
77 (10): B35-36. Heiskanen, W.A. and Vening Meinesz, F.A., 1958. The Earth and its Gravity Field. McGraw-Hill, New York, N.Y., 470 pp. Niethammer, Th., 1921. Die Schwerebestimmungen der Schweizerischen Geodatischen Kommission und ihre Ergebnisse. Verhandi. Schweiz. Naturforsch. Ges., 1921: 1-15. Vening Meinesz, F.A., 1964. The Earth’s Crust and Mantle. Elsevier, Amsterdam, 124 pp.
Tectonophysics,
1 (6) (1965) 51’7-519
DAMS DANGEROUS IN TECTONICALLY UNSTABLE AMPLE ON THE ITALIAN SIDE OF THE ALPS)
AREAS (FOR EX-
F.A. VENING MEINESZ Amersfoort
(Received
(The Netherlands)’
March 18, 1965)
SUMMARY The Alpine area is subject to tectonic movements and to stresses of a general as well as of a local character. These stresses will occur as a consequence of the loading and unloading of the crust in specific areas. Changes in ice loads, effects of erosion, sedimentation and the water load behind the dam play an important role in this matter. It will be necessary to reveal these movements by repeated geodetic measurements.
For some time the author has had in mind a paper on the above subject. The appearance of a communication in a recent issue of “De Ingenieur” (Anonymous, 1965) dealing with the causes of dams failing, is a stimulus for taking up this subject, especially as the cause here mentioned was not included in the above paper. It is clear that major movements of crustal blocks in dam areas must constitute a serious menace to these structures. It is also clear that the Alpine area may be subject to such movements. The Alps are generally known to have risen to their great elevation in recent times. Thus they confirm the geophysical views about the history of high folded mountain ranges. This history begins with the down-buckling of a crustal belt, caused by strong horizontal compression in the crust, which leads to the formation of a geosyncline. In this belt the compression brings about strong folding and overthrusting. The belt is accompanied on both sides by upward crustal waves, and the whole structure of geosynclinal downward waves and the associated upward waves is in isostatic equilibrium. When the crustal compression gradually disappears, the three crustal sectors become sufficiently loosened from each other, for each part to regain its own isostatic equilibrium independently of the adjoining sectors. The central part rises to a great height and the adjoinin, = sectors subside. This is the stage in which the Alps are now. The crustal compression was caused by the dra’g on the crust exerted by a great system of convection currents in the mantle, brought about by the cooling at the earth’s surface. These currents make only a half-turn, thus bringing the cooled upper mantle matter down and the high-temperature lower - Postal address: Potgieterlaan Tectonophysics,
1 (6) (19G)
5, Amersfoort
517-519
(The Netherlands).
1’.11.VE:NIN(; MEINb,Si:
518
1na11tle matter up. At present the half-turn current has nearly cumpleted i\> half-turn, but it appears probable that it has not yet completely stopped. ;!lthough the Alps were not prevented from rising. Besides causing the main geosyncline and mountain formation, the mantle current has had an effect on the down-buckled root which became more ductile under the high temperature which it reached by being pushed down. A considerable part of the root was carried along by the current towards the foreland, and this matter lifted up the foreland crust. This created the “Mittelgebirge” in a broad belt on the outside of the Alpine curve. These Mittelgebirge reach as far as the southeastern part of the Netherlands. During and after its rise the Alpine area - and also the uplifted foreland - was subjected to erosion and the eroded material was deposited elsewhere. Probably only a relatively small amount of the detritus reached the oceans. Evidently this process is still continuing. It may also be supposed that the rise of the Alps is still going on. Summarizing, we may assume that the Alpine area is subject to movements and stresses of a general as well as of a local character. This supposition is confirmed by geophysical and geodetic data. Long ago Niethammer (1921) determined the gravity field in Switzerland and found that after isostatical reduction the anomalies showed a negative belt below the main folded range, south of the Rh6ne and Upped Rhine, and positive values on both sides. This indicates that isostatic balance of this main range has not yet been completely attained, and this points towards the continuing of the elevation of the range. The same conclusion may be drawn from the results of the gravimetrical survey of the Eastern Alps, (Heiskanen and Vening Meinesz, 1958, pp.194-195) where the cross-section of the isostatic gravity anomalies (fig.7: 1) shows the same picture of negative values below the main range and positive values on both sides. A more direct indication of the continued rise of the Alps is given by repeated precise levelling. Not much has yet been done in this direction, but some data have been obtained. Through the good offices of Dr. G. Bakker of The Netherlands Geodetic Survey at Delft the following data were provided by Prof. Dr. F. Gassmann of the Technical University at Ziirich. He mentions that in 40 years the area of Sargans in the St. Galler Rhine-valley has risen 3 cm with respect to Altststten; the latter station is about 40 km farther north than the first. A second example is shown in the Rh6ne valley, where during about the same time interval the area of Martigny has risen about 2 cm relative to Aigle and the area of Sion about 3 cm relative to Aigle. The distance between Aigle and the valley axis through Martigny and Sion is about 15 km. In 1964 plans were made for making a more complete investigation of recent crustal movements in Switzerland. So we may hope that in the future more will be known in this connection. In addition to these large size movements we may expect that movoments will occur as a consequence of the loading or unloading of the crust in specific areas. Three kinds of masses must play a part in this way. Firstly, there are changes of ice loads, as are clearly indicated, for example. by the advance or retreat of glaciers. It is well-known that recently the glaciers have withdrawn markedly. Secondly, we can point to the effects of erosion and sedimentation. which continue to affect the whole area of the Alps and their foreland. Tectonophysics,
1 (6) (1965) 517-519
DAMS DANGEROUS IN TECTONICALLY
UNSTABLE
AREAS
519
In the third place the presence of the dam gives rise to a considerable water load. All these factors render it probable that the Alpine area, as well as the foreland on the outside of the Alpine arc, and the dam area itself, are subject to stresses. Obviously, this means danger for the dam structure, and so it seems important to investigate whether these stresses lead to movements. This could be done by repeated geodetic measurements, which could reveal such movements. The following possibilities might be considered: (1) A regularly repeated general survey of the elevations of the Alpine summits and other high geodetic stations, to check whether rise is still going on. (2) A more local repeated survey of triangulation points in dam areas. (3) A more local repeated precise levelling in dam areas. The results of these observations may be expected to show whether the dams and their associated water reservoirs can be retained.
REFERENCES
Anonymous, 1965. De oorzaken van het bezwijken van stuwdammen. Ingenieur (Utrecht),
77 (10): B35-36. Heiskanen, W.A. and Vening Meinesz, F.A., 1958. The Earth and its Gravity Field. McGraw-Hill, New York, N.Y., 470 pp. Niethammer, Th., 1921. Die Schwerebestimmungen der Schweizerischen Geodatischen Kommission und ihre Ergebnisse. Verhandi. Schweiz. Naturforsch. Ges., 1921: 1-15. Vening Meinesz, F.A., 1964. The Earth’s Crust and Mantle. Elsevier, Amsterdam, 124 pp.
Tectonophysics,
1 (6) (1965) 51’7-519