GYÖRGY LOVÁSZ1
Introduction
Ca. 16 per cent of the 93,000 km2 area of Hungary is covered by blown sand periodically accumulated since the end of the Pleistocene. Hills also occupy large areas (approx. I 1 per cent). The valleys in the area of uplift since the Late Pannonian are cut mainly into Upper Pannonian sandy clay, and partly into Oligocene Schlier. The Upper Pannonian and Oligocene deposits are prone to sliding. On ca. 27 per cent of the area blown-sand accumulation as well as mass movements and sheet wash are the prevailing geomorphic processes.
The studies of landslides and blown sand features have a great tradition in Hungarian geomorphological research. The processes and the stages of development of the features are well known.
This publication was supported by the National Scientific Research Fund (OTKA), Project No. TO 15482.
Discussion
Landslides were most active during the Preboreal stage (10,200-9,000 yr B.P.) of the Holocene epoch (JUHÁSZ, Á. 1972., SZABÓ, J. 1996.). Three major blown sand accumulation periods are known. The first one is the last stadial of the Würm glacial (Late Gacial or Upper Pleniglacial). The movement took place around 20,000 B.P. The second stage was the Boreal (9,000-7,500 yr B.P.) and the third one occurred under the Ottoman rule, in the 16th and 17lh centuries A.D. This last period is considered to be of human origin.
The local population lied into the swamps and forests from the Turkish conquerors. As a consequence, sands started to move on the abandoned farmlands (BORSY, Z. 1977).
According to the investigations of Holocene vegetation and paleoclimate (ZÓLYOMI, B. 1952, JÁRAINÉ KOMLÓDI, M. 1966, 1969"kORDOS L. 1977, 1988, etc.), the periodicity of the landslides, migration of sand and sheet wash caused by precipitation can be traced well. These processes are largely dependent on climate.
The climate changes of the last 2000 years can be mainly studied and restored from historical records and dendrochronological investigations (RÁCZ, L. 1990, 1993, GRYNAEUS, A. 1997, LAMB, H. H. 1982, PFISTER." C. 1994, etc.). The last 160 years,
1 Institute of Geography, University of Pécs, Hungary
i.e. the climate changes from 1841 until present can be best reconstructed from the data series of the Hungarian Meteorological Service. The geomorphological processes of the last
10,200 years can be investigated by the methods mentioned above.
On the basis of the investigations on vegetation history the Preboreal stage can be considerably low. Since climate was similar to present-day tundra conditions, the sand movement could not be intense on the wet surface resulted from the low evaporation rale.
The Boreal stage lasted longer, from 9,000 to 7,500 B.P. Climate was not uniform all through this 1500-year interval. Only the vegetation of the second half of the interval showed warm and dry character, when climatic conditions of intense sand motion were provided. Due to the lack of the precipitation water erosion did not play an important role with the exception of scattered heavy showers characteristic under steppe climate.
Considering landslides the conditions must have been even less favourable for triggering them at that time than before.
The 2500-year interval of the Atlantic phase (7,500-5,000 B.P.) was the Holocene climatic optimum in the Carpathian Basin. It is indicated by the dominance of oak pollen.
The precipitation radically increased in this period and climatic conditions were ideal for intensive sheet wash. There was a substantial deepening of valleys, and landslide processes gained in intensity. However, high annual precipitation did not allow large-scale sand movements.
During the 2500-year long Subboreal phase (5,000-2,500 B.P.) considerable climatic change occurred. At the beginning the climate substantially deteriorated, which is evidenced by the occurrence of spruce, fir and larch pollen. Ideal conditions for landslides prevailed at that time. Sheet wash and blown sand movements were not particularly intense.
According to the pollen analytical investigations, warming and increasing of precipitation happened ca. 3,000 years ago. At that time climatic conditions began to favour sheet wash compared to the former phase. The inclination to landslide increased in spring and autumn. The conditions of sand migration did not improve radically.
At the end of the Subboreal phase, ca. 2,750 years ago, another cool spell is indicated by pollen analysis. No data are available on the amount of precipitation, but it is likely that climate did not favour sand motion. The conditions and geomorphic processes at that time were similar to those of the Subboreal phase. The climatic deterioration was followed by rising temperatures at the beginning of the Subatlantic phase (2,500 B.P. to present). The Mediterranean climate probably supported wet winters. In this so-called Roman climatic optimum stage conditions for sand movements were favourable during the dry and warm summers. The humid summers and intense showers enhanced the processes of sheet wash.
The most characteristic climatic deterioration in the Holocene lasted for 300 years according to LAMB H. H. (1982), and for 560 years by PFISTER Ch. (1984). Both researchers agree that this so called 'Little Ice Age' terminated in the middle of the 19lh century.
The climate of Hungary then bore resemblance to that of the glacial stages of the Pleistocene. As a result of the moderately wet and cold winters the inclination to landslides increased. It is likely that the freeze-thaw alternations accelerated infiltration and triggered
landslides. Although the mentioned favourable climatic conditions existed during the cool and wet summers, landslide activity was reduced by increasing water loss by evaporation.
Sheet wash was limited to short and wet summers. These climatic conditions were extremely unfavourable for sand movement.
The climatic oscillations of the past 160 years (from 1841 to present) can be best investigated with the help of meteorological data provided by the National Meteorological Service. The tendencies in the alterations of summer and winter half-year temperatures and precipitation were studied by trend analysis. Since the geomorphic processes highly differ in summer and winter, their separate investigation seems to be essential. Landslides are particularly common increasing principally in early and late winter, under wet and mild conditions. Under favourable climatic conditions the summer half-year was characterised by sheet wash and sand movement. Trend analysis, however, presents tendencies through ten- year moving averages and conceals possible extremities. As confirmed by present-day experiences, the intensity of geomorphic processes may substantially vary in years deviating from the trend.
During the past 160 years four distinct intervals can be identified, when climatic conditions in the summer and the winter half-years considerably differed from the previous or following intervals.
The 20 years between the mid-1850’s and mid-1870’s were characterised by warm and dry Mediterranean summers. The conditions for sand migration were favourable at that time. Sheet wash was uncommon during these dry summers. The winter half-years, however, were milder and drier. These conditions did not promote landslide generation.
In the 30-year interval from the mid-1870’s to the beginning of the 1900’s the average. It is likely that these conditions were suitable for landslides.
From the beginning of the 1900’s until the mid 1940’s summer half-years were mainly slightly cooler and more humid than the long-term average. In these Atlantic summers the intensity of sheet wash slightly increased. In contrast, climatic conditions for sand migration were not appropriate. During this interval of nearly 50 year mean winter temperatures were above the long-term average. Annual precipitation was below the average in the first part of this interval but rose above the mean values by the second half of the 1940’s. During this relatively long interval winters changed from semicontinental to meditlcranean. The climatic conditions became more suitable for landslides.
From the 1950’s onward the summer half-years have cooled down considerably and annual precipitation has dropped. Summers have been rather cool and dry, reducing opportunities for sheet wash. Sand migration may intensify. Winter temperatures have shown a decreasing tendency but remain above the long-term average. Annual precipitation has notably reduced compared to the long term average, although short increases occurred occasionally. These climatic modifications indicate that the inclination to landslides has been diminishing for the last 50 years. Although, as it was already mentioned above, due to the smoothing character of the trend analysis, extremely humid or arid years could occur parallel to these tendencies, when the reduction or intensification of landslides might be typical.
Summ ary
The evaluation of the vegetation and climate history and meteorological data from the geomorphological aspect provided an excellent opportunity for the temporal analysis of the sand migration and sheet wash processes. It is likely that suitable conditions for sand migration were restricted to the second half of the Subboreal phase. The occurrence of landslides considerably increased during the Atlantic phase and in the so-called Little Ice Age.
The analysis of the maritime, continental and mediterranean oscillations of the last 160 years contributed to our understanding of the temporal alterations and provided valuable information for recent variations in the intensity of geomorphic processes.
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