Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin

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1 Riscuri şi catastrofe, an XV, vol. 19, nr. 2/2016 Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin Gh. ROȘIAN 1, Cs. HORVATH 2 ABSTRACT. - Correlation between the landslides and the morphological and functional units of slopes in the Transylvanian Basin. The presence of fluvial morphology in the Transylvanian Basin, in form of an alternation of water divides and valley corridors, indicates favourable conditions for the genesis of geomorphologic processes. Under this aspect two sections stand out within this type of processes: river beds and slopes. In this paper, the emphasis is on the processes, developed on slopes. Water erosion and mass movement processes can be observed on their surface. From all mass movement processes, the emphasis will be put on the landslides. They will be observed in correlation with the morphologic and functional units of the slopes from different regional units of Transylvanian Basin. Eight case studies were taken into consideration regarding this aspect. Thus, we noticed that landslides particularly develop in the median part of the slopes which is corresponding to the maximum processual dynamic and transfer unit. Key-words: landslides, geomorphological processes, morphological unit, slope 1. INTRODUCTION From all the geomorphologic processes typical for the slopes from the Transylvanian Basin, a special category is represented by landslides. These are the result of many causes, the most significant being geology, the pre-existent morphology, the land use etc. Geologically speaking, the Transylvanian Basin represents a large inter-carpathian morphotectonic unit, dominated by the geological formations from the Cenozoic Era. It has a base over which sedimentary deposits were laid-down and which currently outcrops. (Vancea, 1960; Sanders et al., 2002). From the latter, particularly noticeable are the quaternary (Holocen, 1 Babeș Bolyai University, Faculty of Environmental Science and Engineering, , Fântânele Street, No. 30, Cluj Napoca, Romania; georgerosian@yahoo.com 2 Babeş-Bolyai University, Faculty of Geography, , Clinicilor Street, No. 5-7, Cluj- Napoca, Romania; hcsaba@gmail.com 105

2 Gh. Roşian, Cs. Horváth Pleistocen) and the Neogen (Pannonian, Sarmatian, Badenian, Burdigalian) deposits to which marginally in contact with Carpathian Mountains, Paleogen, Cretaiceous and Jurassic deposits are added. From the above-mentioned deposits, by their composition and extension the Pannonian, Sarmatian, Badenian, Burdigalian deposits are the most susceptible to landslides (Bălteanu et al., 2010, Petrea et. al., 2014). The deposits have in their composition illite and beidellite type montmorillonite minerals, which have the property to retain water. The Transylvanian Basin s relief is the result of certain geological friable formations modelling as ones mentioned above. This fact conducted to the formation of a fluvial relief under the form of a variation of water divides and valley corridors. The presence of valley corridors accompanied by inclined surfaces, like slopes, are able to offer further favourable conditions to geomorphological processes, mainly landslides. Regarding the land use, in the Transylvanian Basin, the agricultural fields (55.2%) are predominant, followed by forests (38.3%), built territories (6.3%) waters (0.2%), etc. (Roșian, 2011). The most visible changes of the natural landscape from the Transylvanian Basin took place as a consequence of the vegetation cover variation. The vegetation type was gradually modified in time, by clearing and grubbing, the forests being replaced by arable lands, grasslands, hayfields, vineries, orchards, etc. which did not lack in consequences, being perceived as one of the substrates instability source. Based on the local conditions and in relation to morphological and functional units of the slopes, landslides can fall into different categories. For their identification various case studies were selected (Fig. 1). 106 Figure 1. The geographic position of case studies

3 Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin 2. DATABASE AND METHODS To emphasize the correlation between the landslides and the morphological and functional units of the slopes, first the landslides have been vectorized from satellite images so that after the slope units could be identified. Landslide vectorizing and classification has been performed with a GIS software (Esri ArcMap ). The main functions/tools used were: Editor, Statistics, Extract etc. Regarding the morphological and functional slope unit s limit, the study has been conducted based on the current models found in the scientific literature. In the following we will present four of them briefly: The first model belonging to J. F. White (1966) defines that a balanced slope profile has three elements in general which can be considered as morphological and functional units: upper convex element, a middle straight element, and a lower concave element. The second model belonging to L. King (1953) considers that the slope has four morphological elements, each with a different function. These are: waxing slope (on the upper side); free face; debris slope; pediment (lower concave element - usually a gentle concave profile). The third model was proposed by J. B. Dalrymple, R. J. Blong and A. I. Conacher (1968), the authors indicate the presence of nine slope units: interfluve or eluvial process unit; the distribution unit; convex slope or the organisational unit of areal and linear erosion; fall face or insertion slope of areal and linear erosion forms; midslope or the maximum mobility morphodynamic unit; colluvial footslope or the redeposit unit of the diluvio-colluvial deposits; meadow or the alluvial proluvial unit; channel wall or the lateral erosion unit and also the crumbling-collapsing unit; channel bed or the unit where riverbed processes take place as a result of river dynamics. The fourth model, derived from the previous one, was proposed by Mac, L. Blaga (2004). According to the quoted authors, the following morphological and functional units can be generalized: interfluve or alluvial process unit (U I ); the possible morphological processes programming unit (U II ); process initiation unit (U III ); maximum processual dynamic and transfer unit (U IV ); re-depositing unit (U V ); river bed (U VI ). Therefore, it is a model with six morphological and functional units defined according to occurrences and manifestation conditions. In following this last model will be taken into consideration in order to establish the correlation between the landslides and the morphological and functional units of the slopes. It is worth mentioning that depending on the local conditions some morphological and functional units of the slopes will not be encountered in the field. Indifferently if referring to the models with less morphological and functional units such as the first two presented or to those with more (the last two), it can be 107

4 Gh. Roşian, Cs. Horváth noticed the existence of a sector or slope strip, which features active morphological processes. In this category fall the processes related to slope runoff dynamics and those related to mass movement. Further, from the mentioned processes, only the distribution of landslides on functional and morphological units will be observed. 3. RESULTS AND DISCUSSION As a result of vectorisation landslides from satellite images, it has been emphasized that in the Transylvanian Basin there are Regarding their distribution on regional (Table 1) unit there are noticeable some significant differences. Table 1. Regional units landslide distribution In order to establish the correlation between the landslide and morphological and functional slope units, 8 case studies have been taken into consideration. They are the following: the left slope of Feiurdeni Valley at Câmpenești (Someşan Plateau); left slope of Long Valley river at Lechința (Transylvania Plain); the left slope of Band Valley at Band (Transylvania Plain); the left slope of Târnava Mare at Copșa Mică (Small Târnava Hills) the left slope of Cergău Valley at Cergău Mare (Secaș Plateau); the left slope of Secașul Mic at Secășel (Secaș Plateau); the left slope of Calva Valley at Buia (Hârtibaciu Plateau); left slope of Rora Valley at Merghindeal (Hârtibaciu Plateau). Following the correlation between the landslides and the functional and morphological units of the slopes at the geomorphological profile level points out the fact that in most of the cases, landslides are located in the median part of the slope which is also matching the maximum process and transfer dynamic unit (Table 2). This is valid not only for the presented case studies but also for other slopes from the Transylvania Basin which are affected by landslide. 108

5 Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin Table 2. The correlation between the morphological and functional units of the slopes and the landslides From the chosen case studies, three of them shall be detailed: the right slope of Band Valley at Band (Transylvania Plain); the right slope of Cergău Valley at Cergău Mare (Secașelor Plateau) and the right slope of Secașul Mic at Secășel (Secașelor Plateau). The morphological and functional units of the right slope of the Band Valley at Band. The pairing mode of the asymmetric interfluves and slopes that have evolved under the influence of massive glimee-type landslides and vide alluvial plains, from the southern half of Transylvania Plain, had also consequences on the distribution of morphological and functional units in the valley-slope system. From this perspective, the specificity is rendered by the withdrawal of the active section on the upper part of the slope, while their median and base sector is represented by accumulated glacis deposits resulted from the slopes materials, a direct consequence of the massive landslides (Fig. 2). Also, the alluvial plain is well developed. In this category fall most of the valley-slope systems from the Eastern part of the Sărmaşului Plain and the Western part of Mădăraş Fărăgău Hills. 109

6 Gh. Roşian, Cs. Horváth 110 Figure 2. The landslides from Band With reference to the analysis of morphological and functional units, it is worth mentioning that the following are noticed on the transversal profile (Fig. 3): an upper steep sector (generally represented by the detachment cornices as a result of the massive landslides), a median transition sector (slightly convex where are cumulated the deposit resulted from the current morphodynamics) and a concave inferior sector (as a consequence of the deposits accumulated mostly as a result of massive landslides). In this classes, despite the fact that the slopes underwent complex modelling, from the profile shape it can be observed that landslides represent the main processes. The maximum processual dynamic and transfer unit presence on the upper side of the slope, at the contact with the water divide, practically integrates the possible processes programming and the process initiation unit. Despite the fact that the existence of these two units it is accepted, they appear as narrow strips practically under the influence of the maximum processual dynamic and transfer unit. Therefore, we opted for a model with only four morphological and functional units. This slope evolution mod, from this part of the Transylvania Plain, explains the considerable width of the current valleys and the slopes cross section shape, which are not only the result of the fluvial processing. To the formation of these valleys contributed the geomorphological processes which affected the slopes during the superior Pleistocen Era and inferior Holocen Era, when most of the massive landslide processes took place. They led to the withdrawal of the active sections of the slopes, which had as a consequence glacis type accumulation units.

7 Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin Figure 3. The morphologic and functional units of the right slope of the Band Valley from Band. The morphologic and functional units of the right slope of the Cergău Valley at Cergău Mare. In this case it is about a slope which is affected by cyclical landslides, thus it evolved by accumulation, forming a base glacis. The materials which form the glacis are landslides parts which were later deposited on the inferior side of the slope. As it can be observed under the aspect of the morphologic and functional units, in the geomorphological cross section profile (Fig. 5) all six units can be identified. Figure 4. The landslides from Cergău Mare and Secășel 111

8 Gh. Roşian, Cs. Horváth From all these, the maximum procedural and transfer dynamic unit is positioned in the median side of the slope (at m), with a 160 m width and an average inclination of From all the existing geomorphological processes, the landslides stand out favoured by the presence in the substrate of the Pannonian clay and by the land use (pastures). This type of evolutional situation is specific to the majority of the slopes form the Secașelor Plateau. Figure 5. The morphological and functional units of the right slope of Cergău Valley at Cergău Mare The morphologic and functional units of the right slope of the Secașul Mic at Secășel. In this case, also the presence of the old massive landslides to which superficial new ones where added determined certain particularities in the arrangement of the morphological and functional units of the slope. Considering the number and the distribution of the morphological and functional units (Fig. 6), we can notice the absence of two of them (the possible processes programming unit and the processes initiation unit) which, through the expansion of the detachment cornices of the landslides to the contact with the water divide, were gradually diminished from the surface until now it is impossible to identify them in the field. The maximum procedural and transfer dynamic unit is placed on the superior side of the slope (at m), with a 130 m width and an average inclination of It overlaps the detachment cornice of old glimee-type landslide which affected the slope extending to the contact water divide. Also, as presented, there are superficial landslides on the surface of this morphological and functional unit. 112

9 Corelations between the landslides and the morphological and functional units of slopes in the Transylvanian Basin Figure 6. The morphological and functional units of the right slope of Secașului Mic at Secășel 4. CONCLUSIONS Following the field observations and the analysis of the presented case studies it can be said that the landslides from the Transylvanian Basin are overlapping particularly the median part of the slopes. Correlating their position with the morphological and functional units of the slope, which were presented in the scientific literature, clearly points out that the landslides are overlapping with maximum dynamic procedural sectors of the slopes regardless of the name that they have received from authors who have studied them (a middle linear element, the midslope, the maximum processual and transfer dynamic unit etc.). Knowing the landslides position on the slope is meant to provide valuable information in actions against them. Taking into account the growing vulnerability of different territories to landslides, these data can serve for proposing preventing measures to landslides in the future. Additionally, understanding the slope sectors susceptible to landslides is beneficial not only for planning actions and territorial development but also for avoiding the settlement of buildings on unstable areas. Understanding the vulnerability to landslides could be helpful for taxation differentiation by the local administrations or for the insurance values of insurance companies. 113

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