The phenomenon of excessive erosion in the area of Slani potok (Vinodol valley)

Abstract

The Vinodol Valley has an irregular elliptical shape with a maximum total length of up to 23 km and a width of up to 4 km, and a broken extension in the main direction NW-SE from the Bakar Bay in the northwest to the hinterland of Novi Vinodolski in the southeast. In the Vinodol Valley area, there are lithofaciesally very different carbonate and clastic sedimentary rocks of the Cretaceous and Paleogene periods, as well as Plioquaternary formations that form a partial or complete cover on the rock mass. The northeastern slopes and the coastal ridge are formed in Upper Cretaceous and Paleogene carbonate rocks. The flysch rock complex of Eocene age builds the lower parts of the northeastern slope and the valley floor. The older part of the flysch is formed by a typical alternation of siltstone and sandstone, and in a subordinate share of marl, breccia conglomerate and limestone. The younger part, which lies unconformably on the older one, consists of conglomerates, conglomeratic biocalcarenites and lithic arenites (BLASKOVIC & TISLJAR, 1983). The most widespread type of cover on the flysch is deluvial-colluvial formations, the thickness of which is greater on the northeastern slope. Quaternary scree breccias form convex plates on the flysch slope, which is generally of uniform slope. They are more common on the northeastern slope of the Vinodol Valley, where they are located in two hypsometric levels (BLASKOVIC, 1983). Tectonic relations and morphogenesis can be observed through the process of continental subduction with the formation of tangential folded and fault structures, during which the Vinodol Valley is unevenly narrowed. A synclinoric structural form of the Middle Eocene flysch basin can be assumed with the appearance of recent folds with axes lying parallel to diagonally in relation to the extension of the valley. The northeastern contact between flysch and carbonate rocks is a reverse fault along which the carbonate rock mass was thrust onto the flysch. The tectonic contact in the southwestern part of the Vinodol Valley is also a reverse fault, but with different kinematics and therefore often subvertical or, on the contrary, oppositely inclined paraclases. Younger, transverse and diagonal faults are markedly outlined in the carbonate slopes of the valley. They have incised relatively rigid carbonate blocks into ductile flysch deposits, so the surfaces of the reverse faults, which frame the Vinodol Valley, are no longer complete in many places (BLASKOVIC, 1999). The most intense tectonic movements, crucial for the final formation of the valley, occurred during the Quaternary. At the same time, a large amount of scree was deposited, from which scree breccias were formed through lithification. They were partially deformed during recent tectonic movements when the accumulation of scree continued at the foot of the cliffs on the northeastern side of the valley. The central part of the Vinodol Valley belongs to the basin of the Dubracina River. The valley is limited on the northeastern side by steep cliffs, whose peaks in places exceed an altitude of 700 m. On the southwestern side, the valley is bordered by the slopes of the coastal ridge, whose peaks reach 250 to 350 m above sea level. In this part, the Vinodol Valley has an asymmetrical transverse profile, with a significantly longer northeastern and shorter southwestern slope, so the Dubracina bed is located for the most part along the coastal ridge. During rainy periods, a network of watercourses is formed due to the low permeability of both the parent flysch rock and the zones of physical and chemical decomposition of various lithogenetic types of deluvial-colluvial formations. Therefore, erosion by washing is locally pronounced. A characteristic of flysch slopes in the Vinodol Valley is the alternation of surfaces where the cover has a protective function from erosion and bare parts, where the parent rocks are attacked by pluvial erosion. Cultivable surfaces have the form of terraces with a network of drainage channels to reduce the harmful effects of erosion and increase the stability of the slopes. Abandoning land cultivation leads to economic degradation of the terrain - the collapse of terrace walls and the filling of channels. Erosion then intensifies, and washing takes on drastic proportions in places, creating terrains of the “badlands” type. Just such an example of the combined effect of erosion with accompanying landslide phenomena is the Slani potok basin, and partly also the basins of the torrential watercourses of Mala Dubracina and Kucina. These are the left tributaries of Dubracina in the central part of the Vinodol Valley. The focus of this phenomenon is in the Slani potok basin. The terrain affected by excessive (excessive) erosion has dimensions of 600 m along the axis and 250 m wide. The Mala Dubracina and Kucina stream basins have not yet been affected by the highest level of erosion, so isolated bare areas are only visible in places. The accompanying phenomena around the erosion focus, as well as in it itself, are numerous landslides as a result of the weathering of the parent rocks of the flysch complex and the conversion of the rock into engineering soil. The affected area is about 3 km2, so the surrounding settlements of Belgrad, Baretici, Grizane and Kamenjak, as well as the surrounding roads, are at risk. The retentions are almost completely filled with sediment, mainly silt. The flysch complex in the Slani potok basin is composed of siltstone and silty marl with rare sandstone layers. The material in the focus is characterized by its dispersion due to the presence of nanometer-sized particles and minerals prone to swelling. This is the reason for its excessive erodibility, which is also contributed by the tectonic deformation of the flysch. The soil contains an increased amount of dissolved sodium ions in the pore water, and the indicator is the mineral thenardite. The erosion process occurs due to the transition of colloidal clay particles into suspension in practically stagnant water. The phenomenon of efflorescence or “blooming” is especially noticeable during the dry season, when clusters of white powder, which is composed of the mineral thenardite, are visible on the surface (MILEUSNIC et al., 2004). The powder has a bitter-salty taste, which justifies the name of the location - Salty Stream. The cover on the parent rock (weathering zone and slope formations) acts as a protector. However, once exposed, the flysch parent rock remains exposed to the effects of weathering for a long time, i.e. repeated wetting and drying, which probably causes a change in the volume of individual lithological flysch members. This process leads to the cross-linking of the rock with drying cracks through which precipitation water penetrates. The final consequence is the loosening and decomposition of the rocks, as well as their gradual transformation into engineering soil (categories CW and RS). The deluvial material thus formed has the property of deformability by plastic flow, especially when saturated with water. Under certain circumstances, these deposits are unstable and landslides similar to those at other locations in the Vinodol Valley occur. Despite numerous rehabilitation measures carried out throughout the 20th century, the process of general terrain degradation is still pronounced, and it is stated that the condition assumes the characteristics of a “permanent natural disaster” (HOLJEVIC, 1996). REFERENCES BLASKOVIC, I. (1983): On the distribution and position of Pliocene and Quaternary deposits in Vinodol. -Geoloski vjesnik, 36, 27-35, Zagreb. BLASKOVIC, I. & TISLJAR, J. (1983): Prominske and Jelar deposits in Vinodol (Croatian coast). -Geoloski vjesnik, 36, 37-50, Zagreb. BLASKOVIC, I. (1999): Tectonics of Part of the Vinodol Valley within the Model of the Continental Crust Subduction. -Geologia Croatica, 52/2, 153-189, Zagreb. HOLJEVIC, D. (1996): Slani potok and Dubracina landslide. -Hrvatska vodoprivreda, 43, 42-44, Zagreb. MILEUSNIC, M., SLOVENEC, D. & JURAK, V. (2004): Thenardite- efflorescence indicating cause of the Excessive Flysch Erosion, Slani potok, Croatia. -Acta Mineralogica-Petrographica, Abstact Series 4, 75, Szeged.

Goran Petrović

Full Professor | Department of Electrical Measurements

Prof. dr. sc. Goran Petrović is a full professor at the Faculty of Electrical Engineering, Mechanical Engineering and Architecture in Split. His research interests include measurement of electrical and process quantities, analysis of geoelectrical and geothermal features of the soil, instrumentation for smart grids, measurement and application of synchrophasors. He is the author of numerous papers published in top-tier scientific journals and contributed to valuable international and national scientific projects.