Land slides on river banks in the west ern part of Podhale (Cen tral Carpathians, Po land)
Józef KUKULAK1, * and Karol AUGUSTOWSKI1
1 Ped a gog i cal Uni ver sity of Cra cow, In sti tute of Ge og ra phy, Podchor¹¿ych 2, 30-084 Kraków, Po land
Kukulak, J., Augustowski, K., 2016. Land slides on river banks in the west ern part of Podhale (Cen tral Carpathians, Po land).
Geo log i cal Quar terly, 60 (3): 561–571, doi: 10.7306/gq.1273
The au thors stud ied land slides in al lu vial sed i ments on un der cut steep banks of the rivers flow ing in the west ern part of the Podhale re gion. The land slides are of ro ta tional type. Landsliding pro cesses are com plex and they dif fer from those on solid rock slopes be cause of the het er o ge neous li thol ogy of the al lu vial sed i ments in the banks (muds, sandy grav els, clays) and be cause of un even de gree of their con sol i da tion. Their course de pends on the mode rate of wa ter in fil tra tion into the sed i - ments. Multiannual ob ser va tions of chang ing land slide sur faces and mea sure ments of scarp sur face with ero sion pins in di - cate that each type of sed i ment be haves in a dif fer ent way dur ing the slid ing. The lay ers of mud in the high est parts of the banks were slid ing down in blocks and soon be came frag mented, soaked and washed away. Sandy grav els were slid ing as whole lay ers or sep a rate frag ments. The more com pact the al lu vium, the more com pact was the re sult ing col lu vium. The thick lay ers of mas sive claystones un der ly ing the al lu vial sed i ments at the bank bases were mo bi lized by landsliding only to the depth to which they be came plas tic. The claystones ap peared the most re sis tant to landsliding among the bank ma te ri - als. The sur faces of rup ture were shal low and un even where poorly con sol i dated lay ers of al lu vium lied hor i zon tally (land - slides at Chocho³ów and LudŸmierz). In ar eas where al lu vial sed i ments were more con sol i dated and in clined, and the lay ers of clay al ter nated with sands and grav els, the sur face of rup ture oc curred deeper and was smooth (Stare Bystre land slide).
Land slides in un der cut river banks are an im por tant source of de bris in flu vial chan nels.
Key words: land slides, river banks, Podhale, Carpathians.
INTRODUCTION
Land slides are a com mon phe nom e non in the Carpathians.
Con di tions fa vour able for their oc cur rence in clude steep slopes, per me able grounds, and frac tured and fa vour ably in - clined bed rock strata. Trig gers are pro vided by in tense rain falls or re sult ing floods in river chan nels (e.g., Dziuban, 1983;
Kotarba, 1986; Ziêtara, 1988; Wójcik and Zimnal, 1996; Gil, 1997; Margielewski, 1998; Zabuski et al., 1999; R¹czkowski and Mrozek, 2002; Poprawa and R¹czkowski, 2003; Starkel, 2006; Wójcik et al., 2006; R¹czkowski, 2007; Gil et al., 2009).
Wide spread land slides orig i nate on slopes of moun tain ranges and val leys, but more nu mer ous are small slumps in young V-shaped small val leys or on steep banks of larger rivers. Land - slide stud ies are fo cused on those oc cur ring on land ar eas used for ag ri cul ture, trans port or dwell ing, as most dam age takes place there and the sur face area in volved is great est (e.g., R¹czkowski, 2004; Nescieruk and R¹czkowski, 2007;
Grabowski and Przybycin, 2010; Ma³ecki et al., 2012). Less at - ten tion is given to slumps on river banks, es pe cially those which
do not rep re sent im me di ate haz ard for hu mans. Be ing so nu - mer ous these land slides are es sen tial for sta bil ity of river banks and for the sup ply of rock and wood de bris to river chan nels.
Land slides of this type oc cur in Po land mostly on the banks of high land and low land rivers. They are mainly found on the high banks of the Vistula River near Nowe Brzesko, Sandomierz, Pu³awy, at the War saw es carp ment, in Dobrzyñ and Œwiecie (e.g., Banach, 1973, 1977, 1988, 1998; Bijak, 2007; Ilcewicz-Stefa niuk and Stefaniuk, 2007; Ilcewicz-Stefa - niuk et al., 2007; Tyszkowski, 2008, 2012a, 2014). Land slides are es pe cially fre quent on banks of ar ti fi cial dam res er voirs (Spanila, 1996), such as at W³oc³awek (Banach, 1985, 1994, 2004, 2006; Banach et al., 2013), Pakoœæ (Grobelska, 2006) and Jeziorsko (Banach and Grobelska, 2003; Kaczmarek, 2010), or in Si be ria (Shirokov, 1984; Kuskovskiy and Khabidov, 2002; Kozyrieva, 2001; Nazarov, 2006) and China (e.g., Wang et al., 2004; He et al., 2008) but also on the sea cliffs (Lefebre, 1986; Barret et al., 2011). The land slides orig i nated in those places where wa ter was or is now un der cut ting banks of rivers or lakes or where wa ter soaked the banks (Szabó, 2003; Scesi and Gattioni, 2009). Many land slides on val ley slopes in the Pol ish and Slo vak Carpathians reach with their tongues to the river banks (e.g. Dauksza and Kotarba, 1973; Nemèok, 1982;
Modlitba and Klukanova, 1996; Haczewski and Kukulak, 2004;
Lišèák et al., 2010; Baliak and Strièek, 2012; Bednarik et al., 2014; Kopecký et al., 2014). Al lu vial sed i ments in these land - slides are ro tated to gether with the slope sed i ments.
* Corresponding author, e-mail: jkukulak@up.krakow.pl Received: September 24, 2015; accepted: January 5, 2016; first published online: February 2, 2016
Land slides on river banks are pres ent along most of the world’s rivers where their banks rise above the bankfull flow.
They have been de scribed, among oth ers, from the Alps of Swit zer land (Oppikofer et al., 2008) and It aly (Scesi and Gattioni, 2009), from the Carpathians of Ro ma nia (Boengiu et al., 2011), and from the banks of Scan di na vian (Mil let, 2011) and Ca na dian rivers (Béland, 1956; Wil liams et al., 1979). It has been noted that the course of land slide pro cesses in river banks de pends on the geo log i cal struc ture of the banks (Clifton et al., 1981). The banks cut in young, poorly con sol i dated sed i ments (morainic, lac us trine, ae olian, al lu vial) are eas ily sus cep ti ble to land slides that re sult in sig nif i cant mod i fi ca tion of the chan nel re lief and lo cally al ter the course of chan nel pro cesses (Lefebre, 1986; Pécsi et al., 1987; Miller and Sias, 1998; Har ris, 2003; Kukemilks and Saks, 2013; Chen et al., 2015). Land - slides of this type in duce eco nomic loss by de stroy ing build ings and trans port fa cil i ties. Nicolet and Saint Barnabé-Nord land - slides (Jaboyedoff et al., 2009), those along the South Sas - katch e wan River Val ley (Clifton et al., 1981) and those in Win ni - peg (Baracos and Gra ham, 1981) may serve as ex am ples of such prob lems in Can ada.
A dis tinc tive fea ture of most river-bank land slides is that they in volve only or mainly al lu vial sed i ments, of ten of vari able grain-size and con sol i da tion. Con di tions for their or i gin and the mech a nism of trans la tion are some what dif fer ent than in land - slides in volv ing flysch strata or regolith. Pre sented be low are ex am ples of land slides formed on river banks where mainly al - lu vial sed i ments are in volved. The land slides were stud ied with the aim of find ing how vari a tions in grain-size and con sol i da tion of al lu vial sed i ments in flu ence the course and in ten sity of slid - ing. We tried also to de ter mine the sta bil ity of re lief on such land slides.
STUDY AREA
The stud ied land slides are lo cated in Podhale, in the bound - ary zone be tween Orava–Nowy Targ De pres sion, Guba³ówka Foot hills and Pieniny Klippen Belt (Klimaszewski, 1972).
In geo log i cal terms the land slides formed at the con tact of the
sed i ments fill ing the Orava–Nowy Targ De pres sion with the Podhale Synclinorium (com pris ing Oligocene to Early Mio cene sand stones and shales) and the rocks of the Pieniny Klippen Belt (Watycha, 1959, 1976; Mastella, 1975; Mastella and Rybak-Ostrowska, 2012; £oziñski et al., 2015). In Neo gene and Qua ter nary times the Orava De pres sion was the main site of sed i ment de po si tion by the rivers drain ing the West ern Tatra Moun tains (Czarny Dunajec River) and the Guba³ówka Foot - hills (Cichy, Bystry and Czerwony streams). These sed i ments are now partly ex posed along the chan nels in cised in these sed i ments. Neo gene sed i ments in the west ern part of the de - pres sion con sist mainly of claystones; in ter ca la tions of flysch grav els ap pear in the south-east (Plewa, 1969; Watycha, 1976;
Birkenmajer, 1979). The Qua ter nary sed i ments along the Czarny Dunajec are mostly grav els com posed of ma te rial from the Tatra Moun tains (mostly crys tal line rocks), while along the other rivers the grav els are com posed of flysch rocks with ad - mix ture of clasts de rived from the Pieniny Klippen Belt. Grav els in all Pleis to cene flu vial ter races are over lain with clays 1–2 m thick. In the mar ginal zone of the Orava De pres sion, the banks of the men tioned rivers over long dis tances (up to 1.5 km) are built of Neo gene claystones or grav els dis cor dantly over lain with Pleis to cene or Ho lo cene grav els and clays. Many land - slides have formed in these sec tions of the river chan nels where the rivers were un der cut ting the banks. A com mon fea - ture of these land slides is not only the type of sub strate but also their po si tion in the re lief. The grav ity mass move ments in - volved scarps of high flu vial ter races (Czarny Dunajec River and Wielki RogoŸnik Stream) and also un der cut al lu vial fans (Cichy and Bystry streams). The ter races are of com pos ite na - ture, with bed rock of Neo gene claystones and al lu vial cover of loose Qua ter nary grav els and clays. Ac tive slid ing pro cesses are best pro nounced on the banks which are now be ing un der - cut by the rivers, but they also per sist in the land slides which now lie be yond the reach of flood wa ters.
For this study we have cho sen three land slides sit u ated on the banks of the Czarny Dunajec River at Chocho³ów, Bystry Stream at Stare Bystre, and Wielki RogoŸnik Stream at LudŸmierz (Fig. 1). The land slide at Chocho³ów in volved a frag - ment of the 12 m high left-bank Pleis to cene ter race. It is sit u ated
Fig. 1. Location of the landslides studied in Podhale
2.2 km down stream of the bridge over the Czarny Dunajec River on the road to Sucha Hora in Slovakia and its sur face area is ca.
10 ares. On the Bystry Stream, the land slides form a lin ear front that re ju ve nates the re lief on the west side of the Czerwona Góra (696 m a.s.l.) ridge – an ero sional rem nant at the apex of a Neo - gene al lu vial fan. Seven land slides with sur face ar eas about 0.3–0.5 ha each are sit u ated along an ero sional scarp 1.5 km long and 16–22 m high. The land slide se lected for this study lies in the south ern part of this scarp. The land slide at LudŸmierz oc - curs in the right bank of the Wielki RogoŸnik Stream on a sharp bend of the river, and it in volved sed i ments of the 6–7 m high Pleis to cene ter race. All the stud ied land slides lie on ac tive banks of the rivers and their tongues lie within the rich of the me dium wa ter level in the rivers.
METHODS
Our study fo cused on mor pho log i cal, geo log i cal and hy dro - log i cal fea tures of the three land slides. Sur face re lief of the land slides was sur veyed dur ing four suc ces sive years (2012–2015). The sur vey reg is tered the dis tri bu tion of scarps, cracks in the ground, as well as ris ers and de pres sions within the land slide mass. All ev i dence of re ju ve na tion of the crown and frac tures along it were also reg is tered dur ing the four years.
The geo log i cal ob ser va tions in cluded in ves ti ga tion of com plete lithological sec tions of the land slide slopes, at ti tude of strata, den sity and at ti tude of joints in the rocks ex posed in the head scarps, and lo ca tion of the con tact be tween the claystone bed - rock and al lu vial grav els. Mesoscopic char ac ter is tics of the rocks (grain-size, co her ence, plas tic ity) were de scribed at the ex posed frag ments of de tach ment sur faces. Lo ca tion of ground wa ter out flows, marshes, wa ter seeps, ephem eral wa ter courses and ponds was reg is tered (Grabowski et al., 2008).
Per sis tence of the slip sur face of the Chocho³ów land slide (Fig. 1) was eval u ated by mea sur ing ero sion of this sur face af - ter multigelation events dur ing win ter se mes ters us ing gauge rods (Augustowski and Kukulak, 2013). Vol ume of sed i ment
trans ferred to ward the base of the land slide was also cal cu - lated. Mod i fi ca tion of the land slide tongue was ob served af ter suc ces sive floods of the Czarny Dunajec.
Ob ser va tions on the land slide on the Bystry Stream (Fig. 1) con cerned per ma nence of the land slide ponds; reg is tra tion of the lo ca tion and type of al ter ations of the land slide sur face af ter heavy rains and af ter win ters 2012 to 2014, and the prog ress in ero sion of the tongue that was be ing un der cut by the river. Ori - en ta tion of land slide el e ments was com pared with ori en ta tion of joints in the main scarp.
Ob ser va tions on the land slide at LudŸmierz (Fig. 1) con - sisted in mea sur ing ret ro gres sive dis sec tion of the crown by wa - ter flow ing from the peat bog sit u ated above, and in eval u at ing the age of plants over grow ing the col lu vial masses. De gree of weath er ing of the gravel layer ex posed in the main scarp was de - ter mined, and the depth to the claystone base of the ter race was es tab lished us ing probes. Hy dro log i cal data were used to de ter - mine the ex tent of flood wa ters on the tongue of each land slide.
Pro gres sive changes in the re lief of the land slide at Chocho³ów were also reg is tered by com par ing its pho to graphs taken in years 2012–2015. The pho to graphs taken at var i ous sea sons al lowed de lin eat ing per ma nently ac tive, in ter mit tently ac tive and in ac tive zones on the land slide sur face. The pho to - graphs also al lowed de ter mi na tion of the di rec tions and amounts of trans la tions of sep a rate frag ments of the land slide in the stud ied pe riod. A GIS soft ware was used for com par i son of the pho to graphs.
RESULTS
The land slide at Chocho³ów is sit u ated on the slope of a ter - race un der cut by the Czarny Dunajec. The land slide is 20 m long and 50 m wide (Fig. 2A). The ver ti cal drop height of the land slide is 10–12 m, while the in cli na tion of its slope var ies from ver ti cal at the main scarp to 10–15° in the lower part. The crown is on a flat sur face of the ter race and the tongue reaches the river chan nel. The tongue is now ac tively un der cut by the
Fig. 2. Landslide in the bank of the Czarny Dunajec River at Chocho³ów
A – map of landslide; B – geological structure of affected slope; C – cross-section (A–A') through landslide
river. The ter race sur face is oc cu pied by nar row cul ti vated plots, de lim ited by plough ing rills that drain rain fall and thaw wa - ters to the land slide.
The slope of the cut-in-fill ter race is cut in ter race sed i ments (5.5–6 m thick) and the bed rock (strath ter race) of Plio cene clayey-muddy shales (5.5–6 m). The up per part of the al lu vium con sists of clay (1.7 m), and the lower part of gravel and sand (4-4.5 m; Fig. 2B). The clay does not dis play sed i men tary struc - tures. It is cut by ver ti cal con trac tion frac tures and by a dense set of hor i zon tal joints. Lab o ra tory anal y ses of the clay sam ples in di cated that silt frac tion pre dom i nates in the whole sec tion (2/3 of the to tal), the con tent of sandy frac tion is 21%, clay frac - tion 7% and gravel frac tion 6%. The sed i ment is mainly mud;
this is mainly grav elly mud to a depth of 0.7 m, and grav - elly-sandy mud be low that depth (Chmielowska, 2013). The un - der ly ing grav els are rich (up to 45%) in mud and sand in their up per part (1.7 to 2.8 m), are poorly sorted and cha ot i cally ar - ranged. The peb ble size (b-axis) var ies from a cou ple of centi - metres up to 30 cm, 3–8 cm on av er age. Only the low est part of the grav els (2.8–4.5 m) con sists mainly of coarse clasts em - bed ded in coarse-grained sand. The whole se ries of grav els is com posed of crys tal line rocks from the Tatra Moun tains and is fee bly ce mented. At the base of the ter race sed i ments lies a se - ries of slightly fis sile clayey muds. The muds are com pact, slightly plas tic, ir reg u larly frac tured on the dry sur face, and wa - ter logged within the range of cap il lary draw and at the con tact with the river wa ter.
The sur face of the land slide is un even and var ies from part to part. The sur face is con cave in the south ern part and rather uni formly in clined in the north (Fig. 2C). The north ern part of the land slide con sists of large and thick pack ages of clayey-grav - elly sed i ments with un dis turbed in ter nal struc ture, reach ing down to the river chan nel. The slip sur face of the land slide is steeper there and lies at a greater depth (5–7 m). Dis place ment of the land slide masses oc curred as one-time ro ta tional trans la - tion. In the south ern part the slip sur face is shal lower, the col lu - vial mass is more frag mented and its sur face is cov ered with cha ot i cally crowded grav elly-clayey el e va tions sep a rated by de pres sions filled with clay. They are pres ent in the lower, less in clined, part of the land slide.
The re lief of the land slide var ies also along its length. Nu - mer ous sheets of clay with turf are pres ent in the up per part (Fig. 3). One of them has pre served its con ti nu ity along the width of the land slide, and lies on the in clined sur face. The other sheets are de tached from the crown along frac tures and slid down over a dis tance de pend ent on the slope an gle: the steeper the slope, the lon ger the dis tance of trans la tion. There are also small sec ond ary crowns 1–1.5 m high. The ar cu ate crown of the land slide is in cised and re cessed by 1–1.5 m at the out lets of the plough ing rills. Ero sional rills are cut in the land - slide sur face be low these out lets. Cones of washed-out grav els lie within these rills. A dis tinc tive fea ture of the lower part of this land slide is de bris ridges with clay cores coated with gravel.
Some ridges are en tirely built of clay (Fig. 4) and have a form of Fig. 3. Changes in relief of the landslide at Chocho³ów during the period 2012–2015
A – September 2012; B – April 2013; C – September 2014; D – May 2015
a se ries of imbricated folds. This part of the land slide is boggy and the clays are soft and plas tic. The land slide is wet up to the bound ary be tween the grav els and Neo gene claystones. Two ground wa ter out flows are pres ent along the bound ary.
The land slide sur face is dis rupted by sets of cracks di ag o nal or par al lel to the land slide crown or the toe (Fig. 5). Cracks along the toe, where it is be ing un der cut by the river, are ori - ented 5–55° and par al lel to the ero sional scarp, which means that they formed by grav ity. Cracks on the sandy-grav elly col lu - vium in the cen tral and up per parts of the land slide, ori ented 155–170°, are lo cally di ag o nal to the crown.
The crown of this land slide is re ced ing by mass move ments not only in the sum mer se mes ter, dur ing heavy rains and dur ing floods of the Czarny Dunajec, but also in the win ter se mes ter,
dur ing re peated pe ri ods of multigelation. The av er aged re ces - sion of the crown dur ing the win ter se mes ters 2011/12 and 2012/13 was 5–55 cm in clay and 5–30 cm in gravel, with a sta - ble po si tion of the toe in the same pe riod (Augustowski and Kukulak, 2013).
The land slide on Czerwona Góra has a shape of an elon - gated trough, ca. 100 m long, 15–20 m wide and ca. 0.2 ha in sur face area. It cuts across the whole length of the steep slope and its tongue en ters the chan nel of the Bystry Stream (Fig. 6).
The ver ti cal drop height of the land slide is 25 m (688–663 m a.s.l.), and the av er age slope is 25°. This is a quite deep land - slide, the height of the slip sur face ex posed at the crown is 4–6 m, but in the mid dle part the trough is prob a bly even 7–9 m deep, in clud ing the thick ness of the col lu vium within it. The Fig. 4. Compressional fold in clays in the landslide at
Chocho³ów
Fig. 5. Fractures and cracks in colluvium of the landslide at Chocho³ów
Fig. 6. Landslide at Czerwona Góra
A – sketch of land slide; B – joints: on a di rec tional di a gram (with the num ber of mea sure ments) and on a con tour di a gram (equal area plot, pro jec tion of nor mal on un der hemi sphere, con tour in ter vals: 5.0–10.0–15%); C - geo log i cal cross-sec tion at head scarp; other ex pla na tions as in Fig ure 2
whole trough is sur rounded by a rocky scarp, much higher on the right, north ern side (4–5 m) than on the south (1–3 m;
Fig. 7). The crown is ar cu ate in the high est part of the land slide, but its line is bro ken with two kinks at the tran si tion to the lat eral scarp on the right side. The lat eral scarps are straight and par - al lel to one an other.
The land slide was formed in sed i ments of the Neo gene al lu - vial fan. The Czerwona Góra hill is an ero sional rem nant of the api cal part of the Domañski Wierch al lu vial fan, built of flysch grav els interlayered with claystones and tuffites (Plewa, 1969;
Birkenmajer, 1979). A sec tion of the de tached part of the Czerwona Góra slope is well-ex posed in the up per right scarp of the land slide (Figs. 6A and 7). A layer of clay with fine and me dium gravel, 1.0–1.2 m thick, is un der lain by deeply weath - ered and frac tured light claystones with dis cern ible lay er ing, 1.0 m thick. The deeper-ly ing claystones (0.8 m) are less weath - ered and more sandy. They are un der lain by two lay ers of fine-grained sand stones (0.2 m each), fee bly ce mented, with rusty stains, un der lain by hard pan. Blue claystones, 0.3 m thick, with ad mix ture of sand, lie be low a depth of 3.5 m. They pass down ward into claystones with fine mud (1.5 m). The still lower part of the land slide slope is ex posed on the slip sur face of the next land slide to the south. Five in clined lay ers of blue claystones (0.3–0.6 m thick) al ter nate with slightly sandy light claystones (1.0–1.6 m). The whole Neo gene se ries dips gently (8–18°) to the north (88–95°), nearly per pen dic u lar to the course of the Bystry Stream and to the land slide-af fected slope of Czerwona Góra (170–180°). The claystones are densely cut
by joints in the lat eral scarp of the land slide. Two sets of joints and slick en sides pre dom i nate – 100–110° and 120–130°
(subordinately 20°), steeply dip ping (80–88°) mostly to the north (Fig. 6B). Both lat eral scarps of the land slide have sim i lar az i - muths (110–120°).
The land slide tongue is over grown with young for est. The tongue is 3–4 m high at its front. Upon reach ing the river the tongue be comes soaked and plas tic; by frac tur ing it dis in te - grates into blocks. The tongue’s sur face is un even, cov ered over its whole length with many ridges and hol lows, and it has four ac tive steps in the fron tal part. The most ro tated part of the land slide is its lower por tion rep re sented by claystones. The ridges and steps are ar ranged di ag o nally to the long axis of the land slide, at the az i muths of ca. 140–160°, sim i larly as the cracks in the ax ial part. The cracks along the lat eral scarps, es - pe cially on the north ern side, are lon ger, deeper and par al lel to the scarps. The tongue is wa ter logged in its lower part, and the wa ter out flows from it at two places. Ephem eral ponds are pres - ent on the up per ridges, on the crown side.
The land slide at LudŸmierz was formed at the sharp bend of the Wielki RogoŸnik Stream. It is the small est of the three land - slides, more than 100 m wide, up to 20 m long, and has 5–6 m of ver ti cal drop height.
The front of its col lu vial ridge reaches the river. The crown is semi cir cu lar, com posed of three smaller arcs formed by grad ual ex pan sion of the land slide along the river bank (Fig. 8A). The whole land slide con sists of three parts, and ev ery suc ces sive part has youn ger forms of the sur face. Its steep crown with the ex posed slip sur face is well ex pressed in its lon gi tu di nal sec tion and com pressed col lu vial ridges on the nearly hor i zon tal base in the lower part (Fig. 8B). The ridges are higher (up to 2 m) and wider (up to 9 m) in the up per part of the land slide. Cur rently, the river is erod ing in ten sively the lower ridges, weak en ing the nat - u ral sup port of the land slide. The area above the crown is over - grown with young co nif er ous for est, and an ex ten sive blan - ket-bog lies nearby. The bog feeds acidic wa ter drain ing to the crown of the land slide as sur face wa ter courses along plough rills or by as flows through the pip ing sys tems in clay.
The slid ing mass con sists of an al lu vial cover of the Pleis to - cene flu vial ter race com posed of gravel and clay (Fig. 8C). The up per part of ter race al lu vium con sists of a clay layer up to 2 m thick, com posed mainly of silt with ad mix ture of clay and sand and with dis persed coarse and me dium-sized peb bles of gran - ite and quartz ite. The clay is com pact, slightly plas tic and structureless. Its non-uni form com po si tion is in di cated by wa ter seeps and out flows from pip ing chan nels ir reg u larly dis trib uted on the ex posed slip sur face. A se ries of grav els of the Tatra gran ite that un der lies the clay con sists of three lay ers vary ing in size and ar range ment of peb bles. They dif fer also in the de gree of weath er ing, pres ence of in ter ca la tions of sand, and con cen - tra tions of ochre in wet parts. The lower lev els of the gravel se - ries are strongly weath ered and de com posed into a groat-like clay. The grav els along the con tact with clays are wa ter-soaked.
The claystone bed rock of the ter race is ex posed at the very base of the ter race, al most at the river level. Five probes in the col lu vial ridges have shown that the claystone base of the ter - race lies above the river level, but its top is un even with dif fer - ences in el e va tion up to 1.3 m. The wa ter-soaked claystones are soft and plas tic but still quite co her ent. The sur face of land - slide slip runs along their un even con tact with grav els. Lat eral dis place ment of col lu vial masses to ward the river chan nel over the top of the clays at tains 10–20 m.
Fig. 7. Bedrock crown of the landslide at Czerwona Góra
INTERPRETATION AND DISCUSSION
All the stud ied land slides are sit u ated at river bends where in ten sity of lat eral ero sion of con cave banks in creases dur ing floods. The loss of bank sta bil ity by un der cut ting nat u rally fa - voured ini ti a tion of the land slides. Lat eral ero sion of river banks is widely ac cepted as the fac tor ac cel er at ing the rate of landsliding (e.g., Zieliñski, 2001; Tyszkowski, 2012b, 2014).
Even the changes in wa ter level alone may stim u late mass move ments on the un der cut river banks (Ma³ecki et al., 2012).
The tongues of the stud ied land slides are still be ing un der cut, hence all the land slides are in tensely re ac ti vated dur ing ev ery flood of the Wielki RogoŸnik, Bystry streams and Czarny Dunajec River.
Nev er the less, the main fac tor lead ing to the trig ger ing of the land slides was the abun dance of wa ter in al lu vial sed i ments above the river chan nels. Vari able li thol ogy of these sed i ments and the re sult ing dif fer ences in the rate and modes of wa ter cir - cu la tion within them con trolled the course and type of land slide pro cesses on the river banks. The most im por tant fea ture of the river bank struc ture, as com pared with the banks built of flysch rocks, is the pres ence of a thick cover of loose or weekly com - pacted al lu vial sed i ments on a bed rock of com pact claystones.
The Neo gene claystones in Orawa are gen er ally com pact and have low per me abil ity (Watycha, 1976, 1977; Wiewióra and Wyrwicki 1980). Such sys tem, with per me able lay ers ly ing on im per me able rocks, fa vours in fil tra tion of rain wa ter and melt wa - ter above the clayey base of the ter race sed i ments (Zabuski et al., 1999). Wa ter pen e trates quite eas ily into the al lu vial clays and grav els to the top of the Neo gene claystones. The much slower in fil tra tion into the Neo gene claystones re sults in ac cu - mu la tion of wa ter in the zone of con tact be tween the claystones and grav els; this in turn fa vours de tach ment and trans la tion of the over ly ing sed i ments.
A clas si cal ex am ple of this is pro vided by the mech a nism of the for ma tion and evo lu tion of the Chocho³ów land slide. The thick top most low per me abil ity clay re tards in fil tra tion of wa ter, hence most of it drains by plough rills to the up per edge of the land slide crown. Near the crown, the joints in the surficial clays are more dense and ver ti cal, fa cil i tat ing in fil tra tion of wa ter into un der ly ing grav els (Fig. 9). Poor con sol i da tion of the grav els al - lows for deep pen e tra tion. In creased wa ter con tent in gravel low ers their shear strength, mainly by re duc tion of the an gle of in ter nal fric tion (Schuster, 1979). The gravel be comes still less co her ent and slides in lay ers to ward the river. The dis tance of trans la tion does not ex ceed the front of the clayey base of the ter race. The slowly wet ting Neo gene clays swell, and deep slip sur faces do not de velop within them. Changes in com press ibil - ity and shear strength oc cur only to the depth of wet ting (Thiel, 1980; Choma-Moryl, 2001). The clays may, how ever, pro vide a slip sur face for the wa ter logged grav els and surficial clays (R¹czkowski and Mrozek, 2002). Only their soft ened part, ad ja - cent to grav els, is sub ject to trans la tion. It was ca. 1.2–1.5 m thick in the land slide at Chocho³ów, up to 2–3.5 m on Czerwona Góra, and 0.5–0.8 m at LudŸmierz. It is in the layer of the most fre quent for ma tion of folds un der the stresses in duced by the weight and move ment of the over ly ing col lu vium (Nescieruk and R¹czkowski, 2007). This pro cess may ex plain the folds in clayey-grav elly sed i ments at the base of the Chocho³ów land - slide. For the same rea son the lon gi tu di nal sec tion of this land - slide shows marked loss of sed i ments in the clayey-grav elly part and a small one in the base clays (Fig. 10).
Grav ity trans la tions in the in di vid ual lay ers of al lu vium do not pro ceed in a uni form way. In the surficial clay layer this pro - cess runs in two ways. Near the edge of the main scarp, where the clay is cracked by con trac tion, coarse lumps of clay slide down kept to gether by the turf on the up per side. They dis in te - grate with time by des ic ca tion or waterlogging. The clay which is more sandy slides down in a less co her ent way. They fall down Fig. 8. Landslide at LudŸmierz in the bank of the Wielki RogoŸnik
A – map of landslide; B – cross-section through landslide; C – geological cross-section of landslide scarp; other explanations as in Figure 2
into smaller frag ments at start and, when later soaked, they eas ily flow down to ward the base of the scarp. This pro cess is usu ally slowed down by long-last ing soak ing of clay with wa ter and it lasts lon ger than in grav els. This is why these sed i ments cover grav elly colluvia with a clayey blan ket. Clay is creep ing down im me di ately af ter show ers only di rectly from the main scarp. In fil tra tion of wa ter is easy in the layer of fee bly con sol i - dated gravel, and wa ter flow ing over their sur face still low ers their co he sive ness. Shal low sheet-like slides or slow creep of gravel oc cur sub se quently. These move ments are ac tive mostly in win ter, when gravel slides slowly down in a layer of var i ous thick ness over the sur face of frozen deeper ground (Skar¿yñska, 1969).
Tak ing into ac count the dis tri bu tion, struc ture and sur face forms of the land slide at Chocho³ów it may be de scribed as ro - ta tional at its start, later re ac ti vated in the clayey part by falls and in the gravel – by slid ing of loose sed i ments, and in the part be ing un der cut by the river – by falls of the claystone bed rock of the ter race. The deeper ex tent and more sheet-like mode of trans la tion in the north ern part of the land slide are prob a bly caused by the deeper po si tion of the slip sur face and its later or - i gin. The faster and more clock wise ro ta tion of in di vid ual clay-and-turf rafts in the south ern part of the land slide is the re - sult of in creas ing steep ness of the main scarp in this di rec tion.
The pres ence of cracks over the whole sur face of the land slide points to con tin u a tion of the grav ity move ments within it.
The land slide at Czerwona Góra is also ro ta tional, though many of its fea tures are re lated to the struc ture of the bed rock.
Ori en ta tion of its lat eral scarps, step-like out line of the crown, and ori en ta tion of most cracks in the up per part of the tongue, all cor re spond to the ori en ta tions of cracks and frac tures vis i ble in the side walls of the land slide. This land slide is much deeper than that at Chocho³ów. This may be caused by stron ger ce - men ta tion of the rocks in volved. Deeper along the north ern mar gin, the slip sur face is asym met ri cal in cross-sec tion. This may be caused by de tach ment along the sur face of one of the lay ers of blue claystones dip ping to the north like all lay ers in the land slide (Fig. 11). The slip sur face is in clined steeper that the dip of the lay ers, so the slip sur face must cut through suc ces - sive deeper lay ers to ward the base of the land slide. This ex - plains the pre dom i nantly ar gil la ceous com po si tion of the land - slide’s toe, its higher ro ta tion and wet ness, and its per ma nent ten dency to re ac ti va tion.
The land slide at LudŸmierz is of ro ta tional type and is poly gen - etic. It was caused by joint ac tion of ero sional un der cut ting of the ter race at the river bend, os cil la tions of the wa ter level in the river, and ground wa ter ac tion in the area of its head. Near-sur face acidic ground wa ter inflowing from the nearby bog in ten si fies chem i cal weath er ing of clay and gravel and soaks the scarp be - neath the sites of its out flow. The deeply weath ered wa ter-soaked gravel be comes an ac tive layer sus cep ti ble to dis place ment by grav ity. Ac tiv ity of the land slide in creases af ter heavy rain falls and dur ing high wa ter lev els of the river. Wa ter pen e trates sub se - quently into the land slide mass from the river, fa vour ing mo bi li za - tion of the whole land slide (Ma³ecki et al., 2012).
The con tri bu tion of tec tonic pro cesses to the or i gin of the land slides should be also taken into ac count. All the land slides formed within the zone of mar ginal faults of the Orawa De pres - sion. Faults in Neo gene mudstones and claystones and bound - ary faults be tween the Podhale Flysch and Neo gene rocks are ex posed near the Chocho³ów land slide (Kukulak, 1998;
Fig. 12). It is note wor thy that the land slide ac ti vated only a frag - ment of the un der cut ter race of the Czarny Dunajec. The struc - ture of the val ley slope is iden ti cal above it, but mass move ment does not dis turb it; the slope is sta ble de spite ground wa ter out - flows on its sur face. Large faults be long ing to the Krowiarki Fig. 9. Fractures and cracks in silts in the upper part
of the main scarp of the landslide at Chocho³ów
Fig. 10. Neogene claystones eroded by the Czarny Dunajec within the landslide at Chocho³ów
Fig. 11. Discordant directions of the slip surface and attitude of strata at Czerwona Góra
Fault Zone and the west ern ter mi na tion of the Tatra Moun tains (Bac-Moszaszwilli, 1993; Baumgart-Kotarba, 1996, 2001) are ex posed at Czerwona Góra near the stud ied land slide. They dis sect Czerwona Góra into horsts and grabens be low the land - slide. The un usual shape of this land slide and the neigh bour ing ones – nar row, long troughs, at ti tude of joints in sand stones and claystones. and the ori en ta tion of the lat eral scarps seem to fol - low the di rec tions of these faults.
CONCLUSIONS
Landsliding on scarps of flu vial ter races is more com plex than on slopes built of flysch. The land slides are usu ally ini ti ated as ro ta tional ones, but their fur ther evo lu tion is poly gen etic. This is due to a dif fer ence in li thol ogy of the in volved sed i ments, vari - a tions in the prop er ties of al lu vial sed i ments with re spect to their co he sive ness, per me abil ity and plas tic ity, and also to their usu - ally hor i zon tal at ti tude (Oppikofer et al., 2008). The land slides formed in those places where fee bly con sol i dated or loose sed i -
ments (sandy grav els) were ly ing on co he sive im per me able rocks (Neo gene claystones). Such struc ture of sed i ments fa cil i - tated in fil tra tion of wa ter into the al lu vium and a soft plas tic zone could form at the con tact with the Neo gene claystones serv ing as the site of the slip sur face. The depth of the slip sur face in - creased with the in creas ing depth of the claystone base of the ter race and in creas ing con sol i da tion of the al lu vium. The claystone base of al lu via at Chocho³ów and LudŸmierz was in - volved in slid ing only to the depth to which it be came plas tic. In the thick base ments of the ter races with an al lu vial cover, land - slide move ments tend to be shal low, be cause the clays loose strength and be come plas tic at a very slow rate. These move - ments nor mally do not reach down to the chan nel bed (Har ris, 2003; Schwert, 2003). Land slide move ments in the Qua ter nary al lu vium were more dy namic than in the Neo gene claystones, though they were not uni form within each layer. They were slower and lasted lon ger in the lay ers of top most clays and were faster in grav els. Lo cal fac tors that in flu enced the ini ti a tion and course of the land slide move ments in cluded: the po si tion of the al lu vial strata; the pres ence and ori en ta tion of tec tonic struc - tures; ground wa ter out flows on the scarps; and the vari able de - gree of weath er ing of the sed i ments in volved.
Land slides on river banks are per ma nently re ac ti vated be - cause their fronts are eroded by stream cur rents dur ing ev ery flood. This is in con trast with the land slides on slopes. Al though tongues of some slope land slides reach river chan nels and are eroded by the rivers, this does not re sult in re ac ti va tion of the whole land slides (e.g., Dauksza and Kotarba, 1973; Baliak and Strièek, 2012). The eroded col lu vium con trib utes to the sed i - ment load of the river (e.g., Béland, 1956; Van Asch et al., 1999; Scesi and Gattioni, 2009; Kukemilks and Saks, 2013), so that the re la tion be tween the river and the land slide is dy namic and vi tal for the ac tiv ity of the land slide.
Ac knowl edge ments. The study was per formed within the scope of stat u tory re search at the Fac ulty of Ge og ra phy and Bi - ol ogy, Ped a gog i cal Uni ver sity of Cra cow, Po land.
G. Haczewski as sisted with the prep a ra tion of the Eng lish text.
We thank two anon y mous re view ers for their thor ough re vi sion, help ful sug ges tions and com ments.
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