Материалы гляциологических исследований, вып. 102
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Increase of contaminated avalanches in the Giant Mts. (Czech Republic) —
its consequences and causes
M. Kocianova, I.J. Dvorak, V. Spusta, I. Spatenkova, A. Tondrova
Krkonose National Park Administration, Vrchlabi, Czech Republic
По данным 42-летних наблюдений обнаружено заметное увеличение грунтовых лавин в Гигантских горах, начиная с 1982 г. Обсуждается вопрос о длин-нопериодной цикличности лавинной деятельности и возможных свидетельствах климатических изменений.
Introduction
Snow avalanches are the natural phenomenon that has influenced the relief of the Giant Mountains — the highest part of the Middle-European branch of Hercynian Mountains chain (length of 40 km, width of 20 km, max. height 1603 m asl.) since the end of Tertiary (Fig. 1). Regular occurrance of snow patches, cornices and avalanches on the leeward mountain areas were connected probably to the cooling of climate in the end of Tertiary. The avalanche tracks acted as the first forestless oasis of the arctic-alpine vegetation within the continued primaeval Tertiary forest [2].
The glaciological study [9] confirmed the influence of snow avalanches on the morphology of 8 in 37 glacial valleys and nivation niches in territory of the Giant Mountains at least. Snow avalanches were probably the main source of the Turkestan avalanche type of glacier in the Bile Labe valley too [7].
At present, 102 avalanche tracks are known in both parths of the Giant Mountains (Czech and Poland). Most of them (87 tracks) are conjoined with the Pleistocene glacial corries and nivation niches (Fig. 2). For the whole Holocene — possibly except the warmest and the dryest Boreal and Subboreal periods — snow avalanches keep treeless conditions on the avalanche tracks [2].
Since the winter season1961/62, monitoring of the avalanche activity is provided in the Czech part of the Giant Mountains. At about 870 avalanche events were registered in this period [11] — 35 avalanches (e.g. 4%) in them were considered of dirt ground avalanches with significant volume of replaced boulders, soil and organic material. While the part of ground avalanches was 0,6% (merely 4 events) till the 1986/87, their number has increased to 8,4% (31 in 371 avalanches) from the 1987/88 to 2002/03. During winter season 2001/02 sum of these events increase even to 14% (9 in 65 avalanches). This fact led us to evaluation of impact of the ground avalanches on the vegetation cover [4], relief and searching for a cause of their higher frequency in territory of the Giant Mountains.
Methodology
The study is based 1) on calculation of volume of organic and inorganic material transported from the starting and transitional zones to the accumulation zone during one event, 2) on assesment of frequency of big and
small ground avalanches in the Giant Mountains during the Holocene, 3) on calculation of volume of mass relocated in Giant Mountains glacial corries by ground avalanches during the Holocene and 4) on calculation of
Fig.1. Localisation of the Giant Mountains (1) in the Middle-Europe and Karkevagge valley in the north Scandinavia Рис. 1. Местоположение Гигантских гор (1) в Средней Европе и долины Каркеваге в северной Скандинавии
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Fig. 2. Pleistocene glaciation and localisation of avalanche paths in the Giant Mountains at present: 1 — avalanche paths, 2 — moraines, 3 — glaciers and nivation niches of the middle Pleistocene origin, 4 — glaciers of the last Pleistocene glaciation, 5 — altiplanation terraces. 6 — tors, 7 — cryodenic segregation structures, 8 — peneplain, 9 — relicts of Neogene and lower Pleistocene valleys, 10 — glacial cirques (wall), trough escarpment, 11 — nivation niches Рис. 2. Карта плейстоценового оледенения и положения современных лавиносборов в Гигантских горах: 1 — пути схода лавин, 2 — морены, 3 — ледники и нивационные ниши среднего плейстоцена, 4 — ледники последнего плейстоценового оледенения, 5 — нагорные террасы, 6 — торы, 7 — криогенные сегрегационные структуры, 8 — пенеплен, 9 — остатки неогеновых и нижнеплейстоценовых долин, 10 — ледниковые цирки (стенки), бровка трога, 11 — нивационные ниши
retreat of rockwall (corry wall) during the Holocene ad1. The area of starting and transition zone from which soil, rocks and organic material were snatched away, was measured by using GPS in terrain (in three cases) or in laboratory on base of drawing this areas from photos (Fig. 3) into the orthophotomap (Fig. 4) and then calculated to m2 (in six cases). The same method we have used for evaluation of area of accumulation zones.
The volume of relocated material was calculated within avalanche path No. 3 — Upska rokle gully (avalanche events on April 30, 2002 and May 4, 2002) during May and August 2002 on base of classical methods by Rapp [8] — Table 1. Some random plots 10x10 m were chosen in accumulation zone: six plots in its runout part, three in the middle part, one in the upper part:
the number and size of pebbles, cobbles and boulders (length, breadth, height) were measured at each plot. The average volume of rock material per 1 plot was used for calculation of total quantity within accumulation zone;
content of finer material (size less than 10 cm) was estimated by weighting the debris within 5 sample squares 0,5x0,5 m and multiplying the quantity with the total area of accumulation zone;
content of organic matter was determined a) on base of depth of 100 random punctures within fresh deposit some days after fall down of avalanches, and in
Fig. 3. The ground avalanches on periphery of Upska jama corry: A — path No 4 (fell down on March 3, 2002), B — path No 5 (fell down on April 28, 2002), C — path No 5 (fell down on May 2, 2002), D — path No 3 (fell down on April 30, 2002), E — path No 3 (fell down on May 5, 2002). Photo made by M. Tu°ma Рис. 3. Грунтовые лавины на периферии кара Упска-Яма: А — лавиносбор № 4 (сход лавины 3 марта 2002 г.), В — лавиносбор № 5 (сход лавины 2 мая 2002 г.), D — лавиносбор № 3 (сход лавины 30 апреля 2002 г.), E — лавиносбор № 3 (сход лавины 5 мая 2002 г.)
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Материалы гляциологических исследований, вып. 102
Fig. 4. Orthophotomap of area Ùpska jama corry (paths No. 3, 4, 5) and Velka Studnicni jama corry (path No. 7) with drawing of ground avalanches in spring 2002 Рис. 4. Ортофотокарта области кара Упска-Яма (лавиносбо-ры № 3, 4, 5) и кара Велька-Студничны-Яма (лавиносбор № 7) с контурами грунтовых лавин (весна 2002 г.)
August, when the deposit was completely dry, b) on weight of dryed material collected on some randomly choosen plots 0,5x0,5 m.
Results
Volume and number of transported material. On base of measuremnts in field one big ground avalanche event can relocate at about 1,5 m3 rock debris per 10x10m. For example, for the whole accumulation zone of avalanche from the May 2002 in Upska rokle gully (path No. 3) the total amount of 629m3 was extrapolated. The volume of finer (soil) material equals to 881 m3 (Tables 1, 2).
Frequency of big avalanches within Giant Mountains during Holocene. Jenik [2] allowed for the facts, that the avalanche activity was diminished or excluded in warmer and dryer period of Holocene — e.g. in Boreal (8500-7000 BP) and Subboreal (4500-2800 BP),when
the amounth of snow precipitation was probably significantly smaller than at present. The old literature reports, which concern to the ground avalanches snutched away rocks and soil from the whole rocky wall, originated in XIX century. For the XX and XXI centuries, information about three big ground avalanches are known (events in March 1956, 1999, February 1999, and March, April and May 2002).
On the assumption of three big ground avalanches in 100 years, 204 such episodic events happened in the Czech part of the Giant Mountains during 6800 years of Holocene. Another situation concerns to big slab and powder avalanches (sometime ground avalanches too), which couse braiking of forest. The number and frequency of such avalanches is significantly higher. The treeline on the 60% of avalanche paths is permanently lawered, on the 40% of paths it fluctuates in lenght of 200-300 m in period more than 16 years [5].
Role of extreme episodic events. The mean speed of spring wett ground avalanches reaches about 15m/s [6]. Length of avalanche paths in the Giant Mountains is about 500 m — it means, avalanche fall happens in course of 33 seconds. At about 204 such events equal to 112 minutes of avalanche activity during whole Holocene. The volume of mass transfered by avalanches (size like avalanche on May 2nd, 2002) in course of this time equals to 127520 m3 of rock and 179724 m3 of soil and organic material. The retreat of rockwall (corry wall on example of Upska rokle gully) has been estimated at 1,3 m during the Holocene.
In comparison, Rapp [8] calculated the retreat of rockwall in Karkevagge (North Sweden) caused by rockfalls for 1 m during postglacial, and the volume of rock debries carried by one big ground avalanche of lenght 350-450 m at about 200-300 m3. Jonasson et al. [3] extrapolated the time of slush torrent activity in Karkevagge for 33 hours over the entire Holocene.
Is obvious, new knowledge of frequency and consequences of big episodic ground avalanches in the Giant Mountains are in agreement with conclusions of swedish geomorphologists: The activity of several types of geomor-phological processes has been highly episodic, however, their impact on mountain relief development is very signifi-cat, especially in long time span. Due to different geographical, climatic and vegetation conditions between localities in Karkevagge and in the Giant Mountains (polar maritime
Analyses of big ground avalanches
Table 1
Path No:
Date
Area snutched down (m2)
Area of Number of rock Volume of rock
dirt deposit (m2) fragments(12 plots) fragments (m3)
Volume of replaced soil (m3)
09.IV.99 02.V.02
04.IV.99 12.III.02
3264 4250
23026 14895
2695 12596
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