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Floods at the northern foothills of the Tatra Mountains — A Polish-Swiss research project

Kundzewicz Z.W, Pińskwar I., Choryński A., Stoffel M, Ruiz-Villanueva V, Ballesteros-Canovas J.A, Kaczka R.J, Niedźwiedź T, Łupikasza E, Czajka B., Małarzewski Ł., Janecka K, Wyżga B, Mikuś P.

Acta Geophysica

2014

Vol. 62, no. 3

620--641

The present paper introduces the topical area of the Polish–Swiss research project FLORIST (Flood risk on the northern foothills of the Tatra Mountains), informs on its objectives, and reports on initial results. The Tatra Mountains are the area of the highest precipitation in Poland and largely contribute to flood generation. The project is focused around four competence clusters: observation-based climatology, model-based climate change projections and impact assessment, dendrogeomorphology, and impact of large wood debris on fluvial processes. The knowledge generated in the FLORIST project is likely to have impact on understanding and interpretation of flood risk on the northern foothills of the Tatra Mountains, in the past, present, and future. It can help solving important practical problems related to flood risk reduction strategies and flood preparedness.

Analiza dendrogeomorfologiczna drzew różnych gatunków z obszaru osuwiska „L. Sawickiego" w Szymbarku, Beskid Niski, Karpaty Zewnętrzne

Nawrocka N.

Folia Quaternaria

2013

nr 81

175--187

Analizę dendrogeomorfologiczną przeprowadzono na dużym, aktywnym osuwisku w okolicach Szymbarku, zwanym osuwiskiem „L. Sawickiego". Próby pobrano z 63 drzew należących do różnych gatunków, rosnących na obszarze osuwiska oraz w terenie stabilnym. Zastosowano nową metodę, polegającą na obliczeniu wartości indeksu dekoncentryczności (WISTUBA, MALIK 2011). Zidentyfikowano lata o największej liczbie drzew wykazujących sygnały diagnostyczne dla ruchu podłoża, zaznaczające się jako gwałtowne zmiany wartości indeksu dekoncentryczności. Najsilniejszą reakcją drzew charakteryzowały się lata: 1997, 1998 oraz 1968. Wyniki uzyskane za pomocą metody dendrochronologicznej porównano z dostępnymi danymi klimatycznymi: temperaturą i wysokością opadów. Stwierdzono, iż najczęściej intensywne przemieszczenia podłoża były rejestrowane w latach termicznie chłodnych i wilgotnych, charakteryzujących się wysoką sumą opadów. Reakcja drzew na uruchomienie podłoża jest widoczna w latach o wysokich opadach (na przykład: 1983, 1985, 1997) lub z rocznym opóźnieniem w stosunku do sygnału klimatycznego (przykładowo: 1982, 1984, 1993).

Dendrogeomorphological analysis was carried out on a vast, active landslide, called the landslide of L. SAWICKI, situated in the Beskid Niski (Low) Mts, within the Flysch Carpathians. The research material consisted of 126 samples (cores), collected from 63 trees with a Pressler increment borer. The samples were taken from trees growing within the landslide area, as well as around it. Within the landslide area coring was made for 33 coniferous trees, representing the following species: silver fir (Abies alba Mill.) —15 specimens; European larch (Larix decidua Mill.) —11 specimens, and Scots pine (Pinus sylvestris L.) — 7 specimens. The trees were selected; coring was made for specimens with trunks deformed (tilted) in the effect of movements of an unstable ground. A new research method, consisting in calculation of the eccentricity index, was applied (WISTUBA, MALIK 2011). This method comprises measurements of the annual growth width on both sides of trunks (upslope and downslope), and determination of the eccentricity expressed in millimetres and the eccentricity index expressed in per cent. These indicators are determined for every tree as a whole, as well as for every annual growth ring. Beginnings of the mass movements are dated based on indicators of the annual variability of the eccentricity index of annual growth, expressed as a difference of the index values in a given year and in the previous one. On the basis of the annual variability of the eccentricity index values for trees growing in stable terrains, threshold values are determined for dating periods of gravitational movements (WISTUBA, MALIK 2011; WISTUBA ET AL. 2013). Eccentricity of the annual growths and the eccentricity index were calculated for everyone tree as a whole and for every annual growth, for samples taken from all the trees analysed. Eccentricity [mm] calculated for coniferous trees growing within the landslide varied from —125.46 mm to 138.45 mm, and for reference trees from stable areas — from —37.63 mm to 70.14 mm. Eccentricity index values calculated for whole trunks of trees growing in the landslide area varied from —114.29% to 113.06%, and for trees growing in the reference, stable areas — from —16.88% to 33.34%. For the trees growing in the landslide area the index values are high, which indicates that they have been growing on an unstable ground. Identification of years with the highest numbers of trees exhibiting diagnostic signals for a ground movement was based on abrupt changes of the eccentricity index values. The signals connected with gravitational ground movements were determined for all trees from the landslide area, based on the thresholds calculated. Altogether 536 signals were dated, 265 (49.4%) of them were related to the upslope eccentricity, whereas 271 (50.6%) signals — to the downslope one. Variability of numbers of the signals in indivi¬dual years was analysed for the period 1900-2011. The oldest signal of the movement appeared in 1900. The highest numbers of signals of the landslide activity were noted for the years: 1997, 1998 (14 specimens, ca. 42% of trees sampled on the landslide), and 1968 (13 specimens, 39% trees). Relatively high numbers of signals characterized also the years: 1975, 1977, 1978, 1979, 1988 (12 specimens 36% trees), and 1983, 1984, 1985, 1989 (11 specimens — 33% trees). For the period prior to1930 less data were available, which could be a cause of lower numbers of trees exhibiting reactions in these years. Periods of starting ground movements determined with the dendrochronological analyses were compared with accessible climatic data and with results of analyses carried out with other methods. In the period analysed (1900-2011) the highest annual precipitations, over 1000 mm, were noted in Szymbark five times, with the highest values in 1974 (1164 mm) and in 2010 (1137 mm). Also the years: 1970, 1980, 1985, 2001 should be counted to the most humid years in the region. High rainfall, which could brought about revitalization of mass movements in the entire Beskid Niski and Sądecki Mts, occurred in the years 1897, 1906, 1934, 1955, 1958, 1970, 1973, 1983, 1997, 1999, and 2002. Only some of the periods characterized with strong reaction of trees were directly connected with high rainfall in a given year or in the preceding one, registered in the surroundings of Szymbark. The years characterized by extremely strong reaction of trees, i.e. the years 1997-1998 are connected with particularly intensive rainfall registered in the year 1997 (POPRAWA, RĄCZKOWSKI, 2003). That year in the Carpathians numerous floods were recorded, caused by very high summer rainfall. However, in the surroundings of Szymbark it was the year only slightly more humid than average. Among the trees examined the reaction occurred either in the years with high rainfall in the region (1983, 1985, 1997), or with one-year delay with respect to the climatic signal (1971, 1975, 1981, 1998). In the remaining cases destabilization of the slope could be provoked by other factors, difficult to explain on the basis of the available climatic data. Particular attention should be paid for the strong-reaction dry years preceded by dry years (e.g. 1977) or the strong-reaction dry years preceded by average years (e.g. 1982, 1984, 1993). Comparison of the data obtained with the dendrochronological method with climatic records demonstrated connection of the ground movements with extreme rainfall. It turned out that part of the sliding episodes were registered in highly humid years, however the remaining ones did not display such a relationship.